Protocols such as I2C and 1-Wire use open collector outputs for two way communication on a single data wire. In this type of protocol there is no 1/high level without a pull-up resistor. The pull-up resistor connects to a voltage that “pulls” the wire high. Devices use their output to ground the wire and pull it low.

The Bus Pirate has 10KΩ onboard pull-up resistors that can be toggled with the p and P commands.

Enter LED mode

This tutorial uses LED mode configured for the onboard LEDs, see Blink Some LEDs tutorial. In this mode all the Bus Pirate IO pins are free for experimenting.

Voltage Source

The pull-up resistors are powered by the voltage on the VOUT/VREF pin. This can come from two sources:

For this tutorial let’s use the onboard power supply.

Programmable Power Supply Unit

Bus Pirate [/dev/ttyS0]
LED-()>  W 5 100
5.00V requested, closest value: 5.00V
100.0mA requested, closest value: 100.0mA

Power supply:Enabled
Vreg output: 5.00V, Vref/Vout pin: 5.00V, Current: 2.3mA

LED-()>

Enable the power supply, this will power the pull-up resistors and the IO buffers.

Check the live view statusbar at the bottom of the terminal.

  • The top line indicates the power supply is set at 5 volts with a 100mA current limit.
  • The third line shows that 7.8mA is being consumed on VOUT. Since nothing is connected this represents the base current of the PPSU, op-amp offset error and noise in the RPi chip analog to digital converter.
  • The last line shows that VOUT currently measures 5volts, and other pins measure 0 volts.

External Power Supply

Hacking a device with its own power supply? You can use that instead.

  • Connect the device power to the Bus Pirate VOUT/VREF pin
  • Connect the device ground to the Bus Pirate ground pin.

Enable Pull-up Resistors

Bus Pirate [/dev/ttyS0]
LED-()> P
Pull-up resistors: Enabled (10K ohms @ 5.0V)

LED-()>

Type uppercase P followed by enter to activate the pull-up resistors.

In the live view statusbar all pins should be high, close to 5.0 volts.

  • The top line now shows that the pull-up resistors are enabled.
  • The bottom line shows that IO0 to IO7 all measure 4.9 volts.

Controlling IO Pins

Sometimes it’s helpful to toggle or read a pin. The a/A/@ commands toggle the Bus Pirate pins low, high and input.

Auxiliary Pins Low

Bus Pirate [/dev/ttyS0]
LED-()> a 0
IO0 set to OUTPUT: 0

LED-()> a 1; a 2; a 3; 
IO1 set to OUTPUT: 0
IO2 set to OUTPUT: 0
IO3 set to OUTPUT: 0

LED-()>

a 0 configures IO 0 as output and low. Multiple commands can be chained with the ;, || and && operators.

  • Type a 0; a 1; a 2; a 3 and press enter to configure IO0 to IO3 as output and low.

Verify that the pins are low in the live view statusbar if active, or use the v command to view a voltage report.

  • The third line shows that IO0 to IO3 are configured as auxiliary-low (AUXL).
  • The last line shows that IO0 to IO3 now measure close to ground (0.1volt).

Auxiliary Pins Input

Bus Pirate [/dev/ttyS0]
LED-()> @ 0; @ 1; @ 2; @ 3
IO0 set to INPUT: 1
IO1 set to INPUT: 1
IO2 set to INPUT: 1
IO3 set to INPUT: 1

@ X makes the corresponding Bus Pirate pin an input, allowing the pull-up resistors to hold the pin high again.

  • Type @ 0; @ 1; @ 2; @ 3 followed by enter.

Verify that all the pins are around 5volts in the live monitor statusbar, or by using the voltage report command v.

  • All pins should be pulled high again.

Disable Pull-ups

Bus Pirate [/dev/ttyS0]
LED-()> p
Pull-up resistors: Disabled
LED-()>
  • Type lowercase p then hit enter to disable the pull-up resistors.

Verify that all the pins show 0volts in the live monitor statusbar or voltage report.

  • All pins should now be at 0volts because the pull-up are disabled.

Auxiliary Pins High

Bus Pirate [/dev/ttyS0]
LED-()> A 0
IO0 set to OUTPUT: 1
LED-()>

A X sets the corresponding Bus Pirate pin to output and high.

  • Type A 0 followed by enter to set IO0 high.
  • The live view statusbar should show that IO0 is now high (AUXH) and measures 5 volts.