Updated with selectable I2C prescaler.

examples/i2c_oled/README.md

@@ -12,7 +12,12 @@ https://user-images.githubusercontent.com/1132011/230734071-dee305de-5aad-4ca0-a
 ## Build options
 There are a few build-time options in the i2c.h source:
 * I2C_CLKRATE - defines the I2C bus clock rate. Both 100kHz and 400kHz are supported.
-800kHz has been seen to work but 1MHz did not.
+800kHz has been seen to work when I2C_PRERATE is 1000000, but 1MHz did not. To
+use higher bus rates you must increase I2C_PRERATE at the expense of higher power
+consumption.
+* I2C_PRERATE - defines the I2C logic clock rate. Must be higher than I2C_CLKRATE.
+Keep this value as low as possible (but not lower than 1000000) to ensure low power
+operaton.
 * I2C_DUTY - for I2C_CLKRATE > 100kHz this specifies the duty cycle, either 33% or 36%.
 * TIMEOUT_MAX - the amount of tries in busy-wait loops before giving up. This value
 depends on the I2C_CLKRATE and should not affect normal operation.

examples/i2c_oled/i2c.h

@@ -6,8 +6,11 @@
 #ifndef _I2C_H
 #define _I2C_H
 
-// I2C clock rate
-#define I2C_CLKRATE 800000
+// I2C Bus clock rate - must be lower the Logic clock rate
+#define I2C_CLKRATE 1000000
+
+// I2C Logic clock rate - must be higher than Bus clock rate
+#define I2C_PRERATE 2000000
 
 // uncomment this for high-speed 36% duty cycle, otherwise 33%
 #define I2C_DUTY
@@ -40,7 +43,7 @@ void i2c_setup(void)
 	// set freq
 	tempreg = I2C1->CTLR2;
 	tempreg &= ~I2C_CTLR2_FREQ;
-	tempreg |= (APB_CLOCK/1000000)&I2C_CTLR2_FREQ;
+	tempreg |= (APB_CLOCK/I2C_PRERATE)&I2C_CTLR2_FREQ;
 	I2C1->CTLR2 = tempreg;
 	
 	// Set clock config