esp32c3: add support for this chip

This change adds support for the ESP32-C3, a new chip from Espressif. It
is a RISC-V core so porting was comparatively easy.

Most peripherals are shared with the (original) ESP32 chip, but with
subtle differences. Also, the SVD file I've used gives some
peripherals/registers a different name which makes sharing code harder.
Eventually, when an official SVD file for the ESP32 is released, I
expect that a lot of code can be shared between the two chips.

More information: https://www.espressif.com/en/products/socs/esp32-c3

TODO:
  - stack scheduler
  - interrupts
  - most peripherals (SPI, I2C, PWM, etc)

src/runtime/runtime_esp32.go

@@ -6,19 +6,8 @@ import (
 	"device"
 	"device/esp"
 	"machine"
-	"unsafe"
 )
 
-type timeUnit int64
-
-var currentTime timeUnit
-
-func putchar(c byte) {
-	machine.Serial.WriteByte(c)
-}
-
-func postinit() {}
-
 // This is the function called on startup right after the stack pointer has been
 // set.
 //export main
@@ -50,23 +39,15 @@ func main() {
 	// Clear .bss section. .data has already been loaded by the ROM bootloader.
 	// Do this after increasing the CPU clock to possibly make startup slightly
 	// faster.
-	preinit()
+	clearbss()
 
 	// Initialize UART.
 	machine.Serial.Configure(machine.UARTConfig{})
 
-	// Configure timer 0 in timer group 0, for timekeeping.
-	//   EN:       Enable the timer.
-	//   INCREASE: Count up every tick (as opposed to counting down).
-	//   DIVIDER:  16-bit prescaler, set to 2 for dividing the APB clock by two
-	//             (40MHz).
-	esp.TIMG0.T0CONFIG.Set(esp.TIMG_T0CONFIG_T0_EN | esp.TIMG_T0CONFIG_T0_INCREASE | 2<<esp.TIMG_T0CONFIG_T0_DIVIDER_Pos)
-
-	// Set the timer counter value to 0.
-	esp.TIMG0.T0LOADLO.Set(0)
-	esp.TIMG0.T0LOADHI.Set(0)
-	esp.TIMG0.T0LOAD.Set(0) // value doesn't matter.
+	// Initialize main system timer used for time.Now.
+	initTimer()
 
+	// Initialize the heap, call main.main, etc.
 	run()
 
 	// Fallback: if main ever returns, hang the CPU.
@@ -79,44 +60,6 @@ var _sbss [0]byte
 //go:extern _ebss
 var _ebss [0]byte
 
-func preinit() {
-	// Initialize .bss: zero-initialized global variables.
-	// The .data section has already been loaded by the ROM bootloader.
-	ptr := unsafe.Pointer(&_sbss)
-	for ptr != unsafe.Pointer(&_ebss) {
-		*(*uint32)(ptr) = 0
-		ptr = unsafe.Pointer(uintptr(ptr) + 4)
-	}
-}
-
-func ticks() timeUnit {
-	// First, update the LO and HI register pair by writing any value to the
-	// register. This allows reading the pair atomically.
-	esp.TIMG0.T0UPDATE.Set(0)
-	// Then read the two 32-bit parts of the timer.
-	return timeUnit(uint64(esp.TIMG0.T0LO.Get()) | uint64(esp.TIMG0.T0HI.Get())<<32)
-}
-
-func nanosecondsToTicks(ns int64) timeUnit {
-	// Calculate the number of ticks from the number of nanoseconds. At a 80MHz
-	// APB clock, that's 25 nanoseconds per tick with a timer prescaler of 2:
-	// 25 = 1e9 / (80MHz / 2)
-	return timeUnit(ns / 25)
-}
-
-func ticksToNanoseconds(ticks timeUnit) int64 {
-	// See nanosecondsToTicks.
-	return int64(ticks) * 25
-}
-
-// sleepTicks busy-waits until the given number of ticks have passed.
-func sleepTicks(d timeUnit) {
-	sleepUntil := ticks() + d
-	for ticks() < sleepUntil {
-		// TODO: suspend the CPU to not burn power here unnecessarily.
-	}
-}
-
 func abort() {
 	for {
 		device.Asm("waiti 0")