AT89LP428 datasheet(42/149 Pages) ATMEL

42 / 149 page

3654A–MICRO–8/09

AT89LP428/828

11.1

Mode 0 – Variable Width Timer/Counter

Both Timers in Mode 0 are 8-bit Counters with a variable prescaler. The prescaler may vary from

1 to 8 bits depending on the PSC bits in TCONB, giving the timer a range of 9 to 16 bits.

By default the timer is configured as a 13-bit timer compatible to Mode 0 in the standard 8051.

Figure 11-1 shows the Mode 0 operation as it applies to Timer 1 in 13-bit mode. As the count

rolls over from all “1”s to all “0”s, it sets the Timer interrupt flag TF1. The counter 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

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.

Note:

RH1/RL1 are not required by Timer 1 during Mode 0 and may be used as temporary storage

registers.

Figure 11-1. Timer/Counter 1 Mode 0: Variable Width Counter

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 11-1. There are two different GATE bits, one for

Timer 1 (TMOD.7) and one for Timer 0 (TMOD.3).

11.2

Mode 1 – 16-bit Auto-Reload Timer/Counter

In Mode 1 the Timers are configured for 16-bit auto-reload. The Timer register is run with all

16 bits. The 16-bit reload value is stored in the high and low reload registers (RH1/RL1). The

clock is applied to the combined high and low timer registers (TH1/TL1). As clock pulses are

received, the timer counts up: 0000H, 0001H, 0002H, etc. An overflow occurs on the FFFFH-to-

0000H transition, upon which the timer register is reloaded with the value from RH1/RL1 and the

overflow flag bit in TCON is set. See Figure 11-2. The reload registers default to 0000H, which

gives the full 16-bit timer period compatible with the standard 8051. Mode 1 operation is the

same for Timer/Counter 0.

Mode 0:

Time-out Period

256

PSC0

Oscillator Frequency

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

TPS

()

OSC

T1 Pin

TR1

GATE

INT1 Pin

TL1

(8 Bits)

Control

Interrupt

C/T = 0

C/T = 1

PSC1

TH1

(8 Bits)

TF1

÷TPS

Mode 1:

Time-out Period

65536

RH0 RL0

}

–

()

Oscillator Frequency

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

TPS

()

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