UART

The USB UART dongle delivered with the PineNote allows you to have access to its serial console via USB-C Debug Accessory Mode (DAM) without having to disassemble the device. The factory-installed operating system runs the serial console at the 1,500,000 bps speed, 8 data bits and 1 stop bit, no parity, and no flow control. The USB-C male end should go into the PineNote and the female end can be connected with a standard USB-C cable to your computer.

It is relatively easy to build your own UART interface with a USB-C breakout board (for example https://www.ebay.com/itm/275407037613), two resistors and a 3.3V USB serial adapter. It is basically just two 1K pull up resistors (R3, R4), the data sheet values of 10K isn’t whats on the real hardware, see the schematic. The pull ups enable the serial output on SBU1 and SBU2 you can use with any 3.3V USB serial adapter.

The UART dongle is not necessary to flash the PineNote, but is essential if something goes wrong to fix it without having to open the case.

This page contains information on creating and using a UART dongle for the PineNote. The PineNote was shipped with a compatible UART dongle, but replacements are not available to order in case of loss or hardware fault. Thankfully it is not difficult to make your own from easily-acquired components and a small bit of soldering.

Fear not if you’ve never soldered anything before! This serves as an excellent first soldering project. Borrow the tools from a friend, local hackerspace, or tool library. Pine64 also makes a nice soldering iron themselves, the Pinecil.

A PineNote UART dongle enables you to:

1. Interact with the system boot menu
2. Read system events in real time as the PineNote is used
3. Fix the PineNote without opening the case if something goes wrong while flashing it

Since the PineNote is an embedded system, interfacing with it during boot is more complicated than with an ordinary computer. The UART dongle enables you to do this.

The PineNote factory firmware runs UART at a baud rate of 1500000 bps, 8 data bits, 1 stop bit, no parity and no flow control. The process by which the PineNote design was modified to include closed-case UART is documented here.

Stock dongle

The stock UART dongle included with the PineNote was a simple device plugging directly into the PineNote’s USB-C port. The dongle exposed a female USB-C port, which the user plugged into to get UART access. This design unfortunately ruled out passthrough USB connections, where the user connects to the PineNote via UART and USB simultaneously. The dongle is not currently available for purchase.

Creating a dongle

A typical self-built PineNote UART dongle design has the following components:

1. A USB-C breakout board with a male connector exposing the 24 pins of the PineNote’s USB-C port
2. A USB-UART adapter, to plug into a USB port of the computer you’ll use to interface with the PineNote
3. Jump wires to connect specific breakout board pins to pins on the USB-UART adapter
4. Two 1,000 Ohm through-hole resistors to splice into the jump wires
5. Electrical tape or heat shrink to wrap connectors and prevent shorts

You will also need the following tools:

1. Soldering iron with solder
2. Wire cutters & strippers

The PineNote’s internal UART system is documented in this schematic. The purpose of all 24 USB-C pins is documented on the USB-C Wikipedia page.

We are interested in three sets of pins:

1. The SBU1 (A8) and SBU2 (B8) side band use pins
2. The CC1 (A5) and CC2 (B5) configuration channel pins
3. The GND ground return pins (A1, A12, B1, and B12)

In the PineNote UART schematic you can see (on the bottom right diagram labeled USB_TYPEC_Male) the side band pins are given the labels UART2_TX_SUB1 for A8 and UART2_RX_SUB2 for B8. The first (TX) is used for transmitting data and the second (RX) is used for receiving data, from the perspective of the PineNote. Also note the configuration channel pins labeled TYPEC_CC1 for A5 and TYPEC_CC2 for B5. The diagram shows they must be connected to a 3.3V source in parallel, mediated by resistors. Per the USB-C standard, when these pins are pulled high this indicates the device should enter Debug Accessory Mode; connecting them to a voltage source & limiting the current with pull-up resistors accomplishes this. The schematic indicates 10,000 Ohm resistors, but community member Visti Andresen (talpadk) experimented and found 1,000 Ohm resistors work better. Our mission is to wire up pins from a USB-UART adapter to a USB-C breakout board following these requirements.

Buying components

There are many possible USB-C breakout board designs available for purchase online. One particularly useful design is a “passthrough” or “intercept” style, with both male and female USB-C ports. This design is more versatile in case you want to reuse it in other projects, and also enables you to connect to the PineNote via UART and USB at the same time. An example of this product can be found here, although you are encouraged to shop around for cheaper options. If you’re fine with a bit more soldering, there is a very cheap one here.

Similarly, there are many USB-UART adapter designs available. These devices plug into your computer and expose a number of pins themselves, connecting to specific pins on the breakout board with jump wires. It is important to get a 3.3V model, or at least a model with the option of 3.3V, as a 5V source might fry the PineNote. Here is one example with jump wires included, although you are again encouraged to shop around for alternatives.

All other necessary components of our UART dongle are readily & cheaply available in many locations.

Splicing resistors

This is the only difficult part of the whole process. Your goal is to create a Y-shaped jump wire with 1,000 Ohm through-hole resistors spliced into each of the twin arms. The solitary leg will connect to a 3.3V source on your USB-UART adapter. The twin arms will connect to the configuration channel pins on your USB-C breakout board. Per the USB-C standard, when these pins are pulled high this indicates the device should enter Debug Accessory Mode; connecting them to a voltage source & limiting the current with pull-up resistors accomplishes this.

For this project you’ll need:

1. A soldering iron with solder
2. Wire cutters & strippers
3. 2x jumper wires, male/female as compatible with your board designs
4. 2x 1,000 Ohm through-hole resistors
5. Electrical tape or heat shrink wrap

Consider buying extra jumper wires and resistors in case you mess up. Also double-check that you have 1K resistors with this color code calculator. Although the PineNote UART schematic says to use 10K Ohm resistors, community member Visti Andresen (talpadk) experimented and found 1K is more appropriate.

Assemble your Y-cable as follows:

1. Graft one jump wire onto the other to form a Y shape using this technique, ensuring solitary leg end is compatible with your USB-UART adapter
2. Splice resistors into the twin arms using this technique
3. Splice remaining jump wire onto the ends of the resistors, ensuring ends are compatible with your USB-C breakout board
4. Cover all exposed wire & resistors with electrical tape or heat shrink wrap

Assembly

Once you have acquired all necessary components, assemble the UART dongle as follows:

1. Connect a jump wire from the GND pin on the USB-UART adapter to any one of the four GND pins on the USB-C breakout board (A1, A12, B1, or B12)
2. Connect a jump wire from the RXD pin on the USB-UART adapter to the UART2_TX_SUB1 SBU1 pin on the USB-C breakout board (A8)
3. Connect a jump wire from the TXD pin on the USB-UART adapter to the UART2_RX_SUB2 SBU2 pin on the USB-C breakout board (B8)
4. Connect your Y-shaped jump wire from the 3.3V source pin on the USB-UART adapter to the CC1 and CC2 pins on the USB-C breakout board (A5 and B5)
5. Wrap all metal connectors in electrical tape or heat shrink to prevent accidental shorts

Be very certain of your connections for the 3.3V source and its cable, as there is a real risk of irreparably frying your PineNote if they’re wrong! Especially be sure you are connecting to a 3.3V source and not a 5V source.

Note that if you mix up the TX/RX pins, it will still work but the USB-C breakout board will just plug into the PineNote upside down. You can therefore choose which orientation you want by swapping the TX/RX pin connections. Experience shows that RX/TX-TX/RX connections will have the PineNote face down while connected, while RX/RX-TX/TX connections will put the PineNote face up.

Using the dongle

First, use your UART dongle to physically connect your PineNote to your computer:

1. Plug the USB-UART adapter into one of your computer’s USB ports
2. Plug the USB-C breakout board into the USB-C port on the bottom of your PineNote; the orientation matters, so try both and remember which one works

Once the hardware is connected, we need some program on your computer to communicate over that connection with 1500000 (1.5 million) bps, 8 data bits, 1 stop bit, no parity and no flow control.

Here’s how you do that:

1. Identify the USB-UART adapter in your /dev directory by running ls /dev with it plugged in, unplugging it, then again running ls /dev and seeing what changed; it is likely to be called /dev/ttyUSB0
2. Check your permissions; run ls -l /dev/ttyUSB0 to see which groups have access to the dongle (probably dialout or uucp), and add your user to that group; remember you need to log out before group changes take effect
3. Install minicom (or some other option, but the instructions below are written for minicom)
4. In a terminal window, run minicom -D /dev/ttyUSB0 -b 1500000 or run minicom --setup to specify these settings by default

Once the software is set up, power-cycle your PineNote; as the system boots you should see text appearing in your terminal window. You can exit the session with ctrl+a x then pressing Enter to confirm. Run man minicom for more details.

Sending commands

Pressing ctrl+a in minicom enables you to send keystrokes to your PineNote. The most important of these is ctrl+c, which if sent during boot will put you in the U-Boot command prompt. You can then type help to list possible commands.

Troubleshooting

If you don’t see any text in your terminal as the PineNote boots, or the text is garbled, try the following:

• Ensure your GND, RX/TX, and CC jump wires are connected to the correct pins on both the USB-UART adapter and the USB-C breakout board
• Connect your USB-C breakout board to your PineNote in the opposite orientation
• Run minicom as sudo in case your user doesn’t have appropriate permissions
• Ensure you are setting the baud rate to 1500000 (1.5 million), and (less importantly because these are probably used by default) 8 data bits, 1 stop bit, no parity and no flow control

If you can see text but are having trouble sending ctrl+c to the PineNote during boot:

• Be sure you’re typing ctrl+a first, or whatever escape sequence your terminal emulator uses
• Send it right as the PineNote is booting, before the loading bar appears on screen
• Double-check your Y-shaped pull-up resistor cable; if this isn’t working properly you’ll probably be able to read text but not send text

USB passthrough

If your USB-C breakout board has a passthrough/intercept design, you can connect to your PineNote over USB and UART at the same time. This can be useful when you’re doing development work on the PineNote boot process so you don’t have to continually reconnect cables. You’ll need a USB-A to USB-C cable, connecting directly from your computer’s USB-A hub to your USB-C breakout board’s female USB-C port. It’s important to connect directly from USB-A, without any intermediate USB-C components. Note that connecting a live USB cable to your USB-C breakout board in this way dramatically increases the danger of frying your PineNote with a short, so you should only do this if all connectors are safely wrapped in electrical tape.