1. MCO556
Timer System Exercise

2. Timer System Exercise
Using the HCS12 Timer System, generate a ~8 Hz overflow rate which will control the system loop timing. The Timer System must also be used to generate a 200 Hz square wave on PORT T pin 2.
Calculate the Timer System parameters required to configure the system.

3. Timer System Diagram Prescaler Divide by 216 TOF in TFLG2 ~ 8 Hz
Bus Clock
Fbus = 8 MHz
Fct
Fov
DFF P
TCNT
Tcmp
C2F in
TFLG1
DFF
Comparator
PTT.2
Fsw = 200 Hz
TC2
TFF

4. Prescale Calculation 1) Overflow rate is Fov = ~8 Hz
2) The TCNT counter divides its input clock (Fct) by 2^16 to generate Fov
3) TCNT clock frequency is Fct = 216 * Fov
Fct = 2^16 * 8 Hz = ~ Hz
4) The 8 MHz Bus clock (Fbus) is divided by the prescaler (P) to generate Fct.
Fbus / P = Fct
P = Fbus / Fct
P = 8MHz / Hz = 15.26
The nearest available prescale value is 16 = 24.

5. Actual TCNT Clock Tct = 1 / 0.5 MHz = 2 us
1) When using the prescale value of 16, the clock rate into the TCNT counter is
Fct = Fbus / P
Fct = 8 Mhz / 16 = 0.5 MHz
2) Clock period is Tct = 1 / Fct
Tct = 1 / 0.5 MHz = 2 us

6. Compare Event Timing
1) Port T.2 generates a 200 Hz square wave. This requires using Timer channel 2 in Output Compare mode.
2) Square Wave period is
Tsw = 1/ Fsw
Tsw = 1 / 200 Hz = 5 ms
3) The compare period (Tcmp) is half the square wave period.
Tcmp = 0.5 * 5 ms = 2.5 ms

7. Compare Event Interval
1) The Output Compare event occurs every 2.5 ms. This is equivalent to M cycles of the TCNT clock
M = Tcmp / Tct
M = 2.5 ms / 2 us = 1250

8. Timer Code #define PERIOD 1250
void TOC2open(void); // function prototype
// initialization part of main
TSCR1 = 0x80; // initialize the main Timer
TSCR2 = 0x04; // prescale set to divide by 16
TOC2open(); // initialize Output Compare

9. Timer Channel Initialization
// output compare initialization for channel 2
void TOC2open(void) {
TIOS |= 0x04; // output compare mode
TCTL1 = 0;
TCTL2 = 0x10; // toggle PTT.2
TIE |= 0x04; // enable interrupts
TFLG1 = 0x04; // clear event
TC2 = TCNT + PERIOD; }