Make I²C bus clock speed configurable and add notes about the speed

src/config.h

@@ -24,6 +24,11 @@
 // Allow powering off the sonde by pressing the button for over a second (when the sonde is not transmitting)
 #define ALLOW_POWER_OFF false
 
+// Define the I²C bus clock speed in Hz.
+// The default of 100000 (= 100 kHz) should be used with the Si5351 clock generator to allow fast frequency changes.
+// Note that some BMP280 sensors may require decreasing the clock speed to 10000 (= 10 kHz)
+#define I2C_BUS_CLOCK_SPEED 100000
+
 // Enable use of an externally connected I²C BMP280/BME280 atmospheric sensor
 // NOTE: Only BME280 sensors will report humidity. For BMP280 humidity readings will be zero.
 #define SENSOR_BMP280_ENABLE false

src/hal/i2c.c

@@ -17,7 +17,7 @@ i2c_port DEFAULT_I2C_PORT = {
         .i2c = I2C_PORT,
 };
 
-void i2c_init()
+void i2c_init(uint32_t clock_speed)
 {
     GPIO_InitTypeDef gpio_init;
 
@@ -49,7 +49,7 @@ void i2c_init()
     I2C_InitTypeDef i2c_init;
     I2C_StructInit(&i2c_init);
 
-    i2c_init.I2C_ClockSpeed = 100000;
+    i2c_init.I2C_ClockSpeed = clock_speed;
     i2c_init.I2C_Mode = I2C_Mode_I2C;
     i2c_init.I2C_DutyCycle = I2C_DutyCycle_2;
     i2c_init.I2C_AcknowledgedAddress = I2C_AcknowledgedAddress_7bit;

src/hal/i2c.h

@@ -20,7 +20,7 @@ extern i2c_port DEFAULT_I2C_PORT;
 extern "C" {
 #endif
 
-void i2c_init();
+void i2c_init(uint32_t clock_speed);
 void i2c_uninit();
 int i2c_read_bytes(struct _i2c_port *port, uint8_t address, uint8_t reg, uint8_t size, uint8_t *data);
 int i2c_read_byte(struct _i2c_port *port, uint8_t address, uint8_t reg, uint8_t *data);

src/main.c

@@ -84,8 +84,8 @@ int main(void)
         log_info("External USART init\n");
         usart_ext_init(EXTERNAL_SERIAL_PORT_BAUD_RATE);
     } else {
-        log_info("I2C init\n");
-        i2c_init();
+        log_info("I2C init: clock speed %d kHz\n", I2C_BUS_CLOCK_SPEED / 1000);
+        i2c_init(I2C_BUS_CLOCK_SPEED);
     }
     log_info("SPI init\n");
     spi_init();