8051 Counter

8051 timers can also be used as counters to count the external events. When the timer/counter is used as a timer, the 8051′s crystal is used as the source of the frequency. When it is used as a counter, however, it is a pulse outside the 8051 that increments the TH, TL registers.

The C/T bit in the TMOD register decides the source of the clock for the timer. If C/T = 0, the timer gets pulses from the crystal. In contrast, when C/T = 1, the timer is used as a counter and gets its pulses from outside the 8051. Therefore, when C/T = 1, the counter counts up as pulses are fed from pins 14 (Timer 0 input) and 15 (Timer 1 input). In the case of Timer 0, when C/T = 1, pin P3.4 provides the clock pulse and the counter counts up for each clock pulse coming from that pin. Similarly, for Timer 1, when C/T = 1 each clock pulse coming in from pin P3.5 makes the counter count up.

	ORG 0H
MAIN :	MOV TMOD, #01100000B	; counter 1, mode 2
	MOV TH1, #0	; clear TH1
	SETB P3.5	; make T1 input
START :	SETB TR1	; start counter
LOOP :	MOV A, TL1	; get copy of count TL1
	MOV P2, A	; display it on port 2
	JNB TF1, LOOP	; keep doing it if TF = 0
	CLR TR1	; stop the counter
	CLR TF1	; clear TF1
	SJMP START
	END

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Related topics:
8051 Loop   |   8051 Delay   |   8051 IO Port Programming   |   8051 Timer Programming   |   8051 Serial Port Programming   |   8051 Programming Timer Interrupt   |   8051 Programming Serial Interrupt   |   8051 Programming External Interrupt

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List of topics: 8051

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