Attiny25/45/85 FW Development Thread

[ToyKeeper]

@Tom E

Forgot what the #’s, were but someone measured or calc’ed the rate we are using, and it’s pretty insane - think FAST is bout 30-35 kHz? I think you take 4.8->8, and factor up PWM rates the same? Not sure…

I love the 8Mhz operation of timing. On the 13A’s, I’d see some occasional whacky timing inaccuracies for the CPU speed - not sure if it’s the msecs delay function, and possibly the WatchDog interrupt timing. But with the 25/45/85’s, the 8Mhz seems always dead-on accurate.

That was probably me. I found a way to measure it directly.

From post 1801:

These attiny25 MCUs run much more consistently than the tiny13a ones do.

On the more common tiny13a I measured PWM speeds anywhere from 13 kHz to 17 kHz, since the clock speed of the chip varies a lot from unit to unit. And I’ve never seen one which runs at 18.75 kHz (which is what the spec says it should do).

On the tiny25 though, I measured four units and got the following:

• 15.58 kHz
• 15.49 kHz
• 15.65 kHz
• 15.56 kHz

This means that the tiny25 is running anywhere from 7.93 MHz to 8.01 MHz, very very close to the 8MHz spec.

Multiply PWM speeds by 2 to get the FAST version instead of PHASE.

Also, to calculate what the PWM should be…
8 MHz * 1000 kHz/MHz / 256 cycles per PWM loop = 31.25 kHz (fast PMW)
8 MHz * 1000 kHz/MHz / 512 cycles per PWM loop = 15.625 kHz (phase-correct PWM)

… and to get the ideal “bogomips” factor for the delay loop, take the MHz * 1000 / 4. So, for 8 MHz, the bogomips should be 2000. For 4.8 MHz, the bogomips should be 1200. But the attiny13a actually averages closer to 3.8 MHz, so the value is actually more like 950 instead of 1200.