PCA9535: max frequency

Hi Andrew,

Assuming both output ports are being written to and new header information is sent for each write, we can calculate the theoretical maximum blink rate using the given frequency and the size of the overall payload. A two-byte write with a two-byte header contains 36 bits (including 9th bit ACKs) plus 2 start and stop bits for a total size of 38 bits. A clock frequency of 400kHz allows a data rate of 200Kbps (only using half the edges for data). The resulting divide shows that we can change the state of the output bits around 5000 times every second. This is equivalent to blinking at a frequency of 2.5kHz.

Now this is a simplified and optimistic estimation. Most systems will have some inter-frame spacing between write frames, so the total throughput of I2C information will be slower than this. This estimation also will take up 100% of the I2C bandwidth, which will likely not be allowed if the interface shares any other devices. If the goal of the system is to achieve some kind of PWM LED control, it would likely be better to implement a dedicated device for this function. 

Let me know if you have any other questions.

Regards,
Eric Schott