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Clock Stretching and I2C speed

Last Updated: March 13, 2012

How does the I2C clock speed affect the duration of clock stretching introduced by the I2C slave?


Clock stretching is a phenomenon where the I2C slave pulls the SCL line low on the 9th clock of every I2C data transfer (before the ACK stage).  The clock is pulled low when the CPU is processing the I2C interrupt to evaluate either the address or process a data received from Master or to prepare the next data when Master is reading from the slave. 

The time the clock is pull low depends on the time the CPU takes to process the interrupt and hence is dependent on the CPU speed and not the I2C clock speed.


For more information on this, please refer to the article PSoC I2C Block Clock Stretching: Worst Case Duration.

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