Atari 7800 (and 2600) joystick port question

[EricBall]

If you really want to roll your own, then you can take a queue from NES/SNES controllers and use shift registers. A single 8-bit register will get you 8 inputs, or you could use 2 in parallel for 16 inputs, or go whole hog and chain as a bunch serially so you can use a Sinistar 49-direction joystick and 32 buttons!

 

Yeah, but how many cycles would it take to read the joystick?

STX SWCHA # 4 set clock pin high
LDA SWCHA # 4 read data pin
ROL       # 2 shift bit 7 to carry (ROR for player 2)
STY SWCHA # 4 set clock pin low
ROR JOY1  # 5 shift C into ZP register

So that's 19 cycles for one bit, or 158 cycles (and 104 bytes) for all 8 bits. (Adding 4 cycles to set up X & Y and 6 cycles to send the LATCH, but making the first ROL into a STA.) Actually, that's not too bad.

Edited August 27, 2014 by EricBall

[iesposta]

Why isn't RevEng's post #6

http://atariage.com/forums/topic/228835-atari-7800-and-2600-joystick-port-question/?p=3055063

the end of this topic?

 

He shows with an abundantly available 3 button Genesis pad, 2 swapped pins, and a 10K resistor, the Atari 7800 and 2600 can read a joystick, including diagonals, with 4 buttons (button A, B, C, and Select).

Then you don't need a matrix and all the processor time to read it.

[Moonbeam]

Why isn't RevEng's post #6

http://atariage.com/forums/topic/228835-atari-7800-and-2600-joystick-port-question/?p=3055063

the end of this topic?

 

He shows with an abundantly available 3 button Genesis pad, 2 swapped pins, and a 10K resistor, the Atari 7800 and 2600 can read a joystick, including diagonals, with 4 buttons (button A, B, C, and Select).

Then you don't need a matrix and all the processor time to read it.

 

Perhaps it was not the end of the topic because even if it is an excellent solution, depending on what you want to do, another one may be preferred :-)

I'm very glad I've had all your posts, ideas and help. I have learnt some very interesting stuff. Thanks to all of you !