The Flip Flop

I'm flip flopping like a politician on the data read algorithm in the Atmega328P MCU interface to the cartridge port. As described in previous posts, I was using an edge-triggered interrupt to alert the MCU that new data was ready. I chose the interrupt method because the original design was implemented on a single ESP32 that handled both the cartridge and wifi interfaces. The handling of the two interfaces was inherently asynchronous because they were both reacting to their external counterparts. The cartridge side didn't know when the Atari would push data and likewise with wifi and remote server. This didn't work very well given my rudimentary MCU programming skills. Splitting the system into two MCU's fixed the problem. It also allows a flip-flop to be made.

Researching this project, I ran across a website from Poland (that I can't find now) on interfacing to the 6502 bus. They used an SR flip-flop to latch the write enable signal before sending it to an MCU. The MCU could then poll the signal for a state change. While the interrupt method allows for simpler decoding logic, the polling makes for simpler software. And it's faster. The interrupt takes about 3 microseconds (several 6502 clock cycles). The polling can respond in half a microsecond or about a single 6502 cycle. This will add up to many saved clock cycles on the Atari side in the end.

Here's the concept.

The LE line goes low when the 6502 performs a STA $D5xx (described in this RC2019/03 post). The LE signal SETS the SR flip flop and its output Q tells the MCU it's time to read the data and address. The read operation is described here. Note, now the time between the LE going high and the /OEDATA going low is < 1 microsecond. It used to be 3 microseconds. Then the MCU sends a RESET to the SR flip flop and goes on to its next operation ...

... which is what I'm going to do, too.