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Timers

Timers can be used for timing event counting pulse width measurement pulse generation frequency multiplication

There are 8 Timers (T0 – T7) T0 and T1 – CAPCOM1 [CAPture and COMpare] T7 and T8 – CAPCOM2 T2, T3, T4, T5 and T6 – General Purpose Timers (GPT)

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General Purpose Timers (GPTs)

The five GPT’s are 16-bit timers that are grouped into the two timer blocks GPT1 and GPT2.

Block GPT1 contains 3 timers/counters with a maximum resolution of 16 TCL(TCL = 50ns for fCPU = 20MHz)

Block GPT2 contains 2 timers/counters with a maximum resolution of 8 TCL and a 16-bit Capture/Reload register (CAPREL).

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GPT1 BlockFrom a programmer’s point of view, the GPT1 block is composed of a set of SFR’s as summarized below. Those portions of port and direction registers which are used for alternate functions by the GPT1 block are shaded.

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Timers T2, T3 and T4 Four basic modes

Timer Gated timer Counter Incremental interface mode – for incremental encoders

All timers can count up or down When a timer register overflows from FFFFH to 0000H (when

counting up), or when it underflows from 0000H to FFFFH (when counting down), its interrupt request flag (T2IR, T3IR orT4IR) in register TxIC will be set.

Each timer has an alternate input function pin (TxIN) associated with it which serves as the count input in counter mode, or as the gate control in gated timer mode.

x – refers to the timer number

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Timer 2, 3 and 4

Physical microcontroller pin

TxUD – External Timer Up/Down control

TxIN – External clock input

T3OUT – External output pin from Timer3

fcpu – CPU clock input to prescaler (which divides by 8 ,16, 32, …1024)

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T2, T3 and T4 Timers (contd.)

T3 is the called core timer The count direction (Up/Down) may be programmed

via software or may be dynamically altered by a signal at an external control input pin.

Each overflow/underflow of core timer T3 is latched in the toggle Flip-Flop T3OTL and may be indicated on an alternate output function pin. T2 and T4 do not have this feature.

Some timer modes require the use of more than one timer. For example : The auxiliary timers T2 and T4 may additionally be concatenated with the core timer, or used as capture or reload registers for the core timer.

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Timer Control Register

T3CON – Timer 3 Control Register (bit addressable)

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T3 in Timer mode

n = 3..10

Note the x in TxUD, TxUDE, TxI etc. refers to the timer number

The 3 TxI bits in TxCON determine the precaler factor as shown in the table below.

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T3 in Timer mode

UP(0)/DOWN(1)

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Timer 3 generating a fixed delay

// Initialise timer 3 for Timer mode, prescaler 101 (÷256), timer stopped// count down, disable external up/down, T3OTL output disabled

T3CON = 0x0085; //00000 000 10 000 101

// timer period = prescaler / fcpu = 256/20MHz = 12.8uS// delay required = 400000uS. Timer value = 400000/12.8 = 31250// Clear T3 interrupt request bit, load timer value and run timer

T3IR = 0;T3 = 31249; // 1 less than number of counts requiredT3R = 1;

/* T3 is now decremented by hardware every 12.8uS, after 31250 decrements the interrupt request flag is set. The program should poll/test this flag. */ while (T3IR == 0); // wait until interrupt request is set

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#include <stdio.h> /* standard I/O .h-file */#include <reg167.h> /* special function registers for 167’s */

/*************************************************//* Use Timer 3 to generate a delay of 0.4 seconds*//*************************************************/

void main (void) { /* initialize the serial interface */#ifndef MCB167 /* do not initialize with Monitor-166 */ P3 |= 0x0400; /* SET PORT 3.10 OUTPUT LATCH (TXD) */ DP3 |= 0x0400; /* SET PORT 3.10 DIRECTION CONTROL(TXD O/P)*/ DP3 &= 0xF7FF; /* RESET PORT 3.11 DIRECTION CONTROL(RXD i/p)*/ S0TIC = 0x80; /* SET TRANSMIT INTERRUPT FLAG */ S0RIC = 0x00; /* DELETE RECEIVE INTERRUPT FLAG */ S0BG = 0x40; /* SET BAUDRATE TO 9600 BAUD */ S0CON = 0x8011; /* SET SERIAL MODE */#endif . . . see next slide, which goes here. . .}

Example - Timer delay

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//Initialise timer 3 for timer mode, prescaler=256, timer stopped//count down, disable external up/downT3CON = 0x0085; //0000000010 000 101

while (1) //do this endlessly! {

printf("\nPress a key to start timer\n");_getkey(); // special Keil function to wait for a key

// Clear T3 interrupt request bit, load timer value and run timerT3IR = 0;T3 = 31249;T3R = 1;printf("Timing started\n");

// Now wait for T3 interrupt request bit to be set to 1// This should take 31250 x 12.8uS = 400000uS = 400ms = 0.4S// based on a 20MHz cpu clock frequencywhile( T3IR == 0);

T3R = 0; //Stop timer//When loop terminates we know the required delay has elapsedprintf("400 milliseconds has elapsed\n");

}

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Gated Timer Mode

Timer mode bits TxM

TxIN – provides the gating of the clock

This mode is used for measuring the period of pulses

gate open

n = 3..10

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Counter mode

The 3 bits TxI determine the transition for counter increment/decrement

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Timer T2 and T4

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Concatenation of Timers

Concatenation of Core Timer T3 and an Auxiliary Timer