The program itself is small, but a atmega16 is required because of the requirement for 8 inputs and 16 outputs.
A trick is used to increase the number of interrupts: In addition to the INT0 and INT1 signals, the external counter pins are used as interrupts. By preloading the counters with 0xff (8-bit T0) and 0xffff (16-bit T1), the next rising edge seen on the pin will trigger an interrupt.
We initially had problems with the board losing steps. I believe this was fixed by making the correct settings in EMC: The "step" signal is active high, and the direction signal is latched after the "step" signal is asserted.
I don't know for sure how fast this board can go; It obviously depends on the clock speed you choose from the AVR. Based on what I recall about reading the disassembly, a "step" pulse took around 50 AVR cycles to finish being translated. This means that if it sees an edge on all 4 step pins at once, the direction output must still be valid up to 200 AVR cycles after the step, or 12.5us at 16MHz. Similarly, this would indicate a maximum step rate of 80kHz at 16MHz. In practice, I think that Chris's machine was limited to about 12ipm at that time, a step rate of 1600Hz, so we didn't get anywhere near these theoretical limits.
I had one thought when I looked at this program today. With an atmega16 there is no reason to use PROGMEM for the motor stepping pattern.
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Entry first conceived on 15 October 2005, 19:11 UTC, last modified on 15 January 2012, 3:46 UTC
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