95 lines
3.7 KiB
Markdown
95 lines
3.7 KiB
Markdown
## 8-bit Timer/Counter Register Description (see datasheet section 13.8)
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TCCR0A : Timer/Counter 0 Control Register A
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.---------------------------------------------------------------.
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|---------------------------------------------------------------|
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| COM0A | COM0A | COM0B | COM0B | Reserved | WGM1 | WGM0 |
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'---------------------------------------------------------------'
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TCCR0B : Timer/Counter 0 Control Register B
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.---------------------------------------------------------------.
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|---------------------------------------------------------------|
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| FOC0A | FOC0B | Reserved | WGM02 | CS02 | CS01 | CS00 |
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'---------------------------------------------------------------'
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* We want:
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* `COM0A` = `0b00` : Normal port operation, `OC0A` disconnected
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* `COM0B` = `0b00` : Normal port operation, `OC0B` disconnected
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* `WGM` = `0b010` : CTC (see section 13.6.2 "Clear Timer on Compare
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Match (CTC) Mode")
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* `CS` = `0b011` : clk_i/o / 64 (from prescaler)
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TIMSK0 : Timer/Counter 0 Interrupt Mask Register
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.-----------------------------------------------------------------.
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|-----------------------------------------------------------------|
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| Reserved | OCIE0B | OCIE0A | TOIE0 |
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'-----------------------------------------------------------------'
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* We want:
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* `OCIE0A` = `0b1` : enable the Timer/Counter 0 Compare Match A
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interrupt
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* We also want to set `OCR0A` (Output Compare Register A) to `250` for the
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number of "ticks per millisecond" (see below)
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* Since we're using CTC mode with `OCIE0A` enabled, we will be using the
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`TIMER0_COMPA_vect` interrupt vector (see
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[the avr-libc documentation on interrupts]
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(http://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html)).
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## Other Notes
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* References:
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* [Newbie's Guide to AVR Timers]
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(http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=50106)
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: tutorial lby Deana Camera
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* For a CPU frequency of 16 MHz:
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--------------------------------------------
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prescale value ticks per millisecond
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---------------- -----------------------
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1 16000
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8 2000
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64 250
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256 62.5
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1024 15.625
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--------------------------------------------
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* So if we set the prescaler to 64, we can just barely get to a millisecond
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within the range of an 8-bit counter (2^8-1 = 255, and we need 250
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ticks).
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* Setting `COM0A` and `COM0B` to `0` (or, rather, leaving them that way) is
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very important: other things may be operating on those pins (`PB7` and `PD0`
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respectively), and we don't want normal port functionality to be overridden.
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* Abbreviations:
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* `COM`: Compare
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* `CS`: Clock Select
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* `FOC`: Force Output Compare
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* `OCIE`: Output Compare Interrupt Enable
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* `OCR`: Output Compare Register
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* `TCCR: Timer/Counter Control Register
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* `TCCR`: Timer/Counter Control Register
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* `TIMSK`: Timer/Counter Interrupt Mask Register
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* `TOIE`: Timer/Counter Overflow Interrupt Enable
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* `WGM`: Waveform Generation Module
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-------------------------------------------------------------------------------
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Copyright © 2013 Ben Blazak <benblazak.dev@gmail.com>
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Released under The MIT License (see "doc/licenses/MIT.md")
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Project located at <https://github.com/benblazak/ergodox-firmware>
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