From: Jim Brain (brain_at_jbrain.com)
Date: 2004-11-20 06:36:53
Well, I'm finally happy with the code that emulates a paddle on the C64 joystick port. The code is written for the Atmel AVR family of microcontrollers, and can be embedded into a number of projects. The final code runs at 8MHz, sets up a free running 16 bit dual compare match counter at clock speed, and the POT line is fed to the Input Capture pin on the AVR. When the POT line is driven low, the ICP captures the current counter value. In the ISR, both POT lines are driven low. For each POT register, 4096 + 8*<register value> is added to the captured counter value, and an interrupt is set to occur when a match occurs. Then, the counter value is subtracted from the last captured counter value. The result is made positive, and it is checked for sanity. Assuming the number is < 7000, it is checked for equality with 4096. If lower, the onboard oscillator is sped up by adding 1 to oscillator calibration register, if more, the internal oscillator is slowed down. The current captured counter value is saved, and the ISR ends. When the compare matches are made, each ISR brings the respective POT pin high, and then checks if they other match has occured and brings the other line high if so. Features: o Can represent values from 4-255 on POT lines. o Jitter of +1 only at endpoints (4 and 255), and when both POT lines are at the same value (127,127, for example) o Values are dead on over the entire range. If the controller sends $a7, $a7 shows up on C64 o Code synchronizes to speed of target computer. As a result, code should sync for PAL 64s as well as the 264 series, though I still need to test that. o Code is entirely in ISR, so normal task can run during the process. Another ISR of less than ~200*8 cycles can be performed during the discharge portion of the POT cycle. o Minimum parts count (1 resistor for RESET, 1 3.3K resistor per POT line, and CPU). o Code will run on any ATMega part. o ISR takes 64cycles to run and occurs every 4096 cycles. I started to write more complex code to handle the scenario when both POT lines are at the same value, but the code became too unwieldy, and then the compare match ISR routines started taking too long. As soon as World of Commodore is over, which is where I'll demo my project, I'll make the code available online. It's GPL, and it should be easy to build a PS/2 mouse interface, or some other idea. It looks like (if PAL testing proves out my theory) one could use the POT lines for 7 bit data transfer at least, or send any values except numbers < 8 or so. Many thanks to the list for the ideas. Jim -- Jim Brain, Brain Innovations firstname.lastname@example.org http://www.jbrain.com Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times! Message was sent through the cbm-hackers mailing list
Archive generated by hypermail pre-2.1.8.