Auto power on for the JetsonTX2 revision C02
One problem of the C02 revision of the Jetson TX2 dev kit is that auto power on is not easy to activate. It includes desoldering and soldering 0402 resistors. The full process is described here: https://forums.developer.nvidia.com/t/auto-power-on-with-c02-carrier-board/70858
I needed a simpler way of automating the auto power on without the risk of damaging the TX2 by desoldering all other resistors next to R313. Initially I tested the solution posted here: https://medium.com/@dumiloghin/auto-power-on-for-jetson-tx1-tx2-with-arduino-a5bd1b37659e It turns on the TX2 using an Arduino Uno and is an excellent easy solution.
However, I needed a smaller form factor that could be co-hosted with the TX2 in a small steel enclosure.
Additionally, the only power source available to drive any logic was the 20V rail on the TX2. In this project I am using an LDO to regulate the 20V to 5V, power an ATtiny85 microcontroller and use it to turn on the TX2 via the J20 header. The microcontroller is mostly in a sleep state and draws very little power, therefore the LDO won't heat up too much.
The /src/ folder contains the very straigtforward main file for the ATtiny85. The Tiny checks if the Jetson is on, if not it turns on the Jetson. If the Jetson is on then the Tiny goes into sleep mode. The sleep mode is the deepest sleep mode available on the Tiny, but as we enable the interrupts before going to sleep the Tiny will wake up to turn on the Jetson again if it shuts down unexpectedly.
The command line commands for compiling and flashing the ATtiny85 are in the compile.sh.
Depending on your compiler please adjust the -c flag of avrdude.
The images folder contains additional images of the final "product".
An inital version of the PCB can be found in the PCB folder. It was created with KiCad. Unfortunatly, I did not have the time to have it manufactured but made it by hand. Additionally, I am more of a software person than a hardware person. Please let me know if you find any boo-boo's.
To build your own circuit you will need the following material & equipment:
- avr-gcc, avr-objcopy, avrdude
- a bit of soldering knowledge & soldering iron
- a programmer for the ATtiny85. I use the one mentioned in the table below. If you use a different one don't forget to change it in the avrdude command.
- the parts on the list below
- consumables (solder, jumper cables, heat shrink, some thicker (i.e. lower gauge) wire)
| Part description | Number | On Schematic | Datasheet link |
|---|---|---|---|
| ATtiny85 | 1 | U1 | http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf |
| Programmer | 1 | na | e.g. https://www.fischl.de/ispnub/ |
| LDO - L7805 TO-220 | 1 | U2 | https://www.st.com/resource/en/datasheet/l78.pdf |
| BJT - 2N2222 | 1 | Q1 | https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF |
| Capacitor - 0.47μF | 1 | C1 | na |
| Capacitor - 0.22μF | 1 | C2 | na |
| Resistor - 200Ω | 1 | R1 | na |
| Resistor - at least 125Ω | 1 | R2 | na |
| Resistor - 1kΩ | 2 | R3 & R4 | na |
| LED | 1 | D1 | na |
On the Schematic the BJT is a 2N2219, which should also work.
For R2 use something that matches your LED but use at least 125Ω as the Tiny I/O pins support a maximum of 40mA. I used between 500Ω to 1000Ω.
The capacitors I used are electrolytic, it would probably be better to use a ceramic cap for the 0.22μF. On the other hand the capacitors are not essential and it should work without the caps, but keep in mind that the LDO datasheet does recommend them.
The connections on the board look as follows:
There are four connection points to the Jetson - H1 to H4.
- H1 goes to the PWR pin on the J20 header this is the pin that needs to be pulled low for 1.5 to 2 seconds to turn on the Jetson.
- H2 goes to the OFF pin on the J20 header and is used to detect if the Jetson is on or off.
- H3 goes to the 20V rail on the Jetson.
- H4 goes to ground on the Jetson.
More details if you are not such a hardware person (which includes me):
- H3 goes into the LDO.
- The LDO decreases the voltage to 5V.
- The capacitors on the input and output of the LDO are used to decrease noise.
- The 5 volt is used to drive the ATtiny85 via pin 8 (VCC).
- The ATtiny85 checks if the Jetson is turned on or off - via pin 5 (PB0 going to H2).
- If the Jetson is off the ATtiny85 sets pin 3 (PB4) to high which activates the BJT and pulls H1 low. Pulling H1 low is the same as pressing the power button.
- The Tiny is also connected to a status LED via pin 2 (PB3). The status LED either blinks (very) slowly or is on (when the Jetson is on).
- All ground points are connected to each other and are connected to the Jetson ground via H4.
The following image shows the top view of the TX2. The J20 header is circled in yellow. To reach the 20V rail you need to access the bottom of the board and unscrew the 4 screws circled in red.
Solder a thicker wire to the point on the PCB circled in red. The yellow circle shows the location of the wifi antenna connectors. The thicker wire is not essential but it will withstand a bit more physical stress.
The J20 header is shown in the next picture. The red circle shows the actual header, the yellow shows the description off the pins we actually use. The bottom pin is the PWR pin, i.e. we connect this to H1. The middle pin is the ground pin, i.e. we connect H4 here. The top pin is the OFF pin, i.e. we connect H2 here.
I built my boards in such a way that I could heat shrink the three wires going to J20 together. This adds physical stability and also makes it more unlikely for individual wires to detach from the header.
You can find the initial version of the pcb schematics, KiCad project and gerber files in the pcb folder. I have not tried to manufacture these as I ran out of time and I needed the boards asap. So I manufactured them by hand.
I changed all components to be SMD components, with the exception of the ATtiny85.
- leave out the status LED - after the Jetson is turned on this consumes the most energy
I am not responsible for any damages caused to you or your equipment, board etc. When working with electronics proceed with caution. If you do not understand what you are doing, ask for the help of a professional. That said, please do let me know if you have any suggestions!