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Allow a frame rate to be set with a #define. Most of the timing goo is
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determined at compile time, so no extra code is added.  Previously the
frame rate was totally erratic across CPU speeds.

Default frame rate is 80Hz, which is very close to the previous value of
81.7Hz on 16MHz 328 chips.
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root committed Sep 1, 2013
1 parent ac1214d commit 1cd202c
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Showing 2 changed files with 85 additions and 30 deletions.
113 changes: 83 additions & 30 deletions lib/Charliplexing.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -80,36 +80,89 @@ volatile boolean videoFlipTimer = false;
timerInfo timer[2], *frontTimer, *backTimer;


// Record a slow and fast prescaler for later use
typedef struct prescalerInfo {
uint8_t relativeSpeed;
uint8_t tccr2;
};

const prescalerInfo
// Number of ticks of the prescaled timer per cycle per frame, based on the
// CPU clock speed and the desired frame rate.
#define TICKS (F_CPU + 6 * (FRAMERATE << SLOWSCALERSHIFT)) / (12 * (FRAMERATE << SLOWSCALERSHIFT))

// Cutoff below which we need to use a lower prescaler. This is designed
// so that TICKS is always <128, to avoid arithmetic overflow calculating
// individual "page" times.
// TODO: Technically the 128 cutoff depends on SHADES, FASTSCALERSHIFT,
// and the gamma curve.
#define CUTOFF(scaler) ((128 * 12 - 6) * FRAMERATE * scaler)

const uint8_t
#if defined (__AVR_ATmega168__) || defined (__AVR_ATmega48__) || defined (__AVR_ATmega88__) || defined (__AVR_ATmega328P__) || defined (__AVR_ATmega1280__) || defined (__AVR_ATmega2560__) || defined (__AVR_ATmega8__)
# if F_CPU < 8000000UL
fastPrescaler = { 8, _BV(CS20) }, // 1
slowPrescaler = { 1, _BV(CS21) }; // 8
# if F_CPU < CUTOFF(8)
fastPrescaler = _BV(CS20), // 1
slowPrescaler = _BV(CS21); // 8
# define SLOWSCALERSHIFT 3
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(64)
fastPrescaler = _BV(CS21), // 8
slowPrescaler = _BV(CS22); // 64
# define SLOWSCALERSHIFT 6
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(256)
fastPrescaler = _BV(CS22), // 64
slowPrescaler = _BV(CS22) | _BV(CS21); // 256
# define SLOWSCALERSHIFT 8
# define FASTSCALERSHIFT 2
# elif F_CPU < CUTOFF(1024)
fastPrescaler = _BV(CS22) | _BV(CS21), // 256
slowPrescaler = _BV(CS22) | _BV(CS21) | _BV(CS20); // 1024
# define SLOWSCALERSHIFT 10
# define FASTSCALERSHIFT 2
# else
fastPrescaler = { 4, _BV(CS21) }, // 8
slowPrescaler = { 1, _BV(CS21) | _BV(CS20) }; // 32
# error frame rate is too low
# endif
#elif defined (__AVR_ATmega128__)
# if F_CPU < 8000000UL
fastPrescaler = { 8, _BV(CS20) }, // 1
slowPrescaler = { 1, _BV(CS21) }; // 8
# if F_CPU < CUTOFF(8)
fastPrescaler = _BV(CS20), // 1
slowPrescaler = _BV(CS21); // 8
# define SLOWSCALERSHIFT 3
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(64)
fastPrescaler = _BV(CS21), // 8
slowPrescaler = _BV(CS21) | _BV(CS20); // 64
# define SLOWSCALERSHIFT 6
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(256)
fastPrescaler = _BV(CS21) | _BV(CS20), // 64
slowPrescaler = _BV(CS22); // 256
# define SLOWSCALERSHIFT 8
# define FASTSCALERSHIFT 2
# elif F_CPU < CUTOFF(1024)
fastPrescaler = _BV(CS22), // 256
slowPrescaler = _BV(CS22) | _BV(CS20); // 1024
# define SLOWSCALERSHIFT 10
# define FASTSCALERSHIFT 2
# else
fastPrescaler = { 8, _BV(CS21) }, // 8
slowPrescaler = { 1, _BV(CS21) | _BV(CS20) }; // 64
# error frame rate is too low
# endif
#elif defined (__AVR_ATmega32U4__)
# if F_CPU < 8000000UL
fastPrescaler = { 8, _BV(WGM12) | _BV(CS10) }, // 1
slowPrescaler = { 1, _BV(WGM12) | _BV(CS11) }; // 8
# if F_CPU < CUTOFF(8)
fastPrescaler = _BV(WGM12) | _BV(CS10), // 1
slowPrescaler = _BV(WGM12) | _BV(CS11); // 8
# define SLOWSCALERSHIFT 3
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(64)
fastPrescaler = _BV(WGM12) | _BV(CS11), // 8
slowPrescaler = _BV(WGM12) | _BV(CS11) | _BV(CS10); // 64
# define SLOWSCALERSHIFT 6
# define FASTSCALERSHIFT 3
# elif F_CPU < CUTOFF(256)
fastPrescaler = _BV(WGM12) | _BV(CS11) | _BV(CS10), // 64
slowPrescaler = _BV(WGM12) | _BV(CS12); // 256
# define SLOWSCALERSHIFT 8
# define FASTSCALERSHIFT 2
# elif F_CPU < CUTOFF(1024)
fastPrescaler = _BV(WGM12) | _BV(CS12), // 256
slowPrescaler = _BV(WGM12) | _BV(CS12) | _BV(CS10); // 1024
# define SLOWSCALERSHIFT 10
# define FASTSCALERSHIFT 2
# else
fastPrescaler = { 8, _BV(WGM12) | _BV(CS11) }, // 8
slowPrescaler = { 1, _BV(WGM12) | _BV(CS11) | _BV(CS10) }; // 64
# error frame rate is too low
# endif
#else
# error no support for this chip
Expand Down Expand Up @@ -272,14 +325,14 @@ void LedSign::Init(uint8_t mode)
// Then start the display
#if defined (__AVR_ATmega168__) || defined (__AVR_ATmega48__) || defined (__AVR_ATmega88__) || defined (__AVR_ATmega328P__) || defined (__AVR_ATmega1280__) || defined (__AVR_ATmega2560__)
TIMSK2 |= _BV(TOIE2);
TCCR2B = fastPrescaler.tccr2;
TCCR2B = fastPrescaler;
#elif defined (__AVR_ATmega8__) || defined (__AVR_ATmega128__)
TIMSK |= _BV(TOIE2);
TCCR2 = fastPrescaler.tccr2;
TCCR2 = fastPrescaler;
#elif defined (__AVR_ATmega32U4__)
// Enable output compare match interrupt
TIMSK1 |= _BV(OCIE1A);
TCCR1B = fastPrescaler.tccr2;
TCCR1B = fastPrescaler;
#endif
// interrupt ASAP
#if !defined (__AVR_ATmega32U4__)
Expand Down Expand Up @@ -386,8 +439,8 @@ void LedSign::SetBrightness(uint8_t brightness)
// set up page counts
// TODO: make SHADES a function parameter. This would require some refactoring.
uint8_t i;
const uint8_t max = 255;
const unsigned long m = max * brightnessPercent * fastPrescaler.relativeSpeed; /*10b*/
const int ticks = TICKS;
const unsigned long m = (ticks << FASTSCALERSHIFT) * brightnessPercent; /*10b*/
#define C(x) ((m * (unsigned long)(x * 1024) + (1<<19)) >> 20) /*10b+10b-20b=0b*/
#if SHADES == 2
const int counts[SHADES] = {
Expand Down Expand Up @@ -424,13 +477,13 @@ void LedSign::SetBrightness(uint8_t brightness)
for (i = 0; i < SHADES - 1; i++) {
int interval = counts[i + 1] - counts[i];
backTimer->counts[i] = 256 - (interval ? interval : 1);
backTimer->prescaler[i] = fastPrescaler.tccr2;
backTimer->prescaler[i] = fastPrescaler;
}

// Compute off time
int interval = max - counts[i] / fastPrescaler.relativeSpeed;
int interval = ticks - (counts[i] >> FASTSCALERSHIFT);
backTimer->counts[i] = 256 - (interval ? interval : 1);
backTimer->prescaler[i] = slowPrescaler.tccr2;
backTimer->prescaler[i] = slowPrescaler;

if (!initialized)
*frontTimer = *backTimer;
Expand Down
2 changes: 2 additions & 0 deletions lib/Charliplexing.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,8 @@
#define DOUBLE_BUFFER 1 // comment out to save memory
#define GRAYSCALE 2 // comment out to save memory

#define FRAMERATE 80UL // Desired number of frames per second

#define DISPLAY_COLS 14 // Number of columns in the display
#define DISPLAY_ROWS 9 // Number of rows in the display
#ifdef GRAYSCALE
Expand Down

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