rp2040: i2c baud rate handling improvements

src/machine/machine_rp2040_i2c.go

@@ -96,6 +96,7 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) error {
 //  SDA: 2, 6, 10, 14, 18, 26
 //  SCL: 3, 7, 11, 15, 19, 27
 func (i2c *I2C) Configure(config I2CConfig) error {
+	const defaultBaud uint32 = 100_000 // 100kHz standard mode
 	if config.SCL == 0 {
 		// If config pins are zero valued or clock pin is invalid then we set default values.
 		switch i2c.Bus {
@@ -107,6 +108,9 @@ func (i2c *I2C) Configure(config I2CConfig) error {
 			config.SDA = I2C1_SDA_PIN
 		}
 	}
+	if config.Frequency == 0 {
+		config.Frequency = defaultBaud
+	}
 	config.SDA.Configure(PinConfig{PinI2C})
 	config.SCL.Configure(PinConfig{PinI2C})
 	return i2c.init(config)
@@ -116,8 +120,13 @@ func (i2c *I2C) Configure(config I2CConfig) error {
 // enabling the I2C hardware if disabled beforehand.
 //go:inline
 func (i2c *I2C) SetBaudrate(br uint32) error {
-	const freqin uint32 = 125 * MHz
-	// Find smallest prescale value which puts o
+
+	if br == 0 {
+		return ErrInvalidI2CBaudrate
+	}
+
+	// I2C is synchronous design that runs from clk_sys
+	freqin := CPUFrequency()
 
 	// TODO there are some subtleties to I2C timing which we are completely ignoring here
 	period := (freqin + br/2) / br