W79E548 Datasheet, PDF (40/79 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E548/W79L548
11. PROGRAMMABLE TIMERS/COUNTERS
The W79E(L)548 has three 16-bit programmable timer/counters and one programmable Watchdog
timer. The Watchdog timer is operationally quite different from the other two timers.
11.1 Timer/Counters 0 & 1
The W79E(L)548 has two 16-bit Timer/Counters. Each of these Timer/Counters has two 8 bit registers
which form the 16 bit counting register. For Timer/Counter 0 they are TH0, the upper 8 bits register,
and TL0, the lower 8 bit register. Similarly Timer/Counter 1 has two 8 bit registers, TH1 and TL1. The
two can be configured to operate either as timers, counting machine cycles or as counters counting
external inputs.
When configured as a "Timer", the timer counts clock cycles. The timer clock can be programmed to
be thought of as 1/12 of the system clock or 1/4 of the system clock. In the "Counter" mode, the
register is incremented on the falling edge of the external input pin, T0 in case of Timer 0, and T1 for
Timer 1. The T0 and T1 inputs are sampled in every machine cycle at C4. If the sampled value is high
in one machine cycle and low in the next, then a valid high to low transition on the pin is recognized
and the count register is incremented. Since it takes two machine cycles to recognize a negative
transition on the pin, the maximum rate at which counting will take place is 1/24 of the master clock
frequency. In either the "Timer" or "Counter" mode, the count register will be updated at C3.
Therefore, in the "Timer" mode, the recognized negative transition on pin T0 and T1 can cause the
count register value to be updated only in the machine cycle following the one in which the negative
edge was detected.
The "Timer" or "Counter" function is selected by the " C / T " bit in the TMOD Special Function
Register. Each Timer/Counter has one selection bit for its own; bit 2 of TMOD selects the function for
Timer/Counter 0 and bit 6 of TMOD selects the function for Timer/Counter 1. In addition each
Timer/Counter can be set to operate in any one of four possible modes. The mode selection is done
by bits M0 and M1 in the TMOD SFR.
Time-Base Selection
The W79E(L)548 gives the user two modes of operation for the timer. The timers can be programmed
to operate like the standard 8051 family, counting at the rate of 1/12 of the clock speed. This will
ensure that timing loops on the W79E(L)548 and the standard 8051 can be matched. This is the
default mode of operation of the W79E(L)548 timers. The user also has the option to count in the
turbo mode, where the timers will increment at the rate of 1/4 clock speed. This will straight-away
increase the counting speed three times. This selection is done by the T0M and T1M bits in CKCON
SFR. A reset sets these bits to 0, and the timers then operate in the standard 8051 mode. The user
should set these bits to 1 if the timers are to operate in turbo mode.
Mode 0
In Mode 0, the timer/counters act as a 8 bit counter with a 5 bit, divide by 32 pre-scale. In this mode
we have a 13 bit timer/counter. The 13 bit counter consists of 8 bits of THx and 5 lower bits of TLx.
The upper 3 bits of TLx are ignored.
The negative edge of the clock increments the count in the TLx register. When the fifth bit in TLx
moves from 1 to 0, then the count in the THx register is incremented. When the count in THx moves
from FFh to 00h, then the overflow flag TFx in TCON SFR is set. The counted input is enabled only if
TRx is set and either GATE = 0 or INTx = 1. When C / T is set to 0, then it will count clock cycles,
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