EE 308 Spring 2011 • The 9S12 Output Compare Function • Huang Section 8.6 • ECT_16B8C Block User Guide o Interrupts on the MC9S12 o The MC9S12 Output Compare Function o Registers used to enable the Output Compare Function o Using the MC9S12 Output Compare Function o A program to use the MC9S12 Output Compare to generate a square wave o Setting and clearing bits in the Timer Subsystem Ways to implement delays • Software Delay void delay (unsigned int ms) { unsigned int i; while (ms > 0) { i = D_1MS; while (i > 0) { i = i - 1; } ms = ms - 1; } } Cannot do anything while waiting • Timer Overflow or Real Time Interrupt Only fixed delaysEE 308 Spring 2011 The MC9S12 Output Compare Function Want event to happen at a certain time Want to produce pulse with width T Wait until TCNT == 0x0000, then bring PA0 high Wait until TCNT == T, then bring PA0 low while (TCNT != 0x0000) ; PORTA = PORTA | 0x01; while (TCNT != T) ; PORTA = PORTA & ~0x01; Problems: 1) May miss TCNT == 0x0000 or TCNT == T 2) Time not exact −− software delays 3) Cannot do anything else while waitingEE 308 Spring 2011 Want event to happen at a certain time Want to produce pulse with width T Wait until TCNT == 0x0000, then bring PA0 high Wait until TCNT == T, then bring PA0 low Now pulse is exactly T cycles longEE 308 Spring 2011 OUTPUT COMPARE PORT T 0−7 To use Output Compare, you must set IOSx to 1 in TIOSEE 308 Spring 2011 The HCS12 Output Compare Function • The MC9S12 allows you to force an event to happen on any of the eight PORTT pins • An external event is a rising edge, a falling edge, or a toggle • To use the Output Compare Function: – Enable the timer subsystem (set TEN bit of TSCR1) – Set the prescaler – Tell the HCS12 that you want to use Bit x of PORTT for output compare – Tell the HCS12 what you want to do on Bit x of PORTT (generate rising edge, falling edge, or toggle) – Tell the HCS12 what time you want the event to occur – Tell the HCS12 if you want an interrupt to be generated when the event is forced to occur • There are some more complicated features of the output compare subsystem which are activated using registers CFORC, OC7M, OC7D and TTOV. – Writing a 1 to the corresponding bit of CFORC forces an output compare event to occur, the same as if a successful comparison has taken place (Section 8.6.5 of Huang). – Using OC7M and OC7D allow Timer Channel 7 to control multiple output compare functions (Section 8.6.4 of Huang). – Using TTOV allows you to toggle an output compare pin when TCNT overflows. This allows you to use the output compare system to generate pulse width modulated signals. – We will not discuss these advanced features in this class.EE 308 Spring 2011 Write a 1 to Bit 7 of TSCR1 to turn on timer To turn on the timer subsystem: TSCR1 = 0x80; Set the prescaler in TSCR2 Make sure the overflow time is greater than the width of the pulse you want to generate To have overflow rate of 21.84 ms: TSCR2 = 0x03;EE 308 Spring 2011 Write a 1 to the bits of TIOS to make those pins output compare To make Pin 4 an output compare pin: TIOS = TIOS | 0X10; Write to TCTL1 and TCTL2 to choose action to take To have Pin 4 toggle on compare: TCTL1 = (TCTL1 | BIT0) & ~BIT1; Write time you want event to occur to TCn register. To have event occur on Pin 4 when TCNT == 0x0000: TC4 = 0x0000; To have next event occur T cycles after last event, add T to TCn. To have next event occur on Pin 4 500 cycles later: TC4 = TC4 + 500;EE 308 Spring 2011 When TCNT == TCn, the specified action will occur, and flag CFn will be set. To clear the flag, write a 1 to the bit you want to clear (0 to all others) To wait until TCNT == TC4: while ((TFLG1 & BIT4) == 0) ; To clear flag bit for Pin 4: TFLG1 = BIT4; To enable interrupt when compare occurs, set corresponding bit in TIE register To enable interrupt when TCNT == TC4: TIE = TIE | BIT4;EE 308 Spring 2011 Using Output Compare on the MC9S12 1. In the main program: (a) Turn on timer subsystem (TSCR1 reg) (b) Set prescaler (TSCR2 reg) (c) Set up PTx as OC (TIOS reg) (d) Set action on compare (TCTL 1-2 regs, OMx OLx bits) (e) Clear Flag (TFLG1 reg) (f) Enable int (TIE reg) 2. In interrupt service routine (a) Set time for next action to occur (write TCx reg) • For periodic events add time to TCx register (b) Clear flag (TFLG1 reg)EE 308 Spring 2011 #include <hidef.h> /* common defines and macros */ #include "derivative.h" /* derivative-specific definitions */ #include "vectors12.h" #define PERIOD 3000 #define HALF_PERIOD (PERIOD/2) #define disable() __asm(sei) #define enable() __asm(cli) interrupt void toc2_isr(void); void main(void) { disable(); TSCR1 = 0x80; /* Turn on timer subsystem */ TSCR2 = 0x04; /* Set prescaler to 16 (0.666 us) */ TIOS = TIOS | 0x04; /* Configure PT2 as Output Compare */ TCTL2 = (TCTL2 | 0x10) & ~0x20; /* Set up PT2 to toggle on compare */ TFLG1 = 0x04; /* Clear Channel 2 flag */ /* Set interrupt vector for Timer Channel 2 */ UserTimerCh2 = (unsigned short) &toc2_isr; TIE = TIE | 0x04; /* Enable interrupt on Channel 2 */ enable(); while (1) { __asm(wai); } } interrupt void toc2_isr(void) { TC2 = TC2 + HALF_PERIOD; TFLG1 = 0x04; }EE 308 Spring 2011 Capturing the Time of an External Event • One way to determine the time of an external event is to wait for the event to occur, the read the TCNT register: • For example, to determine the time a signal on Bit 0 of PORTB changes from a high to a low: while ((PORTB & BIT0) != 0) ; /* Wait while Bit 0 high */ time = TCNT; /* Read time after goes low */ • Two problems with this: 1. Cannot do anything else while waiting 2. Do not get exact time because of delays in software • To solve problems use hardware which latches TCNT when event occurs, and generates an interrupt. • Such hardware is built into the MC9S12 — called the Input Capture SystemEE 308 Spring 2011 Measure the time between two events How to measure ∆t? Wait until signal goes low, then measure TCNT while ((PORTB & BIT0) == BIT0) ; start = TCNT; while ((PORTB & BIT0) == BIT1) ; end = TCNT; dt = end - start; Problems: 1) May not get very accurate time 2) Can’t do anything while waiting for signal level to change.EE 308 Spring 2011 Measure the time between two
View Full Document
Unlocking...