I2C to 100+ PWM

Question

I'm currently looking for some sort of interface which can generate many (up to 100 potentially) individual PWM outputs. I figure I2C is the best way to go about this, but I'm open to other possibilities.

The only thing I've found so far is the Adafruit PCA9685, but at $15 a pop ($105 for the 7 I'd need,) I'm open to other options.

I don't have particularly high needs in terms of frequency or resolution - a step resolution of 1% (7 bit, I guess), at 10 ms time resolution at 1 Hz update rate would be my lower bounds on performance.

Answer 1

If you're open to using SPI, multiple chains of SPI devices can do this, with a more straightforward driver than I2C. As a bonus, SPI will offer you faster updates than you can achieve with I2C. If your host can do multi-bit SPI, even better.

Adafruit themselves have a 12-channel, 16-bit PWM board for $7.50 which is a bit more affordable than the PCA9685 version.

A possibly even cheaper approach, but with more intense CPU overhead, is to implement the PWM directly with shift registers with something like the 74HC595. With this approach the SPI runs all the time, and you implement PWM by controlling the duty cycle in software. Since SPI uses a fixed frequency clock the PWM timing is deterministic.

More here: https://hackaday.com/2011/11/05/controlling-shift-registers-via-spi/

And here: https://forum.arduino.cc/t/software-pwm-using-daisy-chained-74hc595s-spi/38251

TI offers a shift-register style LED driver with built-in current references, saving you the trouble of individual LED resistors. If you only need one color (or use separate R,G,B drivers) then is is a good option: https://www.ti.com/product/TLC59284

Answer 2

Probably the most capable solution in a single IC is an FPGA or a CPLD.

Cheap parts can easily handle 100 PWM outputs, or as many as it has I/O pins for. You can add an SPI interface to control it, which is simpler than I2C and just as practical for a point-to-point connection.

You'd need to be able to design the firmware for it in VHDL or Verilog, which are straightforward to learn and well documented on the internet. You can use the manufacturer's free development tools to simulate and compile your design. You'll find plenty of low-cost demo' boards you can try out or rig up, with varying amounts of I/O pins.

An FPGA/CPLD is more expensive per part than a microcontroller but there's parts for £5 that can easily do this. And an FPGA/CPLD is far more capable at this sort of task than a microcontroller and much easier to design for it (said from the perspective of being able to do both).

Answer 3

It's not a peripheral, but... 8x RP2040's is $5.6, each can do 14 PWM's using 7 timers. (one of them is for i2c)

Or even cheaper, do the PWM in software, because why not?
Then you can do 28 PWM channels per chip, say 5 chips, $3.5.

Answer 4

Use a PLD, that will probably be the best thing. You'll have to do some HDL, but it would give you the GPIO. Another option is a high GPIO count MCU; the PWM could be bit banged at the rates you have without too much trouble.