stm32f4: refactor GPIO, in prep for adding SPI, I2C, PWM etc

src/machine/machine_stm32.go

@@ -5,3 +5,38 @@ package machine
 // Peripheral abstraction layer for the stm32.
 
 type PinMode uint8
+
+// Peripheral operations sequence:
+//  1. Enable the clock to the alternate function.
+//  2. Enable clock to corresponding GPIO
+//  3. Attach the alternate function.
+//  4. Configure the input-output port and pins (of the corresponding GPIOx) to match the AF .
+//  5. If desired enable the nested vector interrupt control to generate interrupts.
+//  6. Program the AF/peripheral for the required configuration (eg baud rate for a USART) .
+
+// Given that the stm32 family has the AF and GPIO on different registers based on the chip,
+//  use the main function here for configuring, and use hooks in the more specific chip
+//  definition files
+// Also, the stm32f1xx series handles things differently from the stm32f0/2/3/4
+
+// ---------- General pin operations ----------
+
+// Set the pin to high or low.
+// Warning: only use this on an output pin!
+func (p Pin) Set(high bool) {
+	port := p.getPort()
+	pin := uint8(p) % 16
+	if high {
+		port.BSRR.Set(1 << pin)
+	} else {
+		port.BSRR.Set(1 << (pin + 16))
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p Pin) Get() bool {
+	port := p.getPort()
+	pin := uint8(p) % 16
+	val := port.IDR.Get() & (1 << pin)
+	return (val > 0)
+}