Russlk said:
My giving you advice is like the blind leading the blind, but in 40 years I have picked up some tidbits of information. The atmega16 has 32 I/O ports and operaties on +5 volts. RS232 is out because it uses + and - voltage. The only difference between TTL and CMOS is the impedance level. The data sheet (143 pages) will tell you the source and sink capability of the outputs and the voltage and current requirements of the inputs.
Not quite. Rs-232 voltages are +- 12 V , TTL voltages are ground and +5VDC.
The output of the Amtel device ( since the supply is +5VDC ) is limited to that range unless you use either a TTL to CMOS level converter or a TTL input CMOS output level output UART. The TTL signal is used for short range communications, typically < a few meter or so. The 232 is a very old convention, there are new better schema's. I don't know what Your trying for as usual the better the description, the easier to find a solution.