AVR Freaks

One of the first things I tried was: read 8 bits of ADC and put them to OCR0A and vary the output pulsewidth with a pot.

Well as only about 3 registers are involved it's not that complex to see what might be different. The key control register for ADC is ADCSRA. If you compare the mega8 and tiny85 datasheets you wil find that 7 of the 8 bits are identical and in the same position. The only difference is bit 5 which is called ADFR in the mega8 - that is ADC Free Run and simply means that once ADSC is set the ADC will make back to back conversions repeatedly. Bit 5 in ADCSRA for t85 is described as ADATE. This is a bit more complex than simply ADFR as it brings into play a whole new register. When bit 5 of ADCSRA is set in t85 then ADCSRB dictates how the "auto conversion" mode will operate (mega8 doesn't even have ADCSRB). So now read the documentation of ADCSRB in the t85 data and you find that (a) it defaults to 0 and (b) when it holds 0 (so the bottom three ADTS bits are all 0) then this selects "free running" mode. If you didn't know better you could actually believe that Atmel designed it this way on purpose so that (until ADCSRB is changed) it operates identically to old CPUs like mega8.

The beginners code also shows 0 written to ADMUX. So now compare the two datasheets... M8 tells us this will select AREF as a reference, no left shifting and channel 0. The t85 is slightly different in that to select AREF would erquire bit 6 to be set. So you have two choices. Either you leave the code as it is and accept that ADMUX=0 is going to select Vcc as the reference or you change the line:

to be:

Another thing you will want to change is:

at the top to be:

but if you are using AS6 you don't need any .include at all as simply setting "tiny85" in the project options passes a command to the assembler that effectively does that .include anyway.

One other small difference is:

The vector for ADC moves from 0x000E to 0x0008 inthe t85. But it is bad practice to hard code the address anyway (I would have thought avrbeginners.net would have taught this in fact!). It's far better to refer to it symbolically:

where "ADCCaddr" is define in the header for mega8 as 0x000E and in the header for t85 as 0x0008 so the same code would work on both without change if they had done it this way.

Finally the m8 program outputs to eight LEDs on the whole of PORTD (m8 is 28 pins after all). A t85 doesn't have PORTD/DDRD, only DDRB/PORTB and not a full 8 pins for LEDs so you need to change those lines to pick the other port and perhaps just one or two LEDs showing the top 2-3 bits of the ADCH reading (use a shift/and to get this)

As far as I can see that's it.

This analysis I've just done is something you should learn to do yourself. You aren't always going to find code for the tiny85 or mega328 or xmega192A1 or whatever it is you happen to be using. But most of the time it's possible to "port" code from one AVR to another. All it takes is a comparison of the two datasheets to find out how anything being used differs.

Perhaps someone else reading this can double-check my work to see if I got anything wrong or missed anything but I'm pretty confident this will give you a piece of working t85 code.

You know what you did there? (HWMNBN!)

You still need to read the datasheet. Otherwise, how are you going to adjust this from RetroDan?

Indeed, the datasheet is the main resource, but sometimes a bit of code to look at can clarify what the datasheet is trying to tell you.

I sometimes finding myself thinking, "Ok, I've read about all these bits in all these registers, so, um, how do I make this work?" Then I see an example and realize you select the reference, set the clock speed and do it, just like the datasheet has been trying to tell me all along. Once you understand you're dealing with a forest, all the trees start to make sense.

Accept when you get into xmegas or AVR32s, there are no "Totally different" AVRs. The cores are all basically the same. Systems such as I/O ports, ADC and USARTS are pretty much the same for basic usage. Even the most complex peripheral like the timers, only come in a few varieties. I think you are obsessing too much on what bits to set rather than how they work. Once you know how they work, what bits to set is easy to figure out.