I bought a CH341 USB Serial to Audio cable at the same time I ordered the Pinebook Pro. What I discovered was that the CH341 chip had difficulty keeping up with the 1.5Mbit speed the Pinebook Pro communicates at.
I had a FT232RL USB UART device available and built a new cable, this gist describes how.
The minimum materials required are:
- 24 guage stranded wire, about 30 inches (approx 75 cm)
- 1/8 inch (3.5mm) 3-pole male audio plug
- 3.3V FT232rl USB Serial (UART) breakout board with female mini USB connector.
- male mini USB to male type A USB cable (assuming the host has type A USB ports)
What makes it nicer:
- heat shrink tubing or electrical tape
- soldering iron
- lead free solder
- braided desoldering wick
- multimeter or continuity tester
Nice to have but not required:
- wire cutter/stripper
/---- Rx
/ /--- Tx
/ / /-- Gnd
<=:=:==[
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Unscrew the collar from the plug.
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Cut 3 equal lengths approx. 6 inches or 15 cm in length.
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Strip approx. 3/8 or 1 cm from each end of each wire.
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Insert a wire into an eyelet, fold the wire in half onto itself and solder.
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Using a multimeter or continuity tester, place one probe on the Rx end of plug and find it's matching wire. Mark this wire with one line at the end.
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Do the same thing with the Tx end of the plug. Find it's matching wire and mark it with 2 lines at the end.
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Confirm the last wire is attached to Gnd. You do not need to mark this wire.
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If using heat shrink tubing or electrical tape, apply it to the wires from the plug connection leaving about 2 inches or 5 cm from the end of the wires.
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Thread the collar over the wire and back onto the plug.
- Carefully desolder any connectors attached to the serial end of the board. Usually labeled:
- GND
- CTS
- 5V
- TXD
- RXD
- DTR
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Solder the Rx wire (marked with one line) to the TXD hole of the FT232rl board.
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Solder the Tx wire (marked with two lines) to the RXD hole of the FT232rl board.
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Solder the Gnd wire to the GND hole of the FT232rl board.
IMPORTANT: Most of these boards can be configured for 3.3V or 5V using either a switch or a jumper. Ensure it is set to 3.3V.
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Plug the mini USB connector into the FT232rl board and the opposite end into an open USB type A port.
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Make sure the UART switch on the Pinebook Pro motherboard is set to on, towards the hinge.
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Insert the audio plug into the audio jack on the Pinebook Pro.
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Start your serial communications program on the host. If using linux, and picocom:
picocom /dev/ttyUSB# -b 1500000
where # is the number shown in /dev/
This is usually /dev/ttyusb0 if you only have one of these attached.
- Turn on the Pinebook Pro, and you should see u-boot messages, and then kernel messages.
Remember to turn the UART switch off, towards the touchpad.