ssd1306: avoid unnecessary heap allocations (#767)

* ssd1306: avoid unnecessary heap allocations

* ssd1306: extract i2c and spi bus implementations

* ssd1306: refactor tests -- show fps and heap usage

* ssd1306: bring back the lost exported methods

* Adjust examples

* Fix smoketests for ssd1306

ssd1306/ssd1306_spi.go

@@ -0,0 +1,68 @@
+package ssd1306
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers"
+)
+
+type SPIBus struct {
+	wire     drivers.SPI
+	dcPin    machine.Pin
+	resetPin machine.Pin
+	csPin    machine.Pin
+	buffer   []byte // buffer to avoid heap allocations
+}
+
+// NewSPI creates a new SSD1306 connection. The SPI wire must already be configured.
+func NewSPI(bus drivers.SPI, dcPin, resetPin, csPin machine.Pin) *Device {
+	dcPin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	resetPin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	csPin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	return &Device{
+		bus: &SPIBus{
+			wire:     bus,
+			dcPin:    dcPin,
+			resetPin: resetPin,
+			csPin:    csPin,
+		},
+	}
+}
+
+// configure pins with the SPI bus and allocate the buffer
+func (b *SPIBus) configure(address uint16, size int16) []byte {
+	b.csPin.Low()
+	b.dcPin.Low()
+	b.resetPin.Low()
+
+	b.resetPin.High()
+	time.Sleep(1 * time.Millisecond)
+	b.resetPin.Low()
+	time.Sleep(10 * time.Millisecond)
+	b.resetPin.High()
+
+	b.buffer = make([]byte, size+1) // +1 for a command
+	return b.buffer[1:]             // return the image buffer
+}
+
+// command sends a command to the display
+func (b *SPIBus) command(cmd uint8) error {
+	b.buffer[0] = cmd
+	return b.tx(b.buffer[:1], true)
+}
+
+// flush sends the image to the display
+func (b *SPIBus) flush() error {
+	return b.tx(b.buffer[1:], false)
+}
+
+// tx sends data to the display
+func (b *SPIBus) tx(data []byte, isCommand bool) error {
+	b.csPin.High()
+	b.dcPin.Set(!isCommand)
+	b.csPin.Low()
+	err := b.wire.Tx(data, nil)
+	b.csPin.High()
+	return err
+}