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Table of Contents

Hardware

Existing Probe Hardware

Name Comment

Raspberry Pi Debug Probe

the "original"

picoLink

Open Source picoLink: Raspberry Pi RP2040 CMSIS-DAP Debug Probe

tiny-picoprobe

small PCB to mount a regular Pico Pi board to work without breadboard

Pimoroni Probe PCB

converts a Pi Pico into a simple probe (with standard SWD connector)

Pico Probe Holder

Adapter which also provides 3.3V and 5V to the target

Pico Probe Programmer

Another PCB adapter

All unfortunately without level shifters and without the possibility to use YaPicoprobes sigrok features. So there is some space left for other hardware (ideas).

Pin Assignments

Table 1. Pins Assignments Rasberry Pi Pico
Pin Description

GP1

SWDIR

GP2

target SWCLK

GP3

target SWDIO

GP4

target UART-RX

GP5

target UART-TX

GP6

target /RESET (RUN)

GP10..17

sigrok digital inputs

GP26/ADC0

sigrok ADC0

GP27/ADC1

sigrok ADC1

GP28/ADC2

sigrok ADC2
Table 2. Other Pin Assigments Raspberry Pi Pico
Pin Description Pico W

GPIO0

spare

GPIO7..9

spare

GPIO18

spare

GPIO19..21

debug pins (for probe debugging)

GPIO22

spare

GPIO23

power supply control

WL_ON

GPIO24

USB sense

WL_D

GPIO25

LED

WL_CS

GPIO29/ADC3

VSYS/3

WL_CLK

SWDIR / SWDIO

Level shifter must be used to allow different voltage levels on probe and target. There are different switching circuits out there, e.g.

  • TXS0108E (or TXS0102/4E) which allows 3.3V on probe side and up to 5V on target side for up to 8 signals

  • 74LXC1T45 which allows the same voltage levels for a single signal (depending of type)

Because SWDIO is a bidirectional signal, the level shifter must switch between input and output. The TXS010xx does this automatically while the 74LXCxT45 requires an SWDIR signal to control direction.

Drawback of the automatic switching are much lower frequencies (<=24MHz) which may pass the component and the condition Vcca<=Vccb. So the TXS0108E is actually not recommended for this purpose.

For a clean implementation SWDIR has been provided to allow support of the 74LXCxT45. The following image shows the timing of SWDIR, SWCLK and SWDIO.

SWDIR

sigrok

For the sigrok input signals it’s also good practice to use level shifter if the target uses other voltage levels than the probe.

Ideas

The firmware calls for a new hardware. To get a really nice probe, some level shifters , LEDs and power regulator for target supply would be required.

Following parts could be used:

  • use 2x 74LXC1T45 for the SWD IF, largest package: 6 pin SOT-23

  • 7803 for power supply of target

  • 74LVC8T245 level shifter for sigrok input, 24 pin SOIC / SOP packages are visible for soldering