Uploading sketch via RX/TX error: unable to sync

Wondering if you guys can give me some direction.

I have a breadboard 1284p I'm working with. I loaded up a bootloader with my UNO using ArduinoISP. I can upload sketches that way.

However when I connect my CP2102 directly I get programmer not responding / sync errors. I tried both UARTs with the same luck.

Unsure how to adequately test UART, I did at least run the blink sketch on all four of those pins.

Any idea what I am doing wrong or how I can test?

You have DTR from CP2102 going thru a 0.1uF cap to reset pin, with 10K pullup to 5V?

Yes, I tried it with a .1uf tantalum, and a 10k to 5v.

Tried swapping Tx and Rx?

I did on both serial ports.

WAStevenson:
I loaded up a bootloader with my UNO using ArduinoISP. I can upload sketches that way.

The order of your sentences is interesting to me. It makes me think perhaps you uploaded the bootloader, and then uploaded a sketch. When you upload a sketch via ISP it erases the bootloader. So, do you have a bootloader on the processor at the moment you try a serial upload?

Other things to think about:
Your bootloader (don't know what bootloader you may be using) should probably be written to just look for communication from a specific serial port, probably Serial 0. And it is compiled for a specific baud rate and compiled to expect the processor to be running at a specific frequency (8MHz, 16MHz, or whatever). So you need to make sure everything matches. You didn't mention any of this stuff, such as whether or not you have a crystal, if the fuses are set for crystal or internal osc., what bootloader you are using and what parameters (frequency, baud rate) it is compiled for, what your boards.txt entry looks like, and so on. So if you are not cured by the time you read this and look into that stuff, all those massive amounts of detail is what we will need to help. Also, the IDE preferences has a setting for show verbose output during upload, that could be useful to see. And good clear photos showing the neat wiring such that we can see if it looks OK. And does your USB-to-serial adapter pass the loop back test (details in one of those pinned posts at the top of the Installation and Troubleshooting forum)? Whew! :slight_smile: There's other stuff that others may mention, such as decoupling capacitors.

dmjlambert:
When you upload a sketch via ISP it erases the bootloader.

You don't say.......... Problem solved. Thank-you kindly sir.

Heh. I don't know what I was thinking.