@aaronrenny
Configuring TC1 in CTC Mode-4 to genertae ~2 Hz to 5 kHz square wave signal at OC1A-pin (DPin-9) of Fig-1, 2.
Figure-1:
Figure-2:
1. Clear TCCR1A Register as they are initialized to some values by init() function.
Figure-3:
TCCR1A = 0x00;
2. Clear TCCR1B Register for above reason and also to keep TC1 at OFF state.
Figure-4:
TCCR1B = 0x00;
3. Place appropriate values to WGM13 - WGM10 bits of TCCR1A and TCCR1A Registers to set Mode-4 (0100) operation (the CTC Waveform Generation Mode).
bitClear(TCCR1B, WGM13);
bitSet(TCCR1B, WGM12); //equivalent code is: 1<<WGM12
bitClear(TCCR1A, WGM11);
bitClear(TCCR1A, WGM10);
4. Place appropriate values to COM1A1 and COM1A0 bits of TCCR1A Register so that the signal at DPin-9 (OC1A, Fig-1) toggles at each compare match (point-M, N in Fig-2) to deliver square wave signal. Note that the direction of DPin-9 must be set as OUTPUT.
bitClear(TCCR1A, COM1A1);
bitSet(TCCR1A, COM1A0);
pinMode(9, OUTPUT);
5. Accepting this equation for the frequency of the signal at DPin-9: f = 16x10^6/(2N(1+OCR1A)), let us choose 64 for N (TC1 clock prescaler) and calculate the value that is to be loaded into OCR1A Register for an initial f = 10 Hz. It is 12499.
OCR1A = 12499;
Given: //Edit
f = 16x10^6/(2N(1+OCR1A))
==> OCR1A + 1 = 16x10^6/(2Nf)
==> OCR1A + 1 = 16000000/(2x64x5000) //
==> OCR1A + 1 = 12500 //N = 64, f = 5 kHz
==> OCR1A = 12500 - 1
==> OCR1A = 12499
6. Load initial value of 0x0000 into CTCNT1 Register (software namw for TC1).
TCNT1 = 0x0000;
7. Strat TC1 with clocking frequency of 250 kHz (16^6/N = 16^6/64) by placing appropriate values for the CS12 - CS10 bits of TCCR1B Register.
bitClear(TCCR1B, CS12);
bitSet(TCCR1B, CS11);
bitSet(TCCR1B, CS10);
8. Let us put all the above information in the Arduino sketch as follows:
void setup()
{
Serial.begin(9600);
//---- registers are cleared for safety---
TCCR1A = 0x00;
TCCR1B = 0x00;
//----- CTC Mode-4 setting-----
bitClear(TCCR1B, WGM13);
bitSet(TCCR1B, WGM12);
bitClear(TCCR1A, WGM11);
bitClear(TCCR1A, WGM10);
//------ signal at DPin-9 will toggle---
bitClear(TCCR1A, COM1A1);
bitSet(TCCR1A, COM1A0);
pinMode(9, OUTPUT);
//---inital value to set frequency at 10 Hz------
OCR1A = 12499; //12499 for 10 Hz; 62499 for 2 Hz
TCNT1 = 0x0000;
//---- set Tc1 clock prescaler at /64 and start TC1------
bitClear(TCCR1B, CS12);
bitSet(TCCR1B, CS11);
bitSet(TCCR1B, CS10);
}
void loop()
{
}
9. Connect R1-LED1 with DPin-9 of UNO as per Fig-1.
10. Upload the sketch of Step-7.
11. Check that LED1 is running at 10 Hz speed.
12. Connect the wiper-point of a 5k Pot with UNO as per Fig-1.
13. Include the following codes in the loop() function of the sketch of Step-7. The map() function transforms input value (y) from this range: 0 to 1023 into this range: 65535 to 24.
unsigned int y = analogRead(A0);
OCR1A = map(y, 0, 1023, 65535, 24); //value of OCR1A chnages frequency
delay(1000); //test interval
14. Upload the new sketch in the UNO.
15. Slowly vary the Pot and observe that the blink rate of LED1 changes indicating that the frequency is chnaging.
16. To check the symmetry of the ON and OFF period of the signal at DPin-9, connect an scope at DPin-9. You will observe a good square wave signal.
17. Exercises
(1) Pick up all the codes that manipulate the bits of TCCR1A Register and then combine them using | (or) operator and then assign the whole expression to TCCR1A Register.
(2) Repeat Step-17(1) for TCCR1B register.
(3) In Fig-2, the OCF1A (Output Compare Match Flag A of TC1) of TIFR1 Register assumes HIGH when the value of TCNT1 becomes equal to the value of OCR1A Register at M-point.
The above flag can be used to interrupt the MCU by prior activation (enabled) of OCIE1A-bit of TIMSK1 Register and I-Bit of SREG Register.
bistSet(TIMSK1, OCIE1A);
bitSet(SREG, 7); //equivalent code is: sei(); or interrupts();
As a resut, the MCU will enetr into the following ISR Routine where a flag can be set bassed on which the MCU will carry out necessary acctions in the loop() function.
ISR(TIMER1_COMPA_vect)
{
flag = true;
}
Add codes with the sketch of Step-8 so that the oscillator runs at 2 Hz, interrupt logic comes into picture and the message "Match Occurred" appears on Serial Monitor.
