physic: Improve Frequency rounding, add Period() (#386)

- Improve rounding and add unit tests to confirm.
- Period() is the same function than Duration() but better describes what
  is returned.
- Fix support for negative value.
- Upgrade all call sites inside periph.
- Fix Frequency documentation that incorrectly stated it's an int32,
  it's in fact an int64.
- Duration() will be removed in v4.0.0 as part of issue #379.

experimental/devices/ads1x15/ads1x15.go

@@ -424,7 +424,7 @@ func (p *analogPin) ReadContinuous() <-chan analog.Sample {
 	}
 	reading := make(chan analog.Sample, 16)
 	p.stop = make(chan struct{})
-	t := time.NewTicker(p.requestedFrequency.Duration())
+	t := time.NewTicker(p.requestedFrequency.Period())
 
 	go func(s <-chan struct{}) {
 		defer t.Stop()

experimental/devices/bitbang/i2c.go

@@ -52,7 +52,7 @@ func New(clk gpio.PinIO, data gpio.PinIO, f physic.Frequency) (*I2C, error) {
 	i := &I2C{
 		scl:       clk,
 		sda:       data,
-		halfCycle: f.Duration() / 2,
+		halfCycle: f.Period() / 2,
 	}
 	return i, nil
 }
@@ -126,7 +126,7 @@ func (i *I2C) Tx(addr uint16, w, r []byte) error {
 func (i *I2C) SetSpeed(f physic.Frequency) error {
 	i.mu.Lock()
 	defer i.mu.Unlock()
-	i.halfCycle = f.Duration() / 2
+	i.halfCycle = f.Period() / 2
 	return nil
 }
 

experimental/devices/bitbang/spi.go

@@ -73,7 +73,7 @@ func (s *SPI) Connect(f physic.Frequency, mode spi.Mode, bits int) (spi.Conn, er
 	defer s.spiConn.mu.Unlock()
 	s.spiConn.freqDev = f
 	if s.spiConn.freqDev != 0 && (s.spiConn.freqPort == 0 || s.spiConn.freqDev < s.spiConn.freqPort) {
-		s.spiConn.halfCycle = f.Duration() / 2
+		s.spiConn.halfCycle = f.Period() / 2
 	}
 	s.spiConn.mode = mode
 	s.spiConn.bits = bits
@@ -101,7 +101,7 @@ func (s *SPI) LimitSpeed(f physic.Frequency) error {
 	defer s.spiConn.mu.Unlock()
 	s.spiConn.freqPort = f
 	if s.spiConn.freqDev == 0 || s.spiConn.freqPort < s.spiConn.freqDev {
-		s.spiConn.halfCycle = f.Duration() / 2
+		s.spiConn.halfCycle = f.Period() / 2
 	}
 	return nil
 }