I've been looking at BusPirate-v3a-final.sch to try and understand how the Bus Pirate power supplies and pullups work.
Am I right in thinking that:
- The PIC is running at 3v3 from VR2. It doesn't share VR3 so that VR3 can be disabled.
- The AUX and ADC lines can only provide 3v3, not 5v
- When enabled, the 4066D pulls up MOSI, MISO, CLK and CS, but the pullup voltage must be physically wired to VEXTERN?
Why does MOSI have a 2K pullup while the others have 10K?
Thanks.
The pic is indeed running from 3v3, so the max output voltage is 3v3. The pins are 5v tolerant and 3v3 is detected as 1 by (most) 5v logic.
However with the pullups and a different voltage applied to Vpu the output can be much higher (max. 5v5!). See the buspirate manual about this: http://hackaday.com/2009/07/01/mixed-vo ... us-pirate/ (http://hackaday.com/2009/07/01/mixed-voltage-interfacing-with-the-bus-pirate/) and http://dangerousprototypes.com/2009/07/ ... resistors/ (http://dangerousprototypes.com/2009/07/27/bus-pirate-practical-guide-to-pull-up-resistors/)
The reason for the 2k pullup is that 1 wire devices can be parasitic powered (just connect the data and gnd) To give them enough juice the pullup needs to be 2k. See http://dangerousprototypes.com/2009/09/ ... istor-mod/ (http://dangerousprototypes.com/2009/09/02/bus-pirate-2k-pull-up-resistor-mod/)
More info in the buspirate manual: http://dangerousprototypes.com/bus-pirate-manual/ (http://dangerousprototypes.com/bus-pirate-manual/) There is a link to the hw discussion which answers most of your questions.
Thanks for the links
I'd only add that the ADC can't provide any voltage. It's an input only pin, fed through a resistor divider, so ti can measure up to 6volts (6.6 really, but 6 to be safe).