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Bus Pirate: 2K pull-up resistor mod

Posted on Wednesday, September 2nd, 2009 in Bus Pirate by Ian

The Bus Pirate v2go shipped with 10K ohm pull-up resistors (R20-23). We modded our Bus Pirate with a 2K pull-up resistor so we don’t need an external resistor to work with 1-Wire devices. Pull-up resistors larger than 2.2K ohms don’t provide enough current for parasitically powered 1-Wire parts. Since we only need the bigger resistor for the 1-Wire protocol, we only replaced the pull-up resistor on the MOSI pin (R23).

Read more about our mod after the break.

10K is a weak pull-up resistor, but it’s worked for everything except parasitic 1-Wire devices that draw power from the pull-up resistor. The 3EEPROM explorer board demo uses an external 1.8K ohm pull-up resistor to get around this limitation.

A CD4066 switch enables and disables the Bus Pirate’s on-board pull-up resistors. We checked the CD4066 datasheet [PDF] to see now much current it can handle. It should be able to pass up to 10ma on any one pin.

We then used an online Ohm’s Law calculator to find the amount of current that would go through the resistors if they were shorted directly to ground. We calculated currents for the maximum possible pull-up resistor voltage (5volts).

The existing 10K resistors pass a maximum of 0.5mA at 5volts, a 2K resistor allows 2.5mA through. Since we’re only changing the MOSI pull-up resistor, 4mA total draw should be well within the current handling abilities of the CD4066.

Finally, we used a soldering iron to heat both sides of R23 until we could lift it from the PCB with tweezers. We replaced it with a 2K ohm 0805-sized resistor. The left-over solder from the previous resistor was enough to hold the new resistor, we didn’t need extra.

Please ignore our clumsy soldering in the video, it was hard to work around the tripod and camera. The noise is the fume extractor.

2KMOSI.450

This image shows the mod to R23 (2001), and the three original 10K pull-up resistors (103).

Should the Bus Pirate v3 ship with a 2K ohm resistor on the MOSI pin for out-of-the-box compatibility with 1-wire devices?

This entry was posted on Wednesday, September 2nd, 2009 at 1:00 pm and is filed under Bus Pirate. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.

2 Responses to “Bus Pirate: 2K pull-up resistor mod”

  1. James says:

    If it doesn’t affect any other functionality I’d say yes, I’m not going to switch mine out since I’ve never worked with 1-wire devices but it’s good to know about.

  2. Chris says:

    Augh ! You should always add a drop of flux after a rework, or just melt in the smallest amount of 0.015 – 0.020″ flux-cored solder to clean up the joint. This will ensure that you don’t have a potential dry / bad joint…
    I know (first-hand!) these are hard to track down when “something” isn’t working, but you assume the solder joints are all good !

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