[sdiy] Digi-Mod schematic

[Eric Brombaugh]

Seb Francis wrote:

>> - page 1: where are the slave-select signals for the SPI interfaces? 
> 
> Hmm, I thought it's possible to operate SPI in 2 or 3 pin modes.  But 
> reading the dsPIC datasheet again it's not clear whether the SS pin can 
> be disabled when in master mode.  Anyone know this for sure?
> 
> If the SS pin can't be disabled in master mode then I guess I should 
> remove access to SPI1 (no way to use SS1 pin) and just have SPI2, 
> including the SS2 pin.  This is also needed for the EEPROM as you 
> pointed out.  This does mean no external SPI if the EEPROM option is 
> included, but there's always I2C.

I've got a bit of experience with dsPIC SPI ports... :)

The dsPIC SPI port in master mode doesn't control the SS pin via 
hardware (unlike SPI ports on other MCUs like ARMs, etc). You will need 
to 'bit-bang' the CS pin for any slave devices you've got. Note that a 
lot of SPI peripherals absolutely require an edge on their SS inputs to 
signal the start/end of a bus transaction, so leaving this signal out is 
generally not an option.

For the most generic application, bring out the SS pin. If you're in 
master mode it can be driven via GPIO as the CS pin to your slave 
devices. If you're in slave mode you'll want it anyway.

> Perhaps this is OTT for dsPIC ADCs, but I'd like to get the ADC inputs 
> accurate enough for pitch CV so thought I'd try it.  I can also solder 
> in a more basic buffer configuration and compare if it makes a 
> difference.  On the final PCBs I may simplify this if it proves 
> unnecessary.

dsPIC ADC inputs are capable of handling V/Oct CVs for pitch control. A 
good low-impedance drive without fancy topologies is generally sufficient.

Eric