machine: define Serial as the default output

Previously, the machine.UART0 object had two meanings:

  - it was the first UART on the chip
  - it was the default output for println

These two meanings conflict, and resulted in workarounds like:

  - Defining UART0 to refer to the USB-CDC interface (atsamd21,
    atsamd51, nrf52840), even though that clearly isn't an UART.
  - Defining NRF_UART0 to avoid a conflict with UART0 (which was
    redefined as a USB-CDC interface).
  - Defining aliases like UART0 = UART1, which refer to the same
    hardware peripheral (stm32).

This commit changes this to use a new machine.Serial object for the
default serial port. It might refer to the first or second UART
depending on the board, or even to the USB-CDC interface. Also, UART0
now really refers to the first UART on the chip, no longer to a USB-CDC
interface.

The changes in the runtime package are all just search+replace. The
changes in the machine package are a mixture of search+replace and
manual modifications.

This commit does not affect binary size, in fact it doesn't affect the
resulting binary at all.

Makefile

@@ -349,6 +349,8 @@ smoketest:
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=feather-m4-can      examples/caninterrupt
 	@$(MD5SUM) test.hex
+	$(TINYGO) build -size short -o test.hex -target=arduino-nano33      examples/blinky1
+	@$(MD5SUM) test.hex
 	# test pwm
 	$(TINYGO) build -size short -o test.hex -target=itsybitsy-m0        examples/pwm
 	@$(MD5SUM) test.hex

src/examples/echo/echo.go

@@ -9,7 +9,7 @@ import (
 
 // change these to test a different UART or pins if available
 var (
-	uart = machine.UART0
+	uart = machine.Serial
 	tx   = machine.UART_TX_PIN
 	rx   = machine.UART_RX_PIN
 )

src/machine/board_arduino_mkr1000.go

@@ -47,7 +47,9 @@ const (
 	LED = D6
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN Pin = PA24
 	USBCDC_DP_PIN Pin = PA25

src/machine/board_arduino_nano33.go

@@ -46,7 +46,7 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN Pin = PA24
 	USBCDC_DP_PIN Pin = PA25

src/machine/board_arduino_zero.go

@@ -35,6 +35,8 @@ const (
 	LED3 Pin = PB03 // RX LED
 )
 
+var Serial = USB
+
 // ADC pins
 const (
 	AREF Pin = PA03

src/machine/board_atsamd21.go

@@ -74,3 +74,5 @@ const (
 	PB30 Pin = 62
 	PB31 Pin = 63
 )
+
+var Serial = USB

src/machine/board_atsame54-xpro.go

@@ -15,6 +15,8 @@ const (
 	BUTTON = PB31
 )
 
+var Serial = USB
+
 const (
 	// https://ww1.microchip.com/downloads/en/DeviceDoc/70005321A.pdf
 
@@ -152,7 +154,7 @@ const (
 	PIN_USB_ID      = PC19
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_bluepill.go

@@ -50,6 +50,8 @@ const (
 	LED = PC13
 )
 
+var Serial = UART1
+
 // UART pins
 const (
 	UART_TX_PIN     = PA9
@@ -60,22 +62,21 @@ const (
 
 var (
 	// USART1 is the first hardware serial port on the STM32.
-	// Both UART0 and UART1 refer to USART1.
+	UART1  = &_UART1
-	UART0  = &_UART0
+	_UART1 = UART{
-	_UART0 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    stm32.USART1,
 	}
-	UART1  = &_UART1
+	UART2  = &_UART2
-	_UART1 = UART{
+	_UART2 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    stm32.USART2,
 	}
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART1, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_USART1, _UART1.handleInterrupt)
-	UART1.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART1.handleInterrupt)
+	UART2.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART2.handleInterrupt)
 }
 
 // SPI pins

src/machine/board_circuitplay_bluefruit.go

@@ -56,10 +56,7 @@ const (
 	UART_RX_PIN = P0_30 // PORTB
 )
 
-// UART0 is the USB device
+var Serial = USB
-var (
-	UART0 = USB
-)
 
 // I2C pins
 const (

src/machine/board_clue_alpha.go

@@ -103,9 +103,9 @@ const (
 	UART_TX_PIN = D1
 )
 
-// UART0 is the USB device
+// Serial is the USB device
 var (
-	UART0 = USB
+	Serial = USB
 )
 
 // I2C pins

src/machine/board_esp32-coreboard-v2.go

@@ -68,6 +68,8 @@ const (
 	ADC3 Pin = IO39
 )
 
+var Serial = UART0
+
 // UART0 pins
 const (
 	UART_TX_PIN = IO1

src/machine/board_feather-m0.go

@@ -42,7 +42,7 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_feather-m4-can.go

@@ -46,7 +46,9 @@ const (
 	NEOPIXELS = D8
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_feather-m4.go

@@ -39,7 +39,9 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_feather-nrf52840.go

@@ -75,9 +75,9 @@ const (
 	UART_TX_PIN = D1
 )
 
-// UART0 is the USB device
+// Serial is the USB device
 var (
-	UART0 = USB
+	Serial = USB
 )
 
 // I2C pins

src/machine/board_feather-stm32f405.go

@@ -140,7 +140,7 @@ var (
 		TxAltFuncSelector: AF7_USART1_2_3,
 		RxAltFuncSelector: AF7_USART1_2_3,
 	}
-	UART0 = UART1
+	Serial = UART1
 )
 
 func initUART() {

src/machine/board_grandcentral-m4.go

@@ -141,6 +141,8 @@ const (
 	NEOPIXEL = NEOPIXEL_PIN
 )
 
+var Serial = USB
+
 // UART pins
 const (
 	UART1_RX_PIN = D0 // (PB25)

src/machine/board_hifive1b.go

@@ -35,6 +35,8 @@ const (
 	LED_BLUE  = P21
 )
 
+var Serial = UART0
+
 const (
 	// TODO: figure out the pin numbers for these.
 	UART_TX_PIN = D1

src/machine/board_itsybitsy-m0.go

@@ -42,7 +42,7 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_itsybitsy-m4.go

@@ -37,7 +37,9 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_itsybitsy-nrf52840.go

@@ -70,9 +70,9 @@ const (
 	UART_TX_PIN = D1
 )
 
-// UART0 is the USB device
+// Serial is the USB device
 var (
-	UART0 = USB
+	Serial = USB
 )
 
 // I2C pins

src/machine/board_lgt92.go

@@ -54,6 +54,8 @@ const (
 	I2C0_SDA_PIN = PA10
 )
 
+var Serial = UART0
+
 var (
 
 	// Console UART (LPUSART1)

src/machine/board_maixbit.go

@@ -52,6 +52,8 @@ const (
 	LED_BLUE  = D14
 )
 
+var Serial = UART0
+
 // Default pins for UARTHS.
 const (
 	UART_TX_PIN = D5

src/machine/board_metro-m4-airlift.go

@@ -40,7 +40,9 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_microbit-v2.go

@@ -12,6 +12,8 @@ const (
 	BUTTONB Pin = P11
 )
 
+var Serial = UART0
+
 // UART pins
 const (
 	UART_TX_PIN Pin = P34

src/machine/board_microbit.go

@@ -12,6 +12,8 @@ const (
 	BUTTONB Pin = 26
 )
 
+var Serial = UART0
+
 // UART pins
 const (
 	UART_TX_PIN Pin = 24

src/machine/board_nicenano.go

@@ -54,9 +54,9 @@ const (
 	UART_TX_PIN = P0_08
 )
 
-// UART0 is the USB device
+// Serial is the USB device
 var (
-	UART0 = USB
+	Serial = USB
 )
 
 // I2C pins

src/machine/board_nodemcu.go

@@ -20,6 +20,8 @@ const (
 // Onboard blue LED (on the AI-Thinker module).
 const LED = D4
 
+var Serial = UART0
+
 // SPI pins
 const (
 	SPI0_SCK_PIN = D5

src/machine/board_nrf52840-mdk-usb-dongle.go

@@ -23,10 +23,8 @@ const (
 	UART_RX_PIN Pin = NoPin
 )
 
-// UART0 is the USB device
+// Serial is the USB device
-var (
+var Serial = USB
-	UART0 = USB
-)
 
 // I2C pins (unused)
 const (

src/machine/board_nrf52840-mdk.go

@@ -18,10 +18,8 @@ const (
 	UART_RX_PIN Pin = 19
 )
 
-// UART0 is the USB device
+// Serial is the USB device
-var (
+var Serial = USB
-	UART0 = USB
-)
 
 // I2C pins (unused)
 const (

src/machine/board_nucleof103rb.go

@@ -99,17 +99,16 @@ const (
 var (
 	// USART2 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to USART2.
+	UART2  = &_UART2
-	UART0  = &_UART0
+	_UART2 = UART{
-	_UART0 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    stm32.USART2,
 	}
-	UART2 = UART0
+	Serial = UART2
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART0.handleInterrupt)
+	UART2.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART2.handleInterrupt)
 }
 
 // SPI pins

src/machine/board_nucleof722ze.go

@@ -31,19 +31,18 @@ const (
 var (
 	// USART3 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to USART2.
+	UART1  = &_UART1
-	UART0  = &_UART0
+	_UART1 = UART{
-	_UART0 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.USART3,
 		TxAltFuncSelector: UART_ALT_FN,
 		RxAltFuncSelector: UART_ALT_FN,
 	}
-	UART1 = UART0
+	Serial = UART1
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART3, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_USART3, _UART1.handleInterrupt)
 }
 
 // SPI pins

src/machine/board_nucleol031k6.go

@@ -63,15 +63,14 @@ const (
 var (
 	// USART2 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to USART2.
+	UART1  = &_UART1
-	UART0  = &_UART0
+	_UART1 = UART{
-	_UART0 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.USART2,
 		TxAltFuncSelector: 4,
 		RxAltFuncSelector: 4,
 	}
-	UART1 = UART0
+	Serial = UART1
 
 	// I2C1 is documented, alias to I2C0 as well
 	I2C1 = &I2C{
@@ -89,5 +88,5 @@ var (
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART1.handleInterrupt)
 }

src/machine/board_nucleol432kc.go

@@ -65,15 +65,14 @@ const (
 var (
 	// USART2 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to USART2.
+	UART1  = &_UART1
-	UART0  = &_UART0
+	_UART1 = UART{
-	_UART0 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.USART2,
 		TxAltFuncSelector: 7,
 		RxAltFuncSelector: 3,
 	}
-	UART1 = UART0
+	Serial = UART1
 
 	// I2C1 is documented, alias to I2C0 as well
 	I2C1 = &I2C{
@@ -91,5 +90,5 @@ var (
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART1.handleInterrupt)
 }

src/machine/board_nucleol552ze.go

@@ -31,15 +31,14 @@ const (
 var (
 	// LPUART1 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to LPUART1.
+	UART1  = &_UART1
-	UART0  = &_UART0
+	_UART1 = UART{
-	_UART0 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.LPUART1,
 		TxAltFuncSelector: UART_ALT_FN,
 		RxAltFuncSelector: UART_ALT_FN,
 	}
-	UART1 = UART0
+	Serial = UART1
 )
 
 const (
@@ -57,5 +56,5 @@ var (
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_LPUART1, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_LPUART1, _UART1.handleInterrupt)
 }

src/machine/board_p1am-100.go

@@ -66,7 +66,7 @@ const (
 	BASE_ENABLE_PIN       Pin = PB09
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN          Pin = PA24
 	USBCDC_DP_PIN          Pin = PA25

src/machine/board_particle_argon.go

@@ -41,8 +41,7 @@ const (
 
 // UART
 var (
-	Serial = USB
+	Serial = UART0
-	UART0  = NRF_UART0
 )
 
 const (

src/machine/board_particle_boron.go

@@ -41,8 +41,7 @@ const (
 
 // UART
 var (
-	Serial = USB
+	Serial = UART0
-	UART0  = NRF_UART0
 )
 
 const (

src/machine/board_particle_xenon.go

@@ -41,8 +41,7 @@ const (
 
 // UART
 var (
-	Serial = USB
+	Serial = UART0
-	UART0  = NRF_UART0
 )
 
 const (

src/machine/board_pca10031.go

@@ -19,6 +19,8 @@ const (
 	LED_BLUE  Pin = 23
 )
 
+var Serial = UART0
+
 // UART pins
 const (
 	UART_TX_PIN Pin = 9

src/machine/board_pca10040.go

@@ -23,6 +23,8 @@ const (
 	BUTTON4 Pin = 16
 )
 
+var Serial = UART0
+
 // UART pins for NRF52840-DK
 const (
 	UART_TX_PIN Pin = 6

src/machine/board_pca10056.go

@@ -22,6 +22,8 @@ const (
 	BUTTON4 Pin = 25
 )
 
+var Serial = UART0
+
 // UART pins
 const (
 	UART_TX_PIN Pin = 6

src/machine/board_pca10056_baremetal.go

@@ -1,8 +0,0 @@
-// +build nrf52840,pca10056
-
-package machine
-
-// UART0 is the NRF UART
-var (
-	UART0 = NRF_UART0
-)

src/machine/board_pca10059.go

@@ -34,9 +34,9 @@ const (
 	UART_RX_PIN Pin = NoPin
 )
 
-// UART0 is the USB device
+// Serial is the USB device
 var (
-	UART0 = USB
+	Serial = USB
 )
 
 // I2C pins (unused)

src/machine/board_pinetime-devkit0.go

@@ -17,6 +17,8 @@ const (
 	LED3 = LCD_BACKLIGHT_LOW
 )
 
+var Serial = UART0
+
 // UART pins for PineTime. Note that RX is set to NoPin as RXD is not listed in
 // the PineTime schematic 1.0:
 // http://files.pine64.org/doc/PineTime/PineTime%20Port%20Assignment%20rev1.0.pdf

src/machine/board_pybadge.go

@@ -66,7 +66,9 @@ const (
 	BUTTON_B_MASK      = 128
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_pygamer.go

@@ -69,7 +69,9 @@ const (
 	BUTTON_B_MASK      = 128
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_pyportal.go

@@ -94,7 +94,9 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_qtpy.go

@@ -45,7 +45,7 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_reelboard.go

@@ -29,6 +29,8 @@ const (
 	BUTTON Pin = 7
 )
 
+var Serial = UART0
+
 // UART pins
 const (
 	UART_TX_PIN Pin = 6

src/machine/board_reelboard_baremetal.go

@@ -1,8 +0,0 @@
-// +build nrf52840,reelboard
-
-package machine
-
-// UART0 is the NRF UART
-var (
-	UART0 = NRF_UART0
-)

src/machine/board_stm32f4disco.go

@@ -27,19 +27,19 @@ const (
 )
 
 var (
-	UART0  = &_UART0
+	UART1  = &_UART1
-	_UART0 = UART{
+	_UART1 = UART{
 		Buffer:            NewRingBuffer(),
 		Bus:               stm32.USART2,
 		TxAltFuncSelector: AF7_USART1_2_3,
 		RxAltFuncSelector: AF7_USART1_2_3,
 	}
-	UART1 = UART0
+	Serial = UART1
 )
 
 // set up RX IRQ handler. Follow similar pattern for other UARTx instances
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART0.handleInterrupt)
+	UART1.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART1.handleInterrupt)
 }
 
 // SPI pins

src/machine/board_teensy40.go

@@ -136,7 +136,7 @@ const (
 )
 
 var (
-	UART0  = UART1 // alias UART0 to UART1
+	Serial = UART1
 	UART1  = &_UART1
 	_UART1 = UART{
 		Bus:      nxp.LPUART6,

src/machine/board_trinket.go

@@ -33,7 +33,7 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/board_wioterminal.go

@@ -325,7 +325,9 @@ const (
 	OUTPUT_CTR_3V3 = PC15
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PIN_USB_DM
 	USBCDC_DP_PIN = PIN_USB_DP

src/machine/board_x9pro.go

@@ -26,3 +26,5 @@ const (
 )
 
 const HasLowFrequencyCrystal = true
+
+var Serial = UART0

src/machine/board_xiao.go

@@ -48,7 +48,7 @@ const (
 	LED3    = LED_TXL
 )
 
-// UART0 aka USBCDC pins
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25

src/machine/machine_atmega.go

@@ -121,6 +121,9 @@ func (i2c *I2C) readByte() byte {
 	return byte(avr.TWDR.Get())
 }
 
+// Always use UART0 as the serial output.
+var Serial = UART0
+
 // UART
 var (
 	// UART0 is the hardware serial port on the AVR.

src/machine/machine_atsamd21.go

@@ -505,8 +505,7 @@ type UART struct {
 }
 
 var (
-	// UART0 is actually a USB CDC interface.
+	USB = &USBCDC{Buffer: NewRingBuffer()}
-	UART0 = &USBCDC{Buffer: NewRingBuffer()}
 )
 
 const (
@@ -1985,7 +1984,7 @@ func handleUSB(intr interrupt.Interrupt) {
 
 	// Start of frame
 	if (flags & sam.USB_DEVICE_INTFLAG_SOF) > 0 {
-		UART0.Flush()
+		USB.Flush()
 		// if you want to blink LED showing traffic, this would be the place...
 	}
 
@@ -2045,7 +2044,7 @@ func handleUSB(intr interrupt.Interrupt) {
 				setEPINTFLAG(i, sam.USB_DEVICE_EPINTFLAG_TRCPT1)
 
 				if i == usb_CDC_ENDPOINT_IN {
-					UART0.waitTxc = false
+					USB.waitTxc = false
 				}
 			}
 		}
@@ -2359,7 +2358,7 @@ func handleEndpoint(ep uint32) {
 
 	// move to ring buffer
 	for i := 0; i < count; i++ {
-		UART0.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
+		USB.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
 	}
 
 	// set byte count to zero

src/machine/machine_atsamd51.go

@@ -951,8 +951,8 @@ type UART struct {
 }
 
 var (
-	// UART0 is actually a USB CDC interface.
+	// USB is a USB CDC interface.
-	UART0 = &USBCDC{Buffer: NewRingBuffer()}
+	USB = &USBCDC{Buffer: NewRingBuffer()}
 )
 
 const (
@@ -2193,7 +2193,7 @@ func handleUSBIRQ(interrupt.Interrupt) {
 
 	// Start of frame
 	if (flags & sam.USB_DEVICE_INTFLAG_SOF) > 0 {
-		UART0.Flush()
+		USB.Flush()
 		// if you want to blink LED showing traffic, this would be the place...
 	}
 
@@ -2253,7 +2253,7 @@ func handleUSBIRQ(interrupt.Interrupt) {
 				setEPINTFLAG(i, sam.USB_DEVICE_ENDPOINT_EPINTFLAG_TRCPT1)
 
 				if i == usb_CDC_ENDPOINT_IN {
-					UART0.waitTxc = false
+					USB.waitTxc = false
 				}
 			}
 		}
@@ -2567,7 +2567,7 @@ func handleEndpoint(ep uint32) {
 
 	// move to ring buffer
 	for i := 0; i < count; i++ {
-		UART0.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
+		USB.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
 	}
 
 	// set byte count to zero

src/machine/machine_generic.go

@@ -8,6 +8,7 @@ var (
 	SPI0  = SPI{0}
 	I2C0  = &I2C{0}
 	UART0 = &UART{0}
+	USB   = &UART{100}
 )
 
 const (

src/machine/machine_nrf.go

@@ -138,8 +138,8 @@ type UART struct {
 // UART
 var (
 	// UART0 is the hardware UART on the NRF SoC.
-	_NRF_UART0 = UART{Buffer: NewRingBuffer()}
+	_UART0 = UART{Buffer: NewRingBuffer()}
-	NRF_UART0  = &_NRF_UART0
+	UART0  = &_UART0
 )
 
 // Configure the UART.
@@ -165,7 +165,7 @@ func (uart *UART) Configure(config UARTConfig) {
 	nrf.UART0.INTENSET.Set(nrf.UART_INTENSET_RXDRDY_Msk)
 
 	// Enable RX IRQ.
-	intr := interrupt.New(nrf.IRQ_UART0, _NRF_UART0.handleInterrupt)
+	intr := interrupt.New(nrf.IRQ_UART0, _UART0.handleInterrupt)
 	intr.SetPriority(0xc0) // low priority
 	intr.Enable()
 }

src/machine/machine_nrf51.go

@@ -6,10 +6,6 @@ import (
 	"device/nrf"
 )
 
-var (
-	UART0 = NRF_UART0
-)
-
 func CPUFrequency() uint32 {
 	return 16000000
 }

src/machine/machine_nrf52.go

@@ -42,10 +42,6 @@ const (
 	P0_31 Pin = 31
 )
 
-var (
-	UART0 = NRF_UART0
-)
-
 // Get peripheral and pin number for this GPIO pin.
 func (p Pin) getPortPin() (*nrf.GPIO_Type, uint32) {
 	return nrf.P0, uint32(p)

src/machine/machine_nrf52833.go

@@ -58,10 +58,6 @@ const (
 	P1_15 Pin = 47
 )
 
-var (
-	UART0 = NRF_UART0
-)
-
 // Get peripheral and pin number for this GPIO pin.
 func (p Pin) getPortPin() (*nrf.GPIO_Type, uint32) {
 	if p >= 32 {

src/runtime/runtime_atmega.go

@@ -8,11 +8,11 @@ import (
 )
 
 func initUART() {
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 // Sleep for a given period. The period is defined by the WDT peripheral, and is

src/runtime/runtime_atsamd21.go

@@ -30,11 +30,11 @@ func init() {
 	initADCClock()
 
 	// connect to USB CDC interface
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initClocks() {

src/runtime/runtime_atsamd51.go

@@ -30,11 +30,11 @@ func init() {
 	initADCClock()
 
 	// connect to USB CDC interface
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initClocks() {

src/runtime/runtime_esp32.go

@@ -14,7 +14,7 @@ type timeUnit int64
 var currentTime timeUnit
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func postinit() {}
@@ -53,7 +53,7 @@ func main() {
 	preinit()
 
 	// Initialize UART.
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	// Configure timer 0 in timer group 0, for timekeeping.
 	//   EN:       Enable the timer.

src/runtime/runtime_esp8266.go

@@ -14,7 +14,7 @@ type timeUnit int64
 var currentTime timeUnit = 0
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 // Write to the internal control bus (using I2C?).
@@ -37,7 +37,7 @@ func main() {
 	rom_i2c_writeReg(103, 4, 2, 145)
 
 	// Initialize UART.
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	// Initialize timer. Bits:
 	//  ENABLE:   timer enable

src/runtime/runtime_fe310.go

@@ -93,11 +93,11 @@ func initPeripherals() {
 	sifive.RTC.RTCCFG.Set(sifive.RTC_RTCCFG_ENALWAYS)
 
 	// Configure the UART.
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 var timerWakeup volatile.Register8

src/runtime/runtime_k210.go

@@ -105,11 +105,11 @@ func initPeripherals() {
 	// Enable FPIOA peripheral.
 	kendryte.SYSCTL.CLK_EN_PERI.SetBits(kendryte.SYSCTL_CLK_EN_PERI_FPIOA_CLK_EN)
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 var timerWakeup volatile.Register8

src/runtime/runtime_nrf.go

@@ -29,7 +29,7 @@ func main() {
 }
 
 func init() {
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 	initLFCLK()
 	initRTC()
 }
@@ -64,7 +64,7 @@ func initRTC() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func sleepTicks(d timeUnit) {

src/runtime/runtime_stm32f103.go

@@ -33,7 +33,7 @@ func init() {
 		Device:         stm32.TIM3,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB1ENR,
@@ -43,7 +43,7 @@ func init() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 // initCLK sets clock to 72MHz using HSE 8MHz crystal w/ PLL X 9 (8MHz x 9 = 72MHz).

src/runtime/runtime_stm32f405.go

@@ -181,10 +181,10 @@ func initCLK() {
 
 func initCOM() {
 	if machine.NUM_UART_INTERFACES > 0 {
-		machine.UART0.Configure(machine.UARTConfig{})
+		machine.Serial.Configure(machine.UARTConfig{})
 	}
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }

src/runtime/runtime_stm32f407.go

@@ -51,7 +51,7 @@ func init() {
 		Device:         stm32.TIM3,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB1ENR,
@@ -61,7 +61,7 @@ func init() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initCLK() {

src/runtime/runtime_stm32f7x2.go

@@ -50,7 +50,7 @@ func init() {
 		Device:         stm32.TIM3,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB1ENR,
@@ -60,7 +60,7 @@ func init() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initCLK() {

src/runtime/runtime_stm32l0.go

@@ -14,7 +14,7 @@ const (
 type arrtype = uint16
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initCLK() {

src/runtime/runtime_stm32l0x1.go

@@ -34,7 +34,7 @@ func init() {
 		Device:         stm32.TIM22,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB2ENR,

src/runtime/runtime_stm32l0x2.go

@@ -34,7 +34,7 @@ func init() {
 		Device:         stm32.TIM3,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB1ENR,

src/runtime/runtime_stm32l4x2.go

@@ -75,7 +75,7 @@ func init() {
 		Device:         stm32.TIM15,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB2ENR,
@@ -85,7 +85,7 @@ func init() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initCLK() {

src/runtime/runtime_stm32l5x2.go

@@ -51,7 +51,7 @@ func init() {
 		Device:         stm32.TIM15,
 	})
 
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 
 	initTickTimer(&timerInfo{
 		EnableRegister: &stm32.RCC.APB2ENR,
@@ -61,7 +61,7 @@ func init() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func initCLK() {