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| -rw-r--r-- | doc/tach_speedo | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/doc/tach_speedo b/doc/tach_speedo new file mode 100644 index 0000000..959222d --- /dev/null +++ b/doc/tach_speedo @@ -0,0 +1,58 @@ +===== Tachometer ===== + +VDO: 0.5--999 pulses/rev + +Target 0.5--4 pulses/rev + +Max 20,000 rpm = 333 rev/s -> 167Hz--1.332 kHz max + +Target resolution: 100 rpm. Target max: 20,000 rpm. 20,000/100 = 200 = resolution. Need 8 bits. + +PIC16F1459 PWM min frequency with PR2=0: +f = FOSC / (4 * TMR2_prescale) +f_min = 48e6 / (4 * 64) = 187.5 kHz +With 10-bit clock divider (counter): 187.5 KHz / 1024 = 183 Hz +Just above target max frequency of 167 Hz. + +TMR0 +Use TMR0 for control. +3-bit prescaler 2--256 +Min frequency without using TMR0: f = FOSC / (4 * pre) +f_min = 48e6 / (4 * 256) = 46.9 kHz + +Alternative to hardware clock divider: software counter. +Increment on timer interrupt. +When reaches threshold, toggle pin. +Uses more CPU cycles but less board space and money than hardware clock divider. + +TMR1 +Can use with LFINTOSC, FOSC, FOSC/4, or external 32.768 kHz oscillator. +LFINTOSC = 31 kHz. +Pre = {1, 2, 4, 8}. +Without using TMR1H/L: f_min = LFINTOSC / 8 = 31,000 / 8 = 3.875 kHz +Use for both tach and speedo, each with their own software counter? +Then can use TMR1H/L to divide frequency further and use software counters for independent control of tach and speedo. +Use TMR1H/L to divide frequency, and Pre to set pulses/rev? Can't use for both tach and speed then. + +TMR2 +Use PR2 for control. +f = FOSC / (4 * pre * pr2 * post) +pre = {1, 4, 16, 64} +post = [1,16] +Without PR2: f_min = 48e6 / (4 * 64 * 16) ~= 11.7 kHz + + +===== Speedometer ===== + +Stack: 310--248,500 pulses/km + +Target 320 km/h = 320/3600 km/s +f = 27Hz--22kHz min/max + +8-bit resolution is OK. + +Use TMR2 for speedo. +Outputs min 11.7 kHz without using PR2. +Don't need software counter. + +Then can use TMR1 for tacho. |