FYI - a simulation - signal from FTDI to the Rx pin, and Rx coupled via a parasitic capacitance to the XTAL1 pin of the 1284p's oscillator. For 3 parasitic capacitances - 1pF, 5pF, 10pF. You may see the effect on the frequency when the rising edge of the Rx signal comes in. May behave differently - depends on the gate (here 4011 gate model used), xtal (??), etc. parameters used.
Disclaimer: this is only a simple simulation with parts with different parameters as the 1284p probably includes. For demonstration purposes only. No warranties of any kind.
