Oscilloscope RP2040 - OpenHantek protocol

An oscilloscope for the RP2040 that implements the OpenHantek6022 protocol.

Specifications

• Up to 1 MS/s
• 2 channels
• OpenHantek6022-compatible protocol

Usage

1. Upload the oscilloscope.uf2 firmware to the RP2040.

2. Install OpenHantek. OpenHantek6022 needs RP2040 support: OpenHantek6022 fork

   • Build OpenHantek following the project instructions.
   • Or download prebuilt binaries.

3. The external circuit is optional. If you are not using the external circuit, copy RP2040_0_calibration.ini to:

   • Linux: ~/.config/OpenHantek/
   • Windows: %USERPROFILE%\.config\OpenHantek

4. Launch the modified version of OpenHantek6022.

Pins

• Channel 1: GPIO 26
• Channel 2: GPIO 27
• Calibration signal: GPIO 22
• No conversion: GPIO 19
• Debug enable/disable: GPIO 18
• Debug output: GPIO 16

Without the external circuit, the input voltage on GPIO 26 and GPIO 27 must stay between 0 V and 3.3 V.

Debug output is available on GPIO 16 at 115200 bps. To enable debug output, connect GPIO 18 to GND at boot.

The LED is on during capture.

External circuit

The external circuit adapts the input signal from ±5 V to the RP2040 ADC range and adds AC/DC coupling.

Signal conversion:

• Step down from 5 V to 3.3 V
• AC/DC coupling
• Optional gain stage
• Center zero voltage at VCC/2

The RP2040 ADC is 12-bit, while OpenHantek expects 8-bit samples. The firmware scales the ADC value from 12 bits to 8 bits. Maximum digital gain is 16. For higher gains, use an additional analog gain stage before centering the signal.

Materials

• 1 x CD4066 analog switch
• 1 x LM358 op-amp
• 2 x 100 nF ceramic capacitor
• 6 x 10 kΩ resistor
• 2 x 33 kΩ resistor
• 2 x 22 kΩ resistor

If the calibration file was previously copied to the OpenHantek config folder, delete it when using the external circuit.

Pins

• AC/DC coupling channel 1: GPIO 20
• AC/DC coupling channel 2: GPIO 21

Calibration file OpenHantek

If the external circuit is not used, OpenHantek needs a calibration file to map the RP2040 0-3.3 V ADC input to the expected ±5 V range.

Copy RP2040_0_calibration.ini to:

• Linux: ~/.config/OpenHantek/
• Windows: %USERPROFILE%\.config\OpenHantek

The calibration file sets:

• Offset: -127
• Gain: 0.33

If using the external circuit, set offsets to 0 and gains to 1, or delete the calibration file.

Notes

• The firmware uses fixed-length USB transfers. Streaming is implemented at the application level by re-arming transfers.
• Double-buffered USB transfers improve stability and reduce buffer overflows.
• At 1 MS/s, operation is usable but may still show occasional dropped samples due to USB full-speed bandwidth limits.
• For rates above ~500 kS/s, the ADC operates at higher clock speeds, which may slightly affect accuracy.
• Linux: if the device is not detected, install the appropriate udev rules under /etc/udev/rules.d/ and reconnect the device.
• Windows: you may need to install a WinUSB-compatible driver using Zadig.

Example Linux setup:

sudo cp udev/99-openhantek.rules /etc/udev/rules.d/
sudo udevadm control --reload-rules
sudo udevadm trigger

Binaries

• RP2040
• OpenHantek (RP2040)

References

• OpenHantek6022
• libsigrok
• Das Oszi Protocol - eLinux.org
• Usb device library