hd44780: Restructure to implement conn/display/TextDisplay. Refine to use GPIO.Group (#91)

* Add support for Adafruit I2C/SPI Backpack
* Add support for PCF857x Backpacks
* Add support for conn/gpio/Group
* Implement conn/display/TextDisplay
* Add examples
* Cleanups
* Change backlight implementation

go.mod

@@ -12,7 +12,7 @@ require (
 	github.com/mattn/go-colorable v0.1.13
 	golang.org/x/image v0.23.0
 	periph.io/x/conn/v3 v3.7.2
-	periph.io/x/host/v3 v3.8.3
+	periph.io/x/host/v3 v3.8.4
 )
 
 require (

go.sum

@@ -17,5 +17,5 @@ golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
 golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 periph.io/x/conn/v3 v3.7.2 h1:qt9dE6XGP5ljbFnCKRJ9OOCoiOyBGlw7JZgoi72zZ1s=
 periph.io/x/conn/v3 v3.7.2/go.mod h1:Ao0b4sFRo4QOx6c1tROJU1fLJN1hUIYggjOrkIVnpGg=
-periph.io/x/host/v3 v3.8.3 h1:v90ozCFDWgEyfNElZ+JnOvq0jAdW0vmgjCUy8dYXDds=
-periph.io/x/host/v3 v3.8.3/go.mod h1:uKrIpfXjELwHkwGBNe6aos//XiQ/3uxDa1P2BmLV6Ok=
+periph.io/x/host/v3 v3.8.4 h1:QNleTythDd0k6Chu0n+ISrJFlf3LFig9oNbtOIkxoCc=
+periph.io/x/host/v3 v3.8.4/go.mod h1:hPq8dISZIc+UNfWoRj+bPH3XEBQqJPdFdx218W92mdc=

hd44780/README.md

@@ -0,0 +1,60 @@
+# Hitachi HD44780 Package
+
+## Overview
+
+The Hitachi HD44780 is an LCD driver chip. It's used in a variety of text LCD 
+displays. The datasheet is available here:
+
+https://www.sparkfun.com/datasheets/LCD/HD44780.pdf
+
+Generally, there are three kinds of displays that use this chip.
+
+1. A raw display. This is a board with a row of pin connectors at the top. It's
+made for interfacing directly to GPIO pins. This is the most complicated method
+of using the LCD because you have to wire a minimum of 6 GPIO pins (4 data, 1 
+reset, 1 enable) and 2 power pins to make it work.
+
+There are also complex initialization routines that have to be performed. This
+is further complicated by having specific initialization calls depending upon
+whether the device is connected to 4 data lines, or 8 data lines.
+
+2. A backpack display. With this type of LCD display, an I2C, serial, or SPI
+interface is provided. This type of display is easier because there are fewer
+pins, but the complex intialization remains. Examples of backpack interfaces
+include The Adafruit I2C/SPI backpack (MCP23008/74HC595D), the generic 
+LCDXXXX/PCF8547T backpack, etc.
+
+3. The third and final variety is the "Intelligent" display. These displays have
+a micro-controller that is connected to the LCD chip. Typically these intelligent
+displays support multiple I/O methods and the micro-controller handles the
+LCD initialization/communication.
+
+Examples of this kind of display would be the SparkFun SerLCD display, the
+MatrixOrbital LK2047T, the AdaFruit USB+Serial Backpack etc.
+
+## Interfacing Notes
+
+The driver package is designed to use the gpio.Group interface. This allows
+the LCD driver to be agnostic about the physical connection between the display 
+and the host device. Any host/expander that supports the 
+periph.io/x/conn/v3/gpio.Group interface can be used to easily drive the LCD 
+display.
+
+## Hardware Notes
+
+DO NOT attempt to source VCC for the unit backlight, or sink VCC to ground.
+The backlight draws ~250ma of current which exceeds the current capability
+of GPIO pins. It will permanently damage your device. If you would like to 
+control the backlight, connect a GPIO pin through a 1K Ohm resistor to a 
+transistor (2N2222 or equivalent).
+
+## Troubleshooting
+
+If nothing displays at all, check the contrast. Adjust the contrast control
+until the 5x7 dot grid on the display is visible.
+
+If the first row contains blocks of dots, and the other rows are blank, then
+the initialization of the device failed. Check IO pins are connected properly.
+
+If the text is garbled, verify the gpio.Group is configured and the IO Pins
+are connected to the right pins of the LCD display.

hd44780/af_i2c_backpack.go

@@ -0,0 +1,62 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"periph.io/x/conn/v3/gpio"
+	"periph.io/x/conn/v3/i2c"
+	"periph.io/x/conn/v3/spi"
+	"periph.io/x/devices/v3/mcp23xxx"
+	"periph.io/x/devices/v3/nxp74hc595"
+)
+
+const (
+	// Name is the LCD pin name, and the integer value is the GPIO
+	// number (not physical) of the MCP23008 I2C GPIO Expander.
+	d4           = 3
+	d5           = 4
+	d6           = 5
+	d7           = 6
+	rsPin        = 1
+	enablePin    = 2
+	backlightPin = 7
+)
+
+// This function returns a display configured to use the Adafruit I2C/SPI LCD Backpack.
+//
+// # Product Information
+//
+// https://www.adafruit.com/product/292
+//
+// The I2C side of this backpack uses an MCP23008 I/O expander. This function
+// creates an MCP23008 device with the required pin configuration. To use this,
+// get an I2C bus, and call this function with the bus, i2c address, number of
+// rows, and columns.
+func NewAdafruitI2CBackpack(bus i2c.Bus, address uint16, rows, cols int) (*HD44780, error) {
+	mcp, err := mcp23xxx.NewI2C(bus, mcp23xxx.MCP23008, address)
+	if err != nil {
+		return nil, err
+	}
+	gr := *mcp.Group(0, []int{d4, d5, d6, d7, rsPin, enablePin, backlightPin})
+	reset, _ := gr.ByOffset(4).(gpio.PinOut)
+	enable, _ := gr.ByOffset(5).(gpio.PinOut)
+	bl := gr.ByOffset(6).(gpio.PinOut)
+	return NewHD44780(gr, reset, enable, NewBacklight(bl), rows, cols)
+}
+
+// This function returns a display configured to use the SPI side of the Adafruit
+// I2c/SPI backpack. The SPI side uses a 74HC595 Serial->Parallel shift register.
+func NewAdafruitSPIBackpack(conn spi.Conn, rows, cols int) (*HD44780, error) {
+	chip, err := nxp74hc595.New(conn)
+	if err != nil {
+		return nil, err
+	}
+	// The SPI side has the same pins but in reverse order from the I2C side.
+	gr, _ := chip.Group(d7, d6, d5, d4)
+	rs := chip.Pins[rsPin]
+	e := chip.Pins[enablePin]
+	bl := chip.Pins[backlightPin]
+	return NewHD44780(gr, rs, e, NewBacklight(bl), rows, cols)
+}

hd44780/backlight.go

@@ -0,0 +1,40 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"fmt"
+
+	"periph.io/x/conn/v3/display"
+	"periph.io/x/conn/v3/gpio"
+)
+
+// A monochrome backlight  Implements display.Backlight It uses a single
+// GPIO Pin to turn the backlight on or off.
+type GPIOMonoBacklight struct {
+	blPin gpio.PinOut
+}
+
+// Given a GPIO pin that turns the backlight on/off, construct a monobacklight
+// to use with HD44780.
+func NewBacklight(blPin gpio.PinOut) *GPIOMonoBacklight {
+	return &GPIOMonoBacklight{blPin: blPin}
+}
+
+// Turn the display backlight on or off.
+func (bl *GPIOMonoBacklight) Backlight(intensity display.Intensity) (err error) {
+	if intensity == 0 {
+		err = bl.blPin.Out(gpio.Low)
+	} else {
+		err = bl.blPin.Out(gpio.High)
+	}
+	return err
+}
+
+func (bl *GPIOMonoBacklight) String() string {
+	return fmt.Sprintf("%#v", bl)
+}
+
+var _ display.DisplayBacklight = &GPIOMonoBacklight{}

hd44780/dev.go

@@ -0,0 +1,225 @@
+// Copyright 2018 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"fmt"
+	"time"
+
+	"periph.io/x/conn/v3"
+	"periph.io/x/conn/v3/gpio"
+)
+
+// lineTwo offset for the second line in the LCD buffer.
+const lineTwo = 0x40
+
+// Deprecated: Use HD44780
+type Dev struct {
+	// data pins
+	dataPins []gpio.PinOut
+
+	// register select pin
+	rsPin gpio.PinOut
+
+	// enable pin
+	enablePin gpio.PinOut
+}
+
+// Deprecated: Use NewHD44780()
+// New creates and initializes the LCD device
+//
+//	data - references to data pins
+//	rs - rs pin
+//	e - strobe pin
+func New(data []gpio.PinOut, rs, e gpio.PinOut) (*Dev, error) {
+	if len(data) != 4 {
+		return nil, fmt.Errorf("expected 4 data pins, passed %d", len(data))
+	}
+	dev := &Dev{
+		dataPins:  data,
+		enablePin: e,
+		rsPin:     rs,
+	}
+	if err := dev.Reset(); err != nil {
+		return nil, err
+	}
+	return dev, nil
+}
+
+// Reset resets the HC-44780 chipset, clears the screen buffer and moves cursor to the
+// home of screen (line 0, column 0).
+func (r *Dev) Reset() error {
+	if err := r.clearBits(); err != nil {
+		return err
+	}
+
+	delayMs(15)
+
+	if err := r.rsPin.Out(gpio.Low); err != nil {
+		return err
+	}
+	if err := r.enablePin.Out(gpio.Low); err != nil {
+		return err
+	}
+
+	if err := r.bulkSendData(resetSequence, r.write4Bits); err != nil {
+		return err
+	}
+
+	return r.bulkSendData(initSequence, r.writeInstruction)
+}
+
+func (r *Dev) String() string {
+	return "HD44870, 4 bit mode"
+}
+
+// Halt clears the LCD screen
+func (r *Dev) Halt() error {
+	if err := r.writeInstruction(0x01); err != nil {
+		return err
+	}
+	delayMs(2)
+	return nil
+}
+
+// SetCursor positions the cursor
+//
+//	line - screen line, 0-based
+//	column - column, 0-based
+func (r *Dev) SetCursor(line uint8, column uint8) error {
+	return r.writeInstruction(0x80 | (line*lineTwo + column))
+}
+
+// Print the data string
+//
+//	data string to display
+func (r *Dev) Print(data string) error {
+	for _, v := range []byte(data) {
+		if err := r.WriteChar(v); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// WriteChar writes a single byte (character) at the cursor position.
+//
+//	data - character code
+func (r *Dev) WriteChar(data uint8) error {
+	if err := r.sendData(); err != nil {
+		return err
+	}
+	if err := r.write4Bits(data >> 4); err != nil {
+		return err
+	}
+	if err := r.write4Bits(data); err != nil {
+		return err
+	}
+	delayUs(10)
+	return nil
+}
+
+// service methods
+
+var resetSequence = [][]uint{
+	{0x03, 50}, // init 1-st cycle
+	{0x03, 10}, // init 2-nd cycle
+	{0x03, 10}, // init 3-rd cycle
+	{0x02, 10}, // init finish
+}
+
+var initSequence = [][]uint{
+	{0x14, 0},    // 4-bit mode, 2 lines, 5x7 chars high
+	{0x10, 0},    // disable display
+	{0x01, 2000}, // clear screen
+	{0x06, 0},    // cursor shift right, no display move
+	{0x0c, 0},    // enable display no cursor
+	{0x01, 2000}, // clear screen
+	{0x02, 2000}, // cursor home
+}
+
+func (r *Dev) bulkSendData(seq [][]uint, f func(_data uint8) error) error {
+	for _, v := range seq {
+		if err := f(uint8(v[0])); err != nil {
+			return err
+		}
+		if v[1] > 0 {
+			delayUs(v[1])
+		}
+	}
+	return nil
+}
+
+func (r *Dev) clearBits() error {
+	for _, v := range r.dataPins {
+		if err := v.Out(gpio.Low); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (r *Dev) write4Bits(data uint8) error {
+	for i, v := range r.dataPins {
+		if data&(1<<uint(i)) > 0 {
+			if err := v.Out(gpio.High); err != nil {
+				return err
+			}
+		} else {
+			if err := v.Out(gpio.Low); err != nil {
+				return err
+			}
+		}
+	}
+	return r.strobe()
+}
+
+func (r *Dev) sendInstruction() error {
+	if err := r.rsPin.Out(gpio.Low); err != nil {
+		return err
+	}
+	return r.enablePin.Out(gpio.Low)
+}
+
+func (r *Dev) sendData() error {
+	if err := r.rsPin.Out(gpio.High); err != nil {
+		return err
+	}
+	return r.enablePin.Out(gpio.Low)
+}
+
+func (r *Dev) writeInstruction(data uint8) error {
+	if err := r.sendInstruction(); err != nil {
+		return err
+	}
+	// write high 4 bits
+	if err := r.write4Bits(data >> 4); err != nil {
+		return err
+	}
+	// write low  bits
+	if err := r.write4Bits(data); err != nil {
+		return err
+	}
+	delayUs(50)
+	return nil
+}
+
+func (r *Dev) strobe() error {
+	if err := r.enablePin.Out(gpio.High); err != nil {
+		return err
+	}
+	delayUs(2)
+	return r.enablePin.Out(gpio.Low)
+}
+
+func delayUs(ms uint) {
+	time.Sleep(time.Duration(ms) * time.Microsecond)
+}
+
+func delayMs(ms int) {
+	time.Sleep(time.Duration(ms) * time.Millisecond)
+}
+
+var _ conn.Resource = &Dev{}

hd44780/example_test.go

@@ -0,0 +1,160 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780_test
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"time"
+
+	"periph.io/x/conn/v3/display"
+	"periph.io/x/conn/v3/display/displaytest"
+	"periph.io/x/conn/v3/gpio"
+	"periph.io/x/conn/v3/i2c/i2creg"
+	"periph.io/x/conn/v3/physic"
+	"periph.io/x/conn/v3/spi"
+	"periph.io/x/conn/v3/spi/spireg"
+	"periph.io/x/devices/v3/hd44780"
+	"periph.io/x/devices/v3/pcf857x"
+	"periph.io/x/host/v3"
+	"periph.io/x/host/v3/gpioioctl"
+)
+
+// This example shows using a gpio.Group with an HD44780 display. For this
+// example its using the periph.io/x/host/gpioioctl package to obtain
+// the gpio.Group and pins. You can use an I/O device that implements
+// gpio.Group and gpio.PinOut to drive a display.
+func Example() {
+	var err error
+	if _, err = host.Init(); err != nil {
+		log.Fatal(err)
+	}
+	chip := gpioioctl.Chips[0]
+	var ls gpio.Group
+	// Using a group to obtain the pins. The first 4 pins in the group are the
+	// data pins, and the remaining ones are reset, enable, and backlight.
+	// For 8 bit mode, specify additional data pins.
+	ls, err = chip.LineSet(gpioioctl.LineOutput, gpio.NoEdge, gpio.PullNoChange,
+		"GPIO27", "GPIO22", "GPIO23", "GPIO24", "GPIO17", "GPIO18", "GPIO25")
+	if err != nil {
+		log.Fatal(err)
+	}
+	pins := ls.Pins()
+	reset := pins[4].(gpio.PinOut)
+	enable := pins[5].(gpio.PinOut)
+	bl := hd44780.NewBacklight(pins[6].(gpio.PinOut))
+	lcd, err := hd44780.NewHD44780(ls, reset, enable, bl, 2, 16)
+	if err != nil {
+		log.Fatal(err)
+	}
+	n, err := lcd.WriteString("Hello")
+	time.Sleep(5 * time.Second)
+	fmt.Printf("n=%d, err=%s\n", n, err)
+	fmt.Println("lcd=", lcd.String())
+
+	_ = lcd.Home()
+	_ = lcd.MoveTo(1, 1)
+	_, _ = lcd.WriteString("Line 1")
+	_ = lcd.MoveTo(2, 2)
+	_, _ = lcd.WriteString("Line 2")
+	time.Sleep(5 * time.Second)
+	_ = lcd.Clear()
+
+	fmt.Println("calling TestTextDisplay")
+
+	errs := displaytest.TestTextDisplay(lcd, true)
+	fmt.Println("back from TestTextDisplay")
+	for _, e := range errs {
+		if !errors.Is(e, display.ErrNotImplemented) {
+			log.Println(e)
+		}
+	}
+}
+
+// Create a new HD44780 that uses the Adafruit I2C/SPI Backpack.
+func ExampleNewAdafruitI2CBackpack() {
+	// Make sure periph is initialized.
+	if _, err := host.Init(); err != nil {
+		log.Fatal(err)
+	}
+
+	// Open default I²C bus.
+	bus, err := i2creg.Open("")
+	if err != nil {
+		log.Fatalf("failed to open I²C: %v", err)
+	}
+	defer bus.Close()
+	dev, err := hd44780.NewAdafruitI2CBackpack(bus, 0x20, 4, 20)
+	fmt.Println(dev.String())
+	if err != nil {
+		log.Fatal(err)
+	}
+	_ = dev.Clear()
+	_, _ = dev.WriteString("Hello")
+	fmt.Println("wrote hello")
+	time.Sleep(5 * time.Second)
+	fmt.Println("calling test text display")
+	_ = displaytest.TestTextDisplay(dev, true)
+}
+
+func ExampleNewAdafruitSPIBackpack() {
+	if _, err := host.Init(); err != nil {
+		log.Fatal(err)
+	}
+
+	pc, err := spireg.Open("")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer pc.Close()
+	conn, err := pc.Connect(physic.MegaHertz, spi.Mode1, 8)
+	if err != nil {
+		log.Fatal(err)
+	}
+	display, err := hd44780.NewAdafruitSPIBackpack(conn, 2, 26)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	_ = display.Clear()
+	_, _ = display.WriteString("Hello")
+	time.Sleep(5 * time.Second)
+	_ = displaytest.TestTextDisplay(display, true)
+}
+
+func ExampleNewPCF857xBackpack() {
+	// Make sure periph is initialized.
+	if _, err := host.Init(); err != nil {
+		log.Fatal(err)
+	}
+
+	// Open default I²C bus.
+	bus, err := i2creg.Open("")
+	if err != nil {
+		log.Fatalf("failed to open I²C: %v", err)
+	}
+	defer bus.Close()
+	dev, err := hd44780.NewPCF857xBackpack(bus, pcf857x.DefaultAddress, 4, 20)
+	fmt.Println(dev.String())
+	if err != nil {
+		log.Fatal(err)
+
+	}
+	for range 5 {
+		fmt.Println("toggling backlight")
+		_ = dev.Backlight(0)
+		time.Sleep(500 * time.Millisecond)
+		_ = dev.Backlight(255)
+		time.Sleep(500 * time.Millisecond)
+
+	}
+	_ = dev.Clear()
+	_, _ = dev.WriteString("Hello")
+	fmt.Println("wrote hello")
+	time.Sleep(5 * time.Second)
+	fmt.Println("calling test text display")
+	_ = displaytest.TestTextDisplay(dev, true)
+}

hd44780/hd44780.go

@@ -1,4 +1,4 @@
-// Copyright 2018 The Periph Authors. All rights reserved.
+// Copyright 2025 The Periph Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
@@ -14,216 +14,427 @@ import (
 	"time"
 
 	"periph.io/x/conn/v3"
+	"periph.io/x/conn/v3/display"
 	"periph.io/x/conn/v3/gpio"
 )
 
-// lineTwo offset for the second line in the LCD buffer.
-const lineTwo = 0x40
+type writeMode bool
 
-// Dev is the 4-bit addressing device for HD-44780
-type Dev struct {
-	// data pins
-	dataPins []gpio.PinOut
+type ifMode byte
 
-	// register select pin
-	rsPin gpio.PinOut
+const (
+	modeCommand writeMode = false
+	modeData    writeMode = true
 
-	// enable pin
+	cmdByte byte = 0xfe
+
+	mode4Bit ifMode = 0x04
+	mode8Bit ifMode = 0x08
+)
+
+// HD44780 is an implementation that supports writing to LCD displays using a
+// gpio.Group for the data pins, and discrete pins for the reset, enable, and
+// and backlight pins.
+//
+// Implements periph.io/conn/x/display/TextDisplay and display.DisplayBacklight
+type HD44780 struct {
+	dataPins  gpio.Group
+	resetPin  gpio.PinOut
 	enablePin gpio.PinOut
+	blMono    display.DisplayBacklight
+	blRGB     display.DisplayRGBBacklight
+	mode      ifMode
+	rows      int
+	cols      int
+	on        bool
+	cursor    bool
+	blink     bool
+	lastWrite int64
 }
 
-// New creates and initializes the LCD device
-//
-//	data - references to data pins
-//	rs - rs pin
-//	e - strobe pin
-func New(data []gpio.PinOut, rs, e gpio.PinOut) (*Dev, error) {
-	if len(data) != 4 {
-		return nil, fmt.Errorf("expected 4 data pins, passed %d", len(data))
-	}
-	dev := &Dev{
-		dataPins:  data,
-		enablePin: e,
-		rsPin:     rs,
-	}
-	if err := dev.Reset(); err != nil {
-		return nil, err
+const (
+	delayCommand   time.Duration = 2000
+	delayCharacter time.Duration = 200
+)
+
+var rowConstants = [][]byte{{0, 0, 64}, {0, 0, 64, 20, 84}}
+var clearScreen = []byte{cmdByte, 0x01}
+var goHome = []byte{cmdByte, 0x02}
+var setCursorPosition = []byte{cmdByte, 0x80}
+
+// Return the row offset value
+func getRowConstant(row, maxcols int) byte {
+	var offset int
+	if maxcols != 16 {
+		offset = 1
 	}
-	return dev, nil
+	return rowConstants[offset][row]
 }
 
-// Reset resets the HC-44780 chipset, clears the screen buffer and moves cursor to the
-// home of screen (line 0, column 0).
-func (r *Dev) Reset() error {
-	if err := r.clearBits(); err != nil {
-		return err
-	}
+// NewHD44780 takes a GPIO group, and gpio.PinOut for reset and enable. It
+// returns an HD44780 device in an initialized state and ready for use.
+//
+// The first 4 or 8 pins of the data group must be connected to the data lines
+// To use 4 bit mode, you would connect lines D4-D7 on the display, and for 8
+// bit mode, D0-D7. If dataPinGroup is 8 or more pins, then it's assumed the
+// display is connected using all 8 pins.
+//
+// backlight should implement either display.DisplayBacklight or
+// display.DisplayRGBBacklight. See GPIOMonoBacklight.
+func NewHD44780(
+	dataPinGroup gpio.Group,
+	resetPin, enablePin gpio.PinOut,
+	backlight any,
+	rows, cols int) (*HD44780, error) {
 
-	delayMs(15)
+	mode := mode4Bit
+	if len(dataPinGroup.Pins()) >= 8 {
+		mode = mode8Bit
+	}
 
-	if err := r.rsPin.Out(gpio.Low); err != nil {
-		return err
+	lcd := &HD44780{
+		dataPins:  dataPinGroup,
+		resetPin:  resetPin,
+		enablePin: enablePin,
+		mode:      mode,
+		rows:      rows,
+		cols:      cols,
+		on:        true,
 	}
-	if err := r.enablePin.Out(gpio.Low); err != nil {
-		return err
+	switch bl := backlight.(type) {
+	case display.DisplayBacklight:
+		lcd.blMono = bl
+	case display.DisplayRGBBacklight:
+		lcd.blRGB = bl
 	}
+	return lcd, lcd.init()
+}
 
-	if err := r.bulkSendData(resetSequence, r.write4Bits); err != nil {
-		return err
-	}
+// Not supported by this device. Returns display.ErrNotImplemented
+func (lcd *HD44780) AutoScroll(enabled bool) error {
+	// TODO: Wrap
+	return display.ErrNotImplemented
+}
 
-	return r.bulkSendData(initSequence, r.writeInstruction)
+// Clears the screen and moves the cursor to the first position.
+func (lcd *HD44780) Clear() error {
+	_, err := lcd.Write(clearScreen)
+	return err
 }
 
-func (r *Dev) String() string {
-	return "HD44870, 4 bit mode"
+// Return the number of columns the display supports
+func (lcd *HD44780) Cols() int {
+	return lcd.cols
 }
 
-// Halt clears the LCD screen
-func (r *Dev) Halt() error {
-	if err := r.writeInstruction(0x01); err != nil {
-		return err
+// Set the cursor mode. You can pass multiple arguments.
+// Cursor(CursorOff, CursorUnderline)
+func (lcd *HD44780) Cursor(modes ...display.CursorMode) (err error) {
+	var val = byte(0x08)
+	if lcd.on {
+		val |= 0x04
 	}
-	delayMs(2)
-	return nil
+	for _, mode := range modes {
+		switch mode {
+		case display.CursorOff:
+			// lcd.Write(underlineCursorOff)
+			lcd.blink = false
+			lcd.cursor = false
+		case display.CursorBlink:
+			lcd.blink = true
+			lcd.cursor = true
+			val |= 0x01
+		case display.CursorUnderline:
+			lcd.cursor = true
+			lcd.blink = true
+			// lcd.Write(underlineCursorOn)
+			val |= 0x02
+		case display.CursorBlock:
+			lcd.cursor = true
+			lcd.blink = true
+			val |= 0x01
+		default:
+			err = fmt.Errorf("HD44780 - unexpected cursor: %d", mode)
+			return
+		}
+	}
+	_, err = lcd.Write([]byte{cmdByte, val & 0x0f})
+	return err
 }
 
-// SetCursor positions the cursor
-//
-//	line - screen line, 0-based
-//	column - column, 0-based
-func (r *Dev) SetCursor(line uint8, column uint8) error {
-	return r.writeInstruction(0x80 | (line*lineTwo + column))
+// Move the cursor home (MinRow(),MinCol())
+func (lcd *HD44780) Home() (err error) {
+	_, err = lcd.Write(goHome)
+	return err
 }
 
-// Print the data string
-//
-//	data string to display
-func (r *Dev) Print(data string) error {
-	for _, v := range []byte(data) {
-		if err := r.WriteChar(v); err != nil {
-			return err
-		}
-	}
-	return nil
+// Return the min column position.
+func (lcd *HD44780) MinCol() int {
+	return 1
 }
 
-// WriteChar writes a single byte (character) at the cursor position.
-//
-//	data - character code
-func (r *Dev) WriteChar(data uint8) error {
-	if err := r.sendData(); err != nil {
-		return err
-	}
-	if err := r.write4Bits(data >> 4); err != nil {
-		return err
-	}
-	if err := r.write4Bits(data); err != nil {
-		return err
+// Return the min row position.
+func (lcd *HD44780) MinRow() int {
+	return 1
+}
+
+// Move the cursor forward or backward.
+func (lcd *HD44780) Move(dir display.CursorDirection) (err error) {
+	var val byte = 0x10
+	switch dir {
+	case display.Backward:
+	case display.Forward:
+		val |= 0x04
+	case display.Down, display.Up:
+		fallthrough
+	default:
+		err = fmt.Errorf("hd44780: %w", display.ErrNotImplemented)
+		return
 	}
-	delayUs(10)
-	return nil
+	_, err = lcd.Write([]byte{cmdByte, val})
+	return
 }
 
-// service methods
+// Move the cursor to arbitrary position.
+func (lcd *HD44780) MoveTo(row, col int) (err error) {
+	if row < lcd.MinRow() || row > lcd.rows || col < lcd.MinCol() || col > lcd.cols {
+		err = fmt.Errorf("HD44780.MoveTo(%d,%d) value out of range", row, col)
+		return
+	}
+	var cmd = []byte{cmdByte, setCursorPosition[1]}
+	cmd[1] |= getRowConstant(row, lcd.cols) + byte(col-1)
+	_, err = lcd.Write(cmd)
+	return
+}
 
-var resetSequence = [][]uint{
-	{0x03, 50}, // init 1-st cycle
-	{0x03, 10}, // init 2-nd cycle
-	{0x03, 10}, // init 3-rd cycle
-	{0x02, 10}, // init finish
+// Return the number of rows the display supports.
+func (lcd *HD44780) Rows() int {
+	return lcd.rows
 }
 
-var initSequence = [][]uint{
-	{0x14, 0},    // 4-bit mode, 2 lines, 5x7 chars high
-	{0x10, 0},    // disable display
-	{0x01, 2000}, // clear screen
-	{0x06, 0},    // cursor shift right, no display move
-	{0x0c, 0},    // enable display no cursor
-	{0x01, 2000}, // clear screen
-	{0x02, 2000}, // cursor home
+// Return info about the dsiplay.
+func (lcd *HD44780) String() string {
+	return fmt.Sprintf("HD44780 - Rows: %d, Cols: %d", lcd.rows, lcd.cols)
 }
 
-func (r *Dev) bulkSendData(seq [][]uint, f func(_data uint8) error) error {
-	for _, v := range seq {
-		if err := f(uint8(v[0])); err != nil {
-			return err
-		}
-		if v[1] > 0 {
-			delayUs(v[1])
-		}
+// Turn the display on / off
+func (lcd *HD44780) Display(on bool) error {
+	lcd.on = on
+	val := byte(0x08)
+	if on {
+		val |= 0x04
 	}
-	return nil
+	if lcd.blink {
+		val |= 0x01
+	}
+	if lcd.cursor {
+		val |= 0x02
+	}
+	_, err := lcd.Write([]byte{cmdByte, val})
+	return err
+
 }
 
-func (r *Dev) clearBits() error {
-	for _, v := range r.dataPins {
-		if err := v.Out(gpio.Low); err != nil {
-			return err
-		}
+// Write a set of bytes to the display.
+func (lcd *HD44780) Write(p []byte) (n int, err error) {
+
+	if len(p) == 0 {
+		return
+	}
+	if p[0] == cmdByte {
+		n = len(p) - 1
+		err = lcd.sendCommand(p[1:])
+		return
+	}
+	lcd.delayWrite(delayCommand)
+	err = lcd.resetPin.Out(gpio.Level(modeData))
+	if err != nil {
+		return
 	}
-	return nil
-}
 
-func (r *Dev) write4Bits(data uint8) error {
-	for i, v := range r.dataPins {
-		if data&(1<<uint(i)) > 0 {
-			if err := v.Out(gpio.High); err != nil {
-				return err
+	for _, byteVal := range p {
+		lcd.lastWrite = time.Now().UnixMicro()
+		if lcd.mode == mode4Bit {
+			err = lcd.write4Bits(byteVal >> 4)
+			if err == nil {
+				err = lcd.write4Bits(byteVal & 0x0f)
 			}
 		} else {
-			if err := v.Out(gpio.Low); err != nil {
-				return err
-			}
+			err = lcd.write8Bits(byteVal)
 		}
+		if err != nil {
+			return
+		}
+		n += 1
+		time.Sleep(delayCharacter * time.Microsecond)
 	}
-	return r.strobe()
+	lcd.lastWrite = time.Now().UnixMicro()
+	return
 }
 
-func (r *Dev) sendInstruction() error {
-	if err := r.rsPin.Out(gpio.Low); err != nil {
-		return err
-	}
-	return r.enablePin.Out(gpio.Low)
+// Write a string output to the display.
+func (lcd *HD44780) WriteString(text string) (int, error) {
+	return lcd.Write([]byte(text))
 }
 
-func (r *Dev) sendData() error {
-	if err := r.rsPin.Out(gpio.High); err != nil {
-		return err
+// Halt clears the display, turns the backlight off, and turns the display off.
+// Halt() is called for the data pins gpio.Group.
+func (lcd *HD44780) Halt() error {
+	_ = lcd.Clear()
+	_ = lcd.Backlight(0)
+	_ = lcd.Display(false)
+	return lcd.dataPins.Halt()
+}
+
+// Set the backlight intensity.
+func (lcd *HD44780) Backlight(intensity display.Intensity) error {
+	if lcd.blMono != nil {
+		return lcd.blMono.Backlight(intensity)
+	} else if lcd.blRGB != nil {
+		return lcd.blRGB.RGBBacklight(intensity, intensity, intensity)
 	}
-	return r.enablePin.Out(gpio.Low)
+	return display.ErrNotImplemented
 }
 
-func (r *Dev) writeInstruction(data uint8) error {
-	if err := r.sendInstruction(); err != nil {
-		return err
+// For units that have an RGB Backlight, set the backlight color/intensity.
+// The range of the values is 0-255.
+func (lcd *HD44780) RGBBacklight(red, green, blue display.Intensity) error {
+	if lcd.blRGB != nil {
+		return lcd.blRGB.RGBBacklight(red, green, blue)
+	} else if lcd.blMono != nil {
+		return lcd.blMono.Backlight(red | green | blue)
 	}
-	// write high 4 bits
-	if err := r.write4Bits(data >> 4); err != nil {
-		return err
+	return display.ErrNotImplemented
+}
+
+// delayWrite looks at the time of the last LCD write and if the specified
+// microseconds period has not elapsed, it invokes time.Sleep() with the
+// difference.
+//
+// Some I/O methods, like direct GPIO on a Pi are very fast, while other methods
+// like i2c take longer. Without delays, on very fast I/O paths, the LCD will
+// display garbage. The correct way to handle this would be to read the Busy
+// flag on the LCD display. However, some backpacks don't have the capability to
+// check the Busy flag because the R/W pin isn't connected. So, we can't
+// correctly handle io delays. This handles the very fast interfaces, while not
+// penalizing the slower ones with unnecessary delays.
+//
+// The value of lcd.lastWrite is updated to the current time by the call.
+func (lcd *HD44780) delayWrite(microseconds time.Duration) {
+	diff := microseconds - time.Duration(time.Now().UnixMicro()-lcd.lastWrite)
+	if diff > 0 {
+		time.Sleep(time.Duration(diff) * time.Microsecond)
 	}
-	// write low  bits
-	if err := r.write4Bits(data); err != nil {
-		return err
+	lcd.lastWrite = time.Now().UnixMicro()
+}
+
+// Init the display. The HD44780 has a fairly complex initialization cycle
+// with variations for 4 and 8 pin mode.
+func (lcd *HD44780) init() error {
+	/*
+	   This is the startup sequence for the Hitachi HD44780U chip as
+	   documented in the Datasheet.
+	*/
+	lcd.lastWrite = time.Now().UnixMicro()
+	if lcd.mode == mode4Bit {
+		var lineMode byte = 0x20
+		if lcd.rows > 1 {
+			lineMode |= 0x08
+		}
+		err := lcd.resetPin.Out(gpio.Level(modeCommand))
+		if err != nil {
+			return err
+		}
+		err = lcd.enablePin.Out(gpio.Low)
+		if err != nil {
+			return err
+		}
+		err = lcd.write4Bits(0x03)
+		if err != nil {
+			return err
+		}
+		time.Sleep(4100 * time.Microsecond)
+		_ = lcd.write4Bits(0x03)
+		_ = lcd.write4Bits(0x03)
+		_ = lcd.write4Bits(0x02)
+		_ = lcd.sendCommand([]byte{lineMode})
+	} else {
+		// Init the display for 8 pin operation.
+		lineMode := byte(0x30) // Set the line mode and interface to 8 bits
+		if lcd.rows > 1 {
+			lineMode |= 0x08
+		}
+		err := lcd.resetPin.Out(gpio.Level(modeCommand))
+		if err != nil {
+			return err
+		}
+		err = lcd.enablePin.Out(gpio.Low)
+		if err != nil {
+			return err
+		}
+
+		_ = lcd.write8Bits(0x03 << 4) // Get it's attention
+		time.Sleep(4100 * time.Microsecond)
+		_ = lcd.write8Bits(0x03 << 4)
+		_ = lcd.write8Bits(0x03 << 4)
+		_ = lcd.write8Bits(lineMode)
+		_ = lcd.write8Bits(0x4) // set entry mode
 	}
-	delayUs(50)
+	_ = lcd.Cursor(display.CursorOff)
+	_ = lcd.Display(true)
+	_ = lcd.Clear()
+	_ = lcd.Home()
+	// If there's not a backlight, ignore the error.
+	_ = lcd.Backlight(0xff)
 	return nil
 }
 
-func (r *Dev) strobe() error {
-	if err := r.enablePin.Out(gpio.High); err != nil {
+func (lcd *HD44780) sendCommand(commands []byte) error {
+	lcd.delayWrite(delayCommand)
+	err := lcd.resetPin.Out(gpio.Level(modeCommand))
+	if err != nil {
 		return err
 	}
-	delayUs(2)
-	return r.enablePin.Out(gpio.Low)
+	for _, command := range commands {
+		if lcd.mode == mode4Bit {
+			err = lcd.write4Bits(byte(command >> 4))
+			if err == nil {
+				err = lcd.write4Bits(byte(command))
+			}
+		} else {
+			err = lcd.write8Bits(command)
+		}
+		if err != nil {
+			break
+		}
+
+	}
+	lcd.lastWrite = time.Now().UnixMicro()
+	return err
+}
+
+func (lcd *HD44780) write4Bits(value byte) error {
+	return lcd.writeBits(gpio.GPIOValue(value), 0x0f)
 }
 
-func delayUs(ms uint) {
-	time.Sleep(time.Duration(ms) * time.Microsecond)
+func (lcd *HD44780) write8Bits(value byte) error {
+	return lcd.writeBits(gpio.GPIOValue(value), 0xff)
 }
 
-func delayMs(ms int) {
-	time.Sleep(time.Duration(ms) * time.Millisecond)
+func (lcd *HD44780) writeBits(value, mask gpio.GPIOValue) error {
+	err := lcd.dataPins.Out(value, mask)
+	if err != nil {
+		return err
+	}
+	err = lcd.enablePin.Out(gpio.High)
+	if err == nil {
+		time.Sleep(2 * time.Microsecond)
+		err = lcd.enablePin.Out(gpio.Low)
+	}
+	return err
 }
 
-var _ conn.Resource = &Dev{}
+var _ display.TextDisplay = &HD44780{}
+var _ display.DisplayBacklight = &HD44780{}
+var _ display.DisplayRGBBacklight = &HD44780{}
+var _ conn.Resource = &HD44780{}

hd44780/hd44780_test.go

@@ -0,0 +1,118 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	periphDisplay "periph.io/x/conn/v3/display"
+	"periph.io/x/conn/v3/display/displaytest"
+	"periph.io/x/conn/v3/i2c/i2ctest"
+)
+
+func getLCD(t *testing.T, recordingName string) (*HD44780, error) {
+	bus := &i2ctest.Playback{Ops: recordingData[recordingName], DontPanic: true}
+	dev, err := NewAdafruitI2CBackpack(bus, 0x20, 2, 16)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return dev, err
+}
+
+const (
+	testRows = 2
+	testCols = 16
+)
+
+var liveDevice = false
+
+func TestBasic(t *testing.T) {
+	display, err := getLCD(t, "TestBasic")
+
+	if err != nil {
+		t.Fatal(err)
+	}
+	s := display.String()
+	t.Log(s)
+	if len(s) == 0 {
+		t.Error("display.String()")
+	}
+
+	_, err = display.WriteString("1234567890")
+	if err != nil {
+		t.Error(err)
+	}
+	err = display.MoveTo(2, 2)
+	if err != nil {
+		t.Error(err)
+	}
+	_, err = display.WriteString("2345678901")
+	if err != nil {
+		t.Error(err)
+	}
+	rows := display.Rows()
+	if rows != testRows {
+		t.Errorf("display.Rows() expected %d, received %d", testRows, rows)
+	}
+	cols := display.Cols()
+	if cols != testCols {
+		t.Errorf("display.Cols() expected %d, received %d", testCols, cols)
+	}
+	if liveDevice {
+		time.Sleep(5 * time.Second)
+	}
+	err = display.Halt()
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+func TestInterface(t *testing.T) {
+	display, err := getLCD(t, "TestInterface")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = display.Halt() }()
+	errs := displaytest.TestTextDisplay(display, liveDevice)
+	for _, err := range errs {
+		if !errors.Is(err, periphDisplay.ErrNotImplemented) {
+			t.Error(err)
+		}
+	}
+	if liveDevice {
+		time.Sleep(5 * time.Second)
+	}
+}
+
+func TestBacklights(t *testing.T) {
+	display, err := getLCD(t, "TestBacklights")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		_ = display.Halt()
+	})
+
+	err = display.Backlight(0)
+	if err != nil {
+		t.Error(err)
+	}
+	err = display.Backlight(0xff)
+	if err != nil {
+		t.Error(err)
+	}
+	for ix := range 3 {
+		colors := make([]periphDisplay.Intensity, 3)
+		colors[ix] = 0xff
+		err = display.RGBBacklight(colors[0], colors[1], colors[2])
+		if err != nil {
+			t.Error(err)
+		}
+	}
+}

hd44780/pcf857x_backpack.go

@@ -0,0 +1,50 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"periph.io/x/conn/v3/gpio"
+	"periph.io/x/conn/v3/i2c"
+	"periph.io/x/devices/v3/pcf857x"
+)
+
+const (
+	// Name is the LCD pin name, and the integer value is the GPIO
+	// number (not physical) of the PCF8574 I2C GPIO Expander.
+	pcf_d4           = 4
+	pcf_d5           = 5
+	pcf_d6           = 6
+	pcf_d7           = 7
+	pcf_rsPin        = 0
+	pcf_enablePin    = 2
+	pcf_backlightPin = 3
+	pcf_rwPin        = 1
+)
+
+// This function returns a display configured to use the pcf8574 i2c backpacks.
+//
+// # Product Information
+//
+// https://www.handsontec.com/dataspecs/I2C_2004_LCD.pdf
+//
+// This function creates a PCF8574 backpack device with the required pin
+// configuration. To use this, get an I2C bus, and call this function with the
+// bus, i2c address, number of rows, and columns.
+func NewPCF857xBackpack(bus i2c.Bus, address uint16, rows, cols int) (*HD44780, error) {
+	pcf, err := pcf857x.New(bus, address, pcf857x.PCF8574)
+	if err != nil {
+		return nil, err
+	}
+	// R/W is connected on this backpack. Set it to low.
+	_ = pcf.Pins[pcf_rwPin].Out(gpio.Low)
+
+	// Create our gpio.Group
+	gr, _ := pcf.Group(pcf_d4, pcf_d5, pcf_d6, pcf_d7, pcf_rsPin, pcf_enablePin, pcf_backlightPin)
+	grPins := gr.Pins()
+	reset := grPins[4].(gpio.PinOut)
+	enable := grPins[5].(gpio.PinOut)
+	bl := grPins[6].(gpio.PinOut)
+	return NewHD44780(gr, reset, enable, NewBacklight(bl), rows, cols)
+}