HI Guys,

I have had a need to use a number of USB to TTL modules for a project

Most have either been the PLH2303 or CP2102 modules

one problem i had found was that when the USB module was connected to the a 3.3v pic ( gnd , RX,TX only) and the usb was powered via the usb cable , the pic was not happy

the pics RX pin was at 5v and the PIC would not power down correctly or do some strange things while the usb cable was connected to the PC and the PIC powered down.

to resolve this i placed a 4k7 resistor in line with the USB-RX to the PIC RX input , dropping the voltage to about 1.8v ( when the PIC is powered down, but the USB cable is still connected)


wondered if other have seen this issue when using USB on PIC running at 3.3v ?


also i have found when the PIC was running DT's INT_RX to detect terminal input and the USB cable to the USB -ttl model was not connected the PIC RX input was low , which triggered the ISR , slowing the PIC down to service an invalid input

as a result i placed a 100k resistor to 3.3v to ensure when the USB is not connected the ISR is not triggered
note: prior to using a 100K pullup a 4k7 was tried , but cos of the above fix of 4k7 in line to stop issues with 5v input signal the low going pulse was not strong enough to get consistent inputs for the RX line


I wonder if others have had this problems with interfacing the USB to TTl modules to 3.3v pic application ?

cheers

sheldon