The baud rate is the effective data rate. It still has to have some kind
of frequency driving it. Perhaps the most effective solution is to switch
the supply voltage to the CH340 chip. That is 3.3v at pin 19, I believe.
The chip is tolerant of 5v from the USB connection, but ordinarily operates
at 3.3v. There is no serial converter chip (as shown in the "normal" configuration
in the datasheet. The TXD and RXD lines connect directly to the ATMega chip.
It would be a delicate operation to switch and thus disable the power input line, but
it could be done.
Has anyone investigated that the USB connection might be a source? It has an internal
frequency control in the computer. But it is an unlikely source, especially when disconnected.
john
AD5YE
