Timers
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The 80C51 has two 16-bit Timer/Counter registers: Timer 0 and Timer 1. Both can be configured to operate either as timers or event counters (see Figure). In the “Timer” function, the register is incremented every machine cycle. Thus, one can think of it as counting machine cycles. Since a machine cycle consists of 12 oscillator periods, the count rate is 1/12 of the oscillator frequency.
In the “Counter” function, the register is incremented in response to a 1-to-0 transition at its corresponding external input pin, T0 or T1. In this function, the external input is sampled during S5P2 of every machine cycle. When the samples show a high in one cycle and a low in the next cycle, the count is incremented. The new count value appears in the register during S3P1 of the cycle following the one in which the transition was detected. Since it takes 2 machine cycles (24 oscillator periods) to recognize a 1-to-0 transition, the maximum count rate is 1/24 of the oscillator frequency. There are no restrictions on the duty cycle of the external input signal, but to ensure that a given level is sampled at least once before it changes, it should be held for at least one full cycle. In addition to the “Timer” or “Counter” selection, Timer 0 and Timer 1 have four operating modes from which to select.
Timer 0 and Timer 1
The “Timer” or “Counter” function is selected by control bits C/T in the Special Function Register TMOD. These two Timer/Counters have four operating modes, which are selected by bit-pairs (M1, M0) in TMOD. Modes 0, 1, and 2 are the same for both Timers/Counters.
Mode 3 is different. The four operating modes are described in the following text.
Mode 0
Putting either Timer into Mode 0 makes it look like an 8048 Timer, which is an 8-bit Counter with a divide-by-32 prescaler. In this mode, the Timer register is configured as a 13-bit register. As the count rolls over from all 1s to all 0s, it sets the Timer interrupt flag TF1. The counted input is enabled to the Timer when TR1 = 1 and either GATE = 0 or INT1 = 1. (Setting GATE = 1 allows the Timer to be controlled by external input INT1, to facilitate pulse width measurements). TR1 is a control bit in the Special Function Register TCON (Figure 8). GATE is in TMOD.
The 13-bit register consists of all 8 bits of TH1 and the lower 5 bits of TL1. The upper 3 bits of TL1 are indeterminate and should be ignored. Setting the run flag (TR1) does not clear the registers. Mode 0 operation is the same for the Timer 0 as for Timer 1. Substitute TR0, TF0, and INT0 for the corresponding Timer 1 signals in Figure 7. There are two different GATE bits, one for Timer 1 (TMOD.7) and one for Timer 0 (TMOD.3).
Mode 1
Mode 1 is the same as Mode 0, except that the Timer register is being run with all 16 bits.
Mode 2
Mode 2 configures the Timer register as an 8-bit Counter (TL1) with automatic reload, as shown in Figure 9. Overflow from TL1 not only sets TF1, but also reloads TL1 with the contents of TH1, which is preset by software. The reload leaves TH1 unchanged. Mode 2 operation is the same for Timer/Counter 0.
Mode 3
Timer 1 in Mode 3 simply holds its count. The effect is the same as setting TR1 = 0.
Timer 0 in Mode 3 establishes TL0 and TH0 as two separate counters. TL0 uses the Timer 0 control bits: C/T, GATE, TR0, INT0, and TF0. TH0 is locked into a timer function (counting machine cycles) and takes over the use of TR1 and TF1 from Timer 1. Thus, TH0 now controls the “Timer 1” interrupt. Mode 3 is provided for applications requiring an extra 8-bit timer on the counter. With Timer 0 in Mode 3, an 80C51 can look like it has three Timer/Counters. When Timer 0 is in Mode 3, Timer 1 can be turned on and off by switching it out of and into its own Mode 3, or can still be used by the serial port as a baud rate generator, or in fact, in any application not requiring an interrupt.
BIT SYMBOL FUNCTION
TCON.7 TF1 Timer 1 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor vectors to interrupt routine, or clearing the bit in software.
TCON.6 TR1 Timer 1 Run control bit. Set/cleared by software to turn Timer/Counter on/off.
TCON.5 TF0 Timer 0 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor vectors to interrupt routine, or by clearing the bit in software.
TCON.4 TR0 Timer 0 Run control bit. Set/cleared by software to turn Timer/Counter on/off.
TCON.3 IE1 Interrupt 1 Edge flag. Set by hardware when external interrupt edge detected.
Cleared when interrupt processed.
TCON.2 IT1 Interrupt 1 type control bit. Set/cleared by software to specify falling edge/low level triggered external interrupts.
TCON.1 IE0 Interrupt 0 Edge flag. Set by hardware when external interrupt edge detected.
Cleared when interrupt processed.
TCON.0 IT0 Interrupt 0 Type control bit. Set/cleared by software to specify falling edge/low level triggered external interrupts.
