pca9685: add buffered one shot write

Makefile

@@ -107,6 +107,8 @@ smoke-test:
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=p1am-100 ./examples/p1am/main.go
 	@md5sum ./build/test.hex
+	tinygo build -size short -o ./build/test.hex -target=pico ./examples/pca9685/main.go
+	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=microbit ./examples/pcd8544/setbuffer/main.go
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=microbit ./examples/pcd8544/setpixel/main.go

pca9685/pca9685_buffered.go

@@ -0,0 +1,53 @@
+package pca9685
+
+import (
+	"encoding/binary"
+
+	"tinygo.org/x/drivers"
+)
+
+// 16 PWM channels, 2 2 byte values each (on, off 16bits)
+const buffLen = 16 * 2 * 2
+
+// DevBuffered provides a way of performing one-shot writes
+// on all PWM signals. This is useful when working with systems
+// which require as little as possible I/O overhead.
+type DevBuffered struct {
+	Dev
+	// LED buffer, first value is address, following values correspond to LED registers.
+	//  [0]: LEDSTART register address
+	//  [1:5]: LED0 corresponding to PWM channel 0
+	//  [5:9]: LED1 PWM channel 1
+	//  ...
+	//  [1 + N*4 : 1 + N*4 + 4] : channnel N up to channel 15
+	ledBuf [buffLen + 1]byte
+}
+
+// New creates a new instance of a PCA9685 device. It performs
+// no IO on the i2c bus.
+func NewBuffered(bus drivers.I2C, addr uint8) *DevBuffered {
+	db := &DevBuffered{
+		Dev: New(bus, addr),
+	}
+	db.ledBuf[0] = LEDSTART
+	return db
+}
+
+// PrepSet prepares a value to be written to the
+// channel's PWM register on Update() call.
+func (b *DevBuffered) PrepSet(channel uint8, on uint32) {
+	b.PrepPhasedSet(channel, on, 0)
+}
+
+// PrepPhasedSet prepares a phased PWM value to be written to the
+// channel's register on Update() call.
+func (b *DevBuffered) PrepPhasedSet(channel uint8, on, off uint32) {
+	onLReg := 1 + channel*4
+	binary.LittleEndian.PutUint16(b.ledBuf[onLReg:], uint16(on)&maxtop)
+	binary.LittleEndian.PutUint16(b.ledBuf[onLReg+2:], uint16(off)&maxtop)
+}
+
+// Update writes the prepared values to the PWM device registers in one shot.
+func (b *DevBuffered) Update() error {
+	return b.bus.Tx(uint16(b.addr), b.ledBuf[:], nil)
+}