TFE+ A500 Keyboard Part 2
Posted by retromaster on February 25, 2010
Last night I had some more progress with the TFE+ A500 keyboard interface.
As shown in the schematics, there are two connectors: One for the keyboard, one for the A500 mainboard. All lines of both connectors are directly connected, with the exception of the CLK line. The CLK line goes through the 74LS01 which has one input tied to an ATtiny24 pin. When the TFE+ is supposedly inactive, the keyboard MCU simply monitors for key events, but does not do anything. Once it detects the keys Alt-Amiga-Tab being simultaneously pressed and released (TFE+ is supposed to become active at this point), it lowers the clock enable line, which prevents the clock pulses from the keyboard reaching the Amiga. This means the Amiga does not receive the keypresses, but the ATtiny24 still does. Obviously, in this mode, the acknowledgement must be issued by the ATtiny24 since the Amiga cannot do it.
Right now, the only thing missing here is the communication between the TFE+ main MCU and the keyboard MCU, which I’ll soon implement and test. But before that, I’d like to port the existing TFE firmware from the ATmega32 to the ATmega644.