Monday, May 4, 2020

S-Core 1.5 RELEASE; build notes.

The other half of my blaster management solution:



Firmware (Use the latest version)

The render is a good reference for component placements:

Substitutions and second-sources should be fairly obvious.

The only really notable thing is the DRV8825 being an exposed pad package. The pad must be solidly soldered down in order to thermally connect the device to the copper on the board, which is the primary heatsink. To allow that to be achieved without reflow equipment, there is an unmasked pad on the other side of the board directly under the chip. I generally do these by applying a large amount of flux on the pad and into the vias, soldering the legs down first, then using an 80 watt Weller iron with a large chisel tip to heat and apply solder from the back of the board. The solder will wick down the 20mil vias and wet the pad quickly. Flux is critical, you cannot use enough flux. A trick when doing these is to rest a finger on the top side of the chip while heating from the back. When the temperature on top the package suddenly spikes, you know that the pad has flowed and has thermal contact to the ground planes on both sides. Give it maybe one more second after you can no longer touch it to ensure the pad is wetted, but careful not to cook your DRV8825. I do use a light dimmer on my 80 watt iron - what you want is a massive thermal capacity iron with a massive tip, not a high temperature.

There is an optional TVS diode footprint which is across the DC bus. You would want to use a device with a clamping voltage of approximately 36-40V as 40V is the absolute maximum rating of the AOZ1282.

C11 is the DC link cap for the pusher motor drive and should be a 220uF or larger low-ESR cap. C15 is the filter cap for the logic power supply. It doesn't need to be and perhaps even should not be a low-ESR cap as this results in a larger inrush spike and stresses D2 more at power-up (not that D2 is not specced to handle that anyway). I have specified the appropriate varieties of caps in the BOM.

To flash these, plug in your USBasp or other AVR ISP device to the 6 pin ISP header, open the firmware in Arduino IDE, select Arduino Pro or Pro Mini (ATmega328P, 5V/16MHz), select "Burn bootloader to board" (which is just a convenient way to initially set the fuses; the bootloader itself is not used and will be clobbered by the next step) and then do an "Upload using programmer" (Shift-Upload). Thereafter for firmware updates, you can simply open the firmware in Arduino IDE, be sure that the 5V/16MHz Arduino Pro Mini target is selected, plug in and do an "Upload using programmer".

Note on setting Vref

Measure from the right testpoint with "VREF" next to it to any ground pin, DC bus ground, or a screw hole. (Do not measure between the two testpoints.) The left testpoint next to the Vref trimpot on this board is the DRV8825's onboard 3.3V regulator output.

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