P89C51RB2BASLASH01 datasheet(13/68 Pages) PHILIPS

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Philips Semiconductors

Preliminary data

P89C51RA2/RB2/RC2/RD2xx

80C51 8-bit Flash microcontroller family

8KB/16KB/32KB/64KB ISP/IAP Flash with 512B/512B/512B/1KB RAM

2002 Jul 18

TIMER 0 AND TIMER 1 OPERATION

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. Figure 2

shows the Mode 0 operation.

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 TFn. The counted input is enabled to the Timer when TRn = 1

and either GATE = 0 or INTn = 1. (Setting GATE = 1 allows the

Timer to be controlled by external input INTn, to facilitate pulse width

measurements). TRn is a control bit in the Special Function Register

TCON (Figure 3).

The 13-bit register consists of all 8 bits of THn and the lower 5 bits

of TLn. The upper 3 bits of TLn are indeterminate and should be

ignored. Setting the run flag (TRn) does not clear the registers.

Mode 0 operation is the same for Timer 0 as for Timer 1. 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 (TLn) with

automatic reload, as shown in Figure 4. Overflow from TLn not only

sets TFn, but also reloads TLn with the contents of THn, which is

preset by software. The reload leaves THn unchanged.

Mode 2 operation is the same for Timer 0 as for Timer 1.

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. The logic for Mode 3 on Timer 0 is shown in Figure 5. TL0

uses the Timer 0 control bits: C/T, GATE, TR0, and TF0 as well as

pin INT0. 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.

GATE

C/T

GATE

C/T

BIT

SYMBOL

FUNCTION

TMOD.3/

GATE

Gating control when set. Timer/Counter “n” is enabled only while “INTn” pin is high and

TMOD.7

“TRn” control pin is set. when cleared Timer “n” is enabled whenever “TRn” control bit is set.

TMOD.2/

C/T

Timer or Counter Selector cleared for Timer operation (input from internal system clock.)

TMOD.6

Set for Counter operation (input from “Tn” input pin).

OPERATING

8048 Timer: “TLn” serves as 5-bit prescaler.

16-bit Timer/Counter: “THn” and “TLn” are cascaded; there is no prescaler.

8-bit auto-reload Timer/Counter: “THn” holds a value which is to be reloaded

into “TLn” each time it overflows.

(Timer 0) TL0 is an 8-bit Timer/Counter controlled by the standard Timer 0 control bits.

TH0 is an 8-bit timer only controlled by Timer 1 control bits.

(Timer 1) Timer/Counter 1 stopped.

SU01580

TIMER 1

TIMER 0

Not Bit Addressable

TMOD

Address = 89H

Reset Value = 00H

543

Figure 1. Timer/Counter 0/1 Mode Control (TMOD) Register

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