A nice simple explanation - thanks! So 34 and 35 are effectively output on the 6809, and input on the 6809E. No problem there, but it looks like LIC and TSC (38 and 39) not being available on the 6809 would be an issue. I might take a look over a few designs (Williams WPC, Data East audio processor, Williams audio processor) and see if they make use of those. I expect they do... On 2/07/2014 2:56 p.m., Eric Smith wrote: circuit (pins 38 and 39) and generates the required quadrature clock (E and Q, pins 34 and 35). The 68x09E requires an external quadrature clock input (E and Q, pins 34 and 35), and uses pins 38 and 39 for other functions (LIC and TSC, respectively).