I am running an AtMega328P-PU at 3v3, 8MHz with external crystal successfully. I am able to burn the bootloader using ISP and flash the chip using FTDI and the clock seems right.
The mega is connected to a RFM69HCW, which uses the SPI interface and thus occupies the SCK, MISO, MOSI and SS along with int0 for RFM DIO0 and PD4 for RFM reset.
I am facing a problem where the FTDI interface does not work most of the time when the RFM is connected to the mega. There does not seem to be a pattern to when it works and when it does not, so I guess it might be something with timing (I do not have a scope, so I cannot verify this).
When the FTDI interface does not work (I cannot establish a serial connection - either using a monitor or uploading a program) the TX LED (red) on the USB-2-FTDI board is very dimly lit. I am using a Sparkfun FTDI Basic with the voltage jumper cut and soldered to 3v3. This board has auto-reset, no no need to reset the mega manually during programming (doing so does not change anything).
The FTDI interface works perfectly and reliably if I disconnect the RFM from the mega, so I suppose that it has something to do with this.
I have attached a schematic of my setup (it contains a few more components than my setup, so please disregard connection labels that have no match).
Any help will be much appreciated!