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Arduino UNO clone CH340G Serial design flaw & fix.


I don't know if it's been mentioned that some of the cheap Arduino UNO clones using the CH340G USB-Serial chip have a correctable design flaw!

They've fancied the board up by adding LEDS to the Tx & Rx lines, but in the process they've violated the ATmega328p Input Low Voltage limit on the RXD input, which is now only pulled down to about 1.7V instead of below 1.5V. This can cause intermittent serial operation.

The fix is easy enough...just remove either the Tx LED, or R7. This restores the full 0-5V swing.


Which nano clone are you talking about?  There are a bunch of different designs out there...


--- Quote from: westfw on May 19, 2019, 09:31:03 am ---Which nano clone are you talking about?  There are a bunch of different designs out there...

--- End quote ---

The OP mentioned UNO rather than Nano.  Any of the UNO R3 boards with the CH340 seem to have these LEDs.  The problem is that the design includes a 1k resistor between the CH430 TX output and the AVR RX input pin, so that the AVR pin can be used for other functions without the CH430 output loading it.  Unfortunately the LED has been placed downstream of this resistor, resulting in a much reduced voltage swing.

From this schematic:

it looks like:

CH340TX(low) <-- 1K  <-- LED <-- 1K <-- 5V

What would the voltage be at AVRRX?  If the LED drops 1.8V, then AVRRX is at the middle of a 1K/1K divider between 3.2V and ground, or 1.6V.  Is that right, or is it more complicated?

Edit:  I guess I should have asked what color the LED is.  It looks like this setup would be more likely to work of it's a green LED instead of red.


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