AT89LP51RB2 datasheet(82/254 Pages) ATMEL

82 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

13.4

Mode 2 – 8-bit Auto-Reload Timer/Counter

Mode 2 configures the Timer register as an 8-bit Counter (TL1) with automatic reload, as shown

in Figure 13-3. 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.

The following equation gives the timeout period for Mode 2. In Fast Mode, TPS applies only

when the TnX2 bits in CKCON0 are set. TPS always applies in Compatibility Mode, therefore

setting TnX2 in Compatibility Mode will double the timeout period.

Figure 13-3. Timer/Counter 1 Mode 2: 8-bit Auto-Reload

Note:

RH1/RL1 are not required by Timer 1 during Mode 2 and may be used as temporary storage

registers.

13.5

Mode 3 – 8-bit Split Timer

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 13-4. TL0 uses the Timer 0 control bits: C/T, GATE0, TR0, INT0, and TF0. TH0

is locked into a timer function (counting clock cycles) and takes over the use of TR1 and TF1

from Timer 1. Thus, TH0 now controls the Timer 1 interrupt. While Timer 0 is in Mode 3, Timer 1

will still obey its settings in TMOD but cannot generate an interrupt.

Mode 3 is for applications requiring an extra 8-bit timer or counter. With Timer 0 in Mode 3, the

AT89LP51RB2/RC2/IC2 can appear to have four 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. In this case,

Timer 1 can still be used by the serial port as a baud rate generator or in any application not

requiring an interrupt.

The following equation gives the timeout period for Mode 3. In Fast Mode, TPS applies only

when the TnX2 bits in CKCON0 are set. TPS always applies in Compatibility Mode, therefore

setting TnX2 in Compatibility Mode will double the timeout period.

Mode 2:

Time-out Period

256

THn

–

()

SYS

--------------------------------

TPS

()

CLKSYS

T1 Pin

TR1

GATE1

TF1

TL1

(8 Bits)

TH1

(8 Bits)

Control

Reload

Interrupt

INT0 Pin

C/T = 0

C/T = 1

÷TPS

Mode 3:

Time-out Period

256

SYS

-----------

TPS

()

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