Maybe SX1509?

Published on 2020-12-19 in Servo Breakout for WeMos D1 Mini.

It’s been a long while since I made this shield, and the PCA9685 chip that it uses has been in service for driving servos even longer. Meanwhile, I have been on a lookout for a more modern and smaller chip that could provide a similar function. The PCA9685 is not perfect, because it is actually a LED driver, which just happens to have adjustable PWM frequency allowing us to abuse it to generate a 50Hz servo signal. Still, it contains constant-current drivers that we don’t need, and we only use a small part of its duty cycle range.

So what could be other chips that we could possibly use? If you look around a little for “PWM chips”, it seems that chip manufacturers only recognize two use cases for PWM signal: buck/boost converters and LED drivers, and in both cases they prefer relatively high frequencies, at least 29kHz, if not 33Mhz. That is useless for us, sadly, so we need to look in other places.

Recently I have stumbled upon a really weird chip, SX1509. It’s I²C- controlled GPIO multiplexer, but with some extra features, that include voltage level shifters, a keyboard matrix scanner and, last but not least, a PWM LED driver. So maybe we could use it for our servos?

I ordered a breakout board with this chip somewhere at the beginning of this year, and I have been putting away working on it for a looong time. But now with the holiday break, I decided to finally sit down and try it. So I downloaded the datasheet, got a Trinket M0 running CircuitPython out of one drawer, and a pocket oscilloscope out of another, and I started to poke at the registers.

Sadly, this is another failure, for two reasons: first, the slowest frequency I managed to get for the PWM signal is 121Hz, over twice as much as the required 50Hz; second, it only has 8 bits of resolution for the duty cycle length, and that’s over the whole range, of which we only need a small part, so it would be even worse than the 12 bits we get with the PCA9685.

But I guess I will keep it in my drawer if I ever need to multiplex GPIO pins while level-shifting to two different voltages, scanning a keyboard matrix and blinking LEDs.