>>> > I don't understand scl_period = 1 µs for Fast Mode. For a bus freqency >>> > of 400 kHz we need low + high = 2.5 µs. Is there a factor 10 missing >>> > somewhere? >>> >>> As CCR = SCL_period * I2C parent clk frequency with minimal freq = >>> 2Mhz and SCL_period = 1 we have: >>> CCR = 1 * 2Mhz = 2. >>> But to compute, scl_low and scl_high in Fast mode, we have to do the >>> following thing as Duty=1: >>> scl_high = 9 * CCR * I2C parent clk period >>> scl_low = 16 * CCR * I2C parent clk period >>> In our example: >>> scl_high = 9 * 2 * 0,0000005 = 0,000009 sec = 9 µs >>> scl_low = 16 * 2 * 0.0000005 = 0,000016 sec = 16 µs >>> So low + high = 27 µs > 2,5 µs >> >> For me 9 µs + 16 µs is 25 µs, resulting in 40 kHz. That's why I wondered >> if there is a factor 10 missing somewhere. > > Hum ok. I am going to double-check what is wrong because when I check > with the scope I always reach 400Khz for SCL. > I will let you know. There is one point I miss here that is described in the reference manual: To reach the 400 kHz maximum I²C fast mode clock, the I2C parent rate must be a multiple of 10 MHz. So, contrary to what we said in a previous thread, 400 kHz could not be reached with low frequencies. In that way, we could compute CCR with duty = 0 by default. So, I find another formula very close to the first one I pushed in the first version: In fast mode, we compute CCR with duty = 0: t_scl_high = CCR * I2C parent clk period t_scl_low = 2 *CCR * I2C parent clk period So, CCR = I2C parent rate / 400 kHz / 3 For example with parent rate = 40 MHz: CCR = 40000000 / 400000 / 3 = 33.333333333 = 33 t_scl_high = 33 * (1 / 2000000) = 825 ns > 600 ns t_scl_low = 2 * 16 * (1 / 2000000) = 1650 ns > 1300 ns It seems ok now. Best regards, Cedric -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
