IS89C51 Datasheet, PDF (14/48 Pages) Integrated Silicon Solution, Inc – CMOS SINGLE CHIP 8-BIT MICROCONTROLLER with 4-Kbytes of FLASH

IS89C51
ISSI ^®
TIMER/COUNTERS
The IS89C51 has two 16-bit Timer/Counter registers:
Timer 0 and Timer 1. All two can be configured to operate
either as Timers or event Counters.
As a Timer, the register is incremented every machine cycle.
Thus, the register counts machine cycles. Since a machine
cycle consists of 12 oscillator periods, the count rate is 1/12
of the oscillator frequency.
As a Counter, the register is incremented in response to a
1-to-0 transition at its corresponding external input pin, T0
and T1. 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 two machine cycles (24 oscillator periods) are required
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 it should be
held for at least one full machine cycle to ensure that a given
level is sampled at least once before it changes.
In addition to the Timer or Counter functions, Timer 0 and
Timer 1 have four operating modes: 13-bit timer, 16-bit timer,
8-bit auto-reload, split timer.
Timer 0 and Timer 1
The Timer or Counter function is selected by control bits
C/T in the Special Function Regiser 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 Timer/Counters, but Mode 3 is different.
The four modes are described in the following sections.
Mode 0:
Both Timers in Mode 0 are 8-bit Counters with a divide-by-32
prescaler. Figure 8 shows the Mode 0 operation as it applies
to Timer 1.
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. Gate is in
TMOD.
The 13-bit register consists of all eight bits of TH1 and the
lower five bits of TL1. The upper three 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 Timer 0 as for Timer 1,
except that TR0, TF0 and INT0 replace the corresponding
Timer 1 signals in Figure 8. There are two different GATE
bits, one for Timer 1 (TMOD.7) and one for Timer 0 (TMOD.3).
OSC
(XTAL2)
OSC
ONE MACHINE
CYCLE
ONE MACHINE
CYCLE
S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
S6
P1 P2
S1
P1 P2
DIVIDE 12
C/T = 0
T1 PIN
GATE
C/T = 1
TR1
TL1
TH1
(5 BITS) (8 BITS)
CONTROL
TF1
INTERRUPT
INT1 PIN
Figure 8. Timer/Counter 1 Mode 0: 13-Bit Counter
14
1-800-379-4774 — Integrated Silicon Solution, Inc.
MC016-1C
11/21/98