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October 24, 2011
about 6 years ago
IC3 (on the schematic, unmarked on the SparkFun silkscreen) lets the software switch the data lines' pull-up resistors between the VPU pin or disabled.
One of the big upgrades on the v4 bus pirate is that the software can also switch the VPU line to one of the onboard power supplies, instead of making you manually wire it up yourself. The v4 hardware guide has a good snippet of the schematic on the description of the pull-up circuit, the only difference on the v4 is that the 4066 is named IC2 instead of IC3.
I like the pictures at the manual’s Bus Pirate I/O Pin Descriptions, they’re the most clear to me (there seem to be lots of competing attempts to document the pinout in the manual).
Notice that the pin header’s physical layout is the same across all Bus Pirate 3.Xs, no matter who manufactures them. The top-left pin is GND, the bottom-right pin is MISO.
The only difference is in the plastic IDC shroud. SparkFun (and some hand-built SOIC bus pirates) stick it on with the notch to the right, meaning that the bottom-right pin, MISO is pin 1, real Bus Pirates put the notch to the left, meaning the top-left pin, GND is pin 1.
The firmware always assumes that you have a normal bus pirate, and it shows pin numbering as being pin 1 = GND. Bus Pirate cables also always have brown as pin 1, so on normal bus pirates GND = brown, but if you plug the same cable into a SparkFun pirate, MISO will be brown. Again, the firmware assumes that ground is brown, even on Sparkfun bus pirates.
SparkFun are technically correct here, the original schematics show that MISO is pin 1, and SparkFun started using a keyed shroud before the dangerous prototypes guys added it to the real bus pirate and stuck it on backwards, but their “backwards” ground = pin 1 is how it is now, and SparkFun owners just have to deal with their pirates being reversed from the “standard” everyone now assumes, and the firmware shows.
But it’s really easy to pry the plastic shroud up off your SparkFun-manufactured Bus Pirate, rotate it 180 degrees (notch towards the USB), and pop it back on. Worth doing to make it match the “standard”. Makes the firmware’s number / color hints useful again, and also allows you to use a straight-through IDC ribbon with genuine bus pirate accessories (like the LCD backpack) without shorting anything out.
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