[sdiy] Digi-Mod schematic

[Eric Brombaugh]

> Seb Francis wrote:
>> 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.

A few additional thoughts:

* Just because you can't bring out the SS1 pin doesn't mean you have  
to give up using SPI1 - you can't use it as a slave port, but with any  
other GPIO pin as the /CS you can use it as a master port.

* SPI can be multi-drop if you want to use it that way. You can have  
your EEPROM and several other devices on the same bus - they'll all  
need separate /CS lines, but you can either use separate GPIOs for  
that, or a small external decoder chip. You won't be able to talk to  
them all simultaneously of course, but for some applications that may  
not be an issue.

Eric