Tx pin of Arduino Nano Ch340 version powering other board

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I had just connected Tx pin of nano board A  to Rx pin of nano board B. To my surprise, board B was powered and it was fully operational when I connected Gnd. Not sure how this happened. Verified this with one more board and it is consistent. Burnt LED blink program in board B and works perfectly. Board A is also functional and I used to send some data to board B.

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Draw the circuit and include the input protection diodes that exist on (nearly) every AVR IO pin.

"This forum helps those that help themselves."

"How have you proved that your chip is running at xxMHz?" - Me

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Multi-mpu projects must be carefully designed to prevent parasitic power problems like the one you describe.

 

Jim

 

Mission: Improving the readiness of hams world wide : flinthillsradioinc.com

Interests: Ham Radio, Solar power, futures & currency trading - whats yours?

 

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Is the USART of Arduino A active? The USART Tx pin idles high, so it is not surprising that it can power the other board. Besides, there may be a pull up that can supply current even if the USART is inactive.

 

As to how board B receives power, the answer is in #2.

 

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Good to know.  The Nano can load and run a "blink.ino" program by using the TX line as Vcc.  Then, I assume, when the A-Nano's TX line goes low to send data pulses to the B-Nano's bootloader, the B-Nano's power bypass capacitors (a 10uF and a 0.1uF IIRC) maintain Vcc on the B-Nano.   The B-Nano has its USART enabled by its bootloader.   The A-Nano's TX line can supply 40mA to the B-Nano. The B-Nano is running a 16MHz Mega328P, a CH340G USB-serial, an AMS1117 voltage regulator and a continuous power-on LED.

 

If this is a reliable method, (by which I mean that it works 99% of the time on my prototypes, not that it is legally guaranteed to work 99.99% of the time on 100,000 commercial units in the field; always best to define one's criteria when discussing electronic reliability), then one could use a stereo headphone extension cable to power and communicate to a remote AVR using the tip and ring pins for TX and RX, along with the ground.

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If you need reliable then there are ways to do power and bi-directional comms over just two wires. It needs a few components but nothing too complex or expensive.

"This forum helps those that help themselves."

"How have you proved that your chip is running at xxMHz?" - Me

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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This is (very likely) not a reliable method.

The way this works is that if a signal line to an un powered circuit is high then the top ESD diode of the unpowered circuit shunts current to the power supply caps.

So VCC of the "slave" circuit is probably a diode drop lower as the input signal ( ESD diodes do not have a particularly low voltage drop).

These ESD diodes are specified for about 2mA continuous on an AVR (random datasheet).

Haven't checked the CH340 datasheet.

So if you want to do this then put at least a scottky diode from the signal line to the VCC of the slave circuit.

And check that this does not damage the signal integrity.

Reliability would also change with the duty cycle of your serial signal. (Droppin Vcc).

 

What's the power consumption of the CH340 board?

 

A long time ago this phenomenon was used on purpose by a "printer tester".

A little uC circuit which could be plugged directly in a printer's centronic's port and it started speweing text to make the printer do someting.

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

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This is (very likely) not a reliable method.

Agreed.

 

Running a micro in this manner is out of spec, per the data sheet, both for I/O pin voltage levels with respect to Vcc, and regarding the Vcc power up requirements.

 

JC