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Arduino trick heard on embedded.fm: use down counters in your while loop

Jan Cumps

When listening to this week's embedded.fm episode ( ), I heard this Arduino tip:

If you use downcounters in stead of upcounters in your loop(), you save a clock cycle each time,

because down counting is a core operation and takes 1 cycle. Up takes 2 cycles.

 

For those cases where you have to pinch that last cycle out of your loop().

  • I've not heard of Embedded.fm before, will definitely be checking that out, thanks

  • Yes, I listened to that pod cast. Very interesting. I'm new to the AVR world; so, I was curious to check out the ATmega328 instruction set. My guess is that the advice given is good but for the wrong reason. This puppy can add and subtract in one cycle just fine. The instruction timing diagram shows for such 1 cycle instructions the first third of the clock being taken up with register setup to the ALU. the middle third taken up by the ALU operation, and the final third being taken up with writing the answer back to the register file. This design doesn't really push the ALU. Thus such ALU instructions also have time to figure out appropriate status and write back to the status register.. One of these status flags is the Zero Flag. Thus after decrementing the loop counter and having steered the Zero Flag appropriately, a simple conditional branch if non-zero (BRNE) back to the top of the loop is all that is necessary. This BRNE takes 2 cycles when branching back to the top.  I f one is incrementing the loop variable, then between the ADD of the loop variable and the BRNE back to the top, at the very least one has to insert a CPI instruction testing for the upper bound to get the Zero Flag set appropriately.  Essentially, when counting up, the appropriate setting of the Zero status flag takes at least one extra cycle.

  • Just had a flashback to the days of hand writing for the Z80.

    Decrease, jump non zero was a single instruction. I can't remember if this particular one was a long or short jump.

    The other thing I remember was padding out sound routines with dummy instructions so that each branch took the same number of cycles. Sound on the Spectrum worked by toggling a single bit on one of the output ports.