machine: make I2C usable in the simulator

This fixes/improves a few issues with I2C support:

  * Validate I2C pins, so only pins that are supported by the hardware
    can be used (similar to how it's done with PWM).
  * Add address to Tx API (without it, the simulator can't really
    simulate I2C).
  * Add frequency when configuring. Not currently used, but might be
    useful in the future and adding it now avoids possibly breaking
    changes.

This is a breaking change, but since the simulator doesn't support I2C
yet that seems fine to me. (It does in my local changes, but those need
to be cleaned up before I can push them).

src/machine/machine_generic.go

@@ -4,6 +4,8 @@ package machine
 
 import (
 	"crypto/rand"
+	"errors"
+	"slices"
 )
 
 // Dummy machine package that calls out to external functions.
@@ -227,7 +229,9 @@ func adcRead(pin Pin) uint16
 
 // I2C is a generic implementation of the Inter-IC communication protocol.
 type I2C struct {
-	Bus uint8
+	Bus     uint8
+	PinsSCL []Pin
+	PinsSDA []Pin
 }
 
 // I2CConfig is used to store config info for I2C.
@@ -239,7 +243,21 @@ type I2CConfig struct {
 
 // Configure is intended to setup the I2C interface.
 func (i2c *I2C) Configure(config I2CConfig) error {
-	i2cConfigure(i2c.Bus, config.SCL, config.SDA)
+	if i2c.PinsSCL != nil {
+		matchSCL := slices.Index(i2c.PinsSCL, config.SCL) >= 0
+		matchSDA := slices.Index(i2c.PinsSDA, config.SDA) >= 0
+		if !matchSCL && !matchSDA {
+			return errors.New("i2c: SCL and SDA pins are incorrect for this I2C instance")
+		} else if !matchSCL {
+			return errors.New("i2c: SCL pin is incorrect for this I2C instance")
+		} else if !matchSDA {
+			return errors.New("i2c: SDA pin is incorrect for this I2C instance")
+		}
+	}
+	if config.Frequency == 0 {
+		config.Frequency = 100 * KHz
+	}
+	i2cConfigure(i2c.Bus, config.SCL, config.SDA, config.Frequency)
 	return nil
 }
 
@@ -261,19 +279,29 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) error {
 		rptr = &r[0]
 		rlen = len(r)
 	}
-	i2cTransfer(i2c.Bus, wptr, wlen, rptr, rlen)
-	// TODO: do something with the returned error code.
-	return nil
+	errCode := i2cTransfer(i2c.Bus, addr, wptr, wlen, rptr, rlen)
+	switch errCode {
+	case 0:
+		return nil
+	case 1:
+		return errI2CNoDevices
+	case 2:
+		return errI2CMultipleDevices
+	case 3:
+		return errI2CWrongAddress
+	default:
+		return errI2CBusError // unknown error code
+	}
 }
 
 //export __tinygo_i2c_configure
-func i2cConfigure(bus uint8, scl Pin, sda Pin)
+func i2cConfigure(bus uint8, scl Pin, sda Pin, frequency uint32)
 
 //export __tinygo_i2c_set_baud_rate
 func i2cSetBaudRate(bus uint8, br uint32)
 
 //export __tinygo_i2c_transfer
-func i2cTransfer(bus uint8, w *byte, wlen int, r *byte, rlen int) int
+func i2cTransfer(bus uint8, addr uint16, w *byte, wlen int, r *byte, rlen int) int
 
 type UART struct {
 	Bus uint8
@@ -336,15 +364,6 @@ var (
 	sercomUSART4 = UART{4}
 	sercomUSART5 = UART{5}
 
-	sercomI2CM0 = &I2C{0}
-	sercomI2CM1 = &I2C{1}
-	sercomI2CM2 = &I2C{2}
-	sercomI2CM3 = &I2C{3}
-	sercomI2CM4 = &I2C{4}
-	sercomI2CM5 = &I2C{5}
-	sercomI2CM6 = &I2C{6}
-	sercomI2CM7 = &I2C{7}
-
 	sercomSPIM0 = &SPI{0}
 	sercomSPIM1 = &SPI{1}
 	sercomSPIM2 = &SPI{2}