These two 3D models show my old and new 32xIO. It’s a layout difference, but except for that they are mostly identical. The old board have the DIP selecting CS Pin for SPI, while the new dropped that.
The challenge with these 32xServo ports are that I do not have 32 PWM signals in HW. In theory STM32F405 do have 32 x PWM signals, but you can’t route them all at the same time. At most I get 28 signals if I switch everything else off – which I can’t.
The idea was to use a timer interrupt to bit-bang some of the pins to compensate, but to do that we need to actually use the MCU a bit. The classic RF pulse is 500uS to 1500 uS, so we need something like 50Khz interrupt or preferable 100Khz interrupt to process with some accuracy. I would have preferred using PWM signals only, but that is not possible. Also, we do actually have a powerfully M4 ticking at 168Mhz to support this, so we have plenty of juice – we can do this. The test is to put signals on a Servo and watch their behavior as they hold selected positions. I tried that with SW on a ESP32 and noticed “twitching” – small movements before I switched to HW PWM channels.
The way it is routed on 2.3 means I have:
- 32 x GPIO channels since all are GPIO.
- 17 x 5V compatible GPIO channels
- 15 x ADC channels
- 22 x Hardware PWM Channels.
- 1 x SPI port for backbone bus
- 1 x CAN port
- 1 x USB port
- 1 x User Led
I do however notice that I can optimize this a bit:
- I have used PC1 which is ADC capable for Status Led, while I have PC13,14 and 15 that are GPIO only available. This gives me one extra ADC channel, meaning I get all 16 ADC channels on IO.
- PC13, PC14 and PC15 can be used for Led’s giving me 3 user leds.
- PB12 and PB13 can be used on CAN2 releasing PB9 and PB8 from CAN1. That gives me 2 extra PWM/GPIO channels replacing 2 x GPIO only channels. CAN2 is Slave only thought, but I think I can accept that.
- If I ditch SPI1_MOSI and accept that this board only can be used as Half-Duplex Slave will free a PWM capable pin. I have to think about this one.
- In addition I need to add resistor options on SPI as well as the new SWD connector.
- Replace 2.54 pitch jumper with a 1.27 pitch.
- Move + Power lane away from ground plane looks doable to get a better ground plane.
- Move USB connector to right side closer to PSU.
- I also need to evaluate capacitance on the power connector.
- That leaves one change – I need different PSU on the 16 first versus the 16 last. Or more correctly. I think the solution here is to add an option to select between 3.3V/5V or Servo PSU on the 16 ADC capable channels.
I guess I can as well just start re-wiring this PCB from scratch due to the number of pin changes. But, I think this will be worth it + it is not that much work once I set my mind to do it. It is actually far more work making SW, but again – the changes will only affect the “wire” function and capability list. Done right Rev 1.4 should have:
- 16 ADC/GPIO w/Mixed HW/SW PWM to left
- 16 GPIO w/ HW PWM to right