New Stepper-Board with DRV8876, comments welcomed

It took some time to get so far since I posted here about a CNC servo stepper development. Chip shortage added extra time for changing the design two times…
Now I’d like to share my thoughts and appreciate any comment or improvement proposal - PCBs are not yet ordered. That’s my first lesson learned - get your parts beforehand.
Some key facts:

  • Separate boards for AS5147 Encoder and Driver - space & height restriction at NEMA17 in my CNC mill
  • I will reflow solder myself -> all SMD parts on one side for stencil, only SMT (plugs) on other.
  • For NEMA17 with 2 x max. 2.5A phase current , low resistance, 36V Power Supply
  • 2 x TI DRV8876. Preferred 8874 not available at all, DRV8876 ordered from semi-reliable source…
  • 2 layer, 1 ounce board. -> 2 oz. and/or 4 layers too expensive (checked PCBWAY, Seeed)
  • Mitigate heat problems by mirroring the 1.1mm power tracks on backside and stitching with 0.8mm/0.4mm vias -> how can 3.8A (spec. max) flow through a 0.45mm pad connection?
  • TI PowerPAD specs followed ,but with 0.3mm vias -> 0.2mm holes drive PCB costs
  • Differential 100Ohm RS422 line drivers DS34LV86/DS26LV31 -> 1.5m distance through noisy energy chain. No stock for DS26LV32, equivalent should work
  • use of 8-wire, energy chain qualified CAT5 cable with crimped Molex contacts -> no room for RJ45 plugs
  • ABI encoder signals used. local ATTiny85 to initialize AS5147 over SPI. -> Minimize cabling. STM8: no stock
  • Bluepill clone piggy back, so STM32F103C8T (don’t use USB and 5V supply concurrently - diode missing)! -> no stock for any intended STM32 48pin MCU.
  • Isolated 5V/3.3V converter -> don’t trust the Bluepill converter. And I have a big 5V supply on mill.
  • Isolated interface for STEP/DIR/ENA or SPI input from GRBL controller. Made with ISO7763DBQR
  • non-isolated UART interface for SimpleFOC monitor
  • Inline current measurement with 2 x INA253 to compare with DRV8876 internal measurement

Let me know what you think!


P.S. Testpoints not yet on board.





Very interesting project … willing to see how a CNC machine works with SimpleFOC :slight_smile:

This has step direction interface yes.

yes, it has 3 isolated In and and one isolated Out Port for either Step/Dir/Ena or SPI Interface.
Meanwhile replaced motor connector (no stock) and added test point and added extra 2.54mm connector for unused MCU pins - you never know what you will need to connect…