# Minimum Clock Source for the I2C Driver

I am curious to know which figure in the MCU (microcontroller) datasheet would reveal the minimum clock source you could use to feed the I2C driver so it can generate it's CLK and Data line.

Looking to find an answer for this question, I have bumped into this paragraph here,

Question: I have a 32.768kHz oscillator. Ideally, I'd like to have the I2C bus clock rate run at 400kHz or 100kHz. But even with a value of SSPADD = 0, the (I2C Master mode clock) Fscl is only around 8 kHz

Answer: The I2C clock is generated by dividing down your clock frequency/4, so there is no way it can be faster than a quarter of your clock frequency.

If assumed true, I would like to know the analogy behind this.

In this case I am using Atmel SAMD21, however, I honestly don't mind if you use a different MCU as your example.

In SAMD21 the standard speed runs in 100KHz baud rate and this is the baud rate I am interested to know about unless you would like to expand your answer for the faster speeds.

SAMD21 uses the following calculations to eliminate an incorrect clock source in its I2C driver,

tmp_baud = (int32_t)(div_ceil(
fgclk - fscl * (10 + (fgclk * 0.000000001)* trise), 2 * fscl));

/* For High speed mode, set the SCL ratio of high:low to 1:2. */
if (config->transfer_speed == I2C_MASTER_SPEED_HIGH_SPEED) {
tmp_baudlow_hs = (int32_t)((fgclk * 2.0) / (3.0 * fscl_hs) - 1);
if (tmp_baudlow_hs) {
tmp_baud_hs = (int32_t)(fgclk / fscl_hs) - 2 - tmp_baudlow_hs;
} else {
tmp_baud_hs = (int32_t)(div_ceil(fgclk, 2 * fscl_hs)) - 1;
}
}

/* Check that baudrate is supported at current speed. */
if (tmp_baud > 255 || tmp_baud < 0 || tmp_baud_hs > 255 || tmp_baud_hs < 0) {
/* Baud rate not supported. */
tmp_status_code = STATUS_ERR_BAUDRATE_UNAVAILABLE;
}


with fgclk being the clock source (e.g. 8MHz) and fscl the baud rate of i2c I believe (e.g. 100k) and trise=215, which results to tmp_baud=35 and tmp_baud_hs=0.

• Hi CL. This is what I have found asking around as well. I guess if you wish you could add it as an answer and I'll tick it as one. cheers Mar 9, 2017 at 5:22