AT48801 datasheet(6/17 Pages) ATMEL

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RCLK + TCLK

CP/RL2

TR2

MODE

16 Bit Auto-Reload

16 Bit Capture

Baud Rate Generator

(Off)

Table 3. Timer 2 Operating Modes

Timer 0 and 1

Timer 0 and Timer 1 in the AT48801 operate the same

way as Timer 0 and Timer 1 in the AT89C51.

Timer 2

Timer 2 is a 16 bit Timer/Counter that can operate as

either a timer or an event counter. The type of operation is

selected by bit C/T2 in the SFR T2CON (shown in Table

2). Timer 2 has three operating modes: capture, auto-re-

load (up or down counting), and baud rate generator. The

modes are selected by bits in T2CON, as shown in Table

Timer 2 consists of two 8 bit registers, TH2 and TL2. In the

Timer function, the TL2 register is incremented every ma-

chine cycle. Since a machine cycle consists of 12 oscilla-

tor periods, the count rate is 1/12 of the oscillator fre-

quency.

In the Counter function, the register is incremented in re-

sponse to a l-to-0 transition at its corresponding external

input pin, T2. In this function, 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

which the transition was detected. Since two machine cy-

cles (24 oscillator periods) are required to recognize a 1-

to-0 transition, the maximum count rate is 1/24 of the os-

Figure 1. Timer 2 in Capture Mode

Capture Mode

In the capture mode, two options are selected by bit

EXEN2 in T2CON. If EXEN2 = 0, Timer 2 is a 16 bit timer

or counter which upon overflow sets bit TF2 in T2CON.

This bit can then be used to generate an interrupt. If

EXEN2 = 1, Timer 2 performs the same operation, but a

l-to-0 transition at external input T2EX also causes the

current value in TH2 and TL2 to be captured into RCAP2H

and RCAP2L, respectively. In addition, the transition at

T2EX causes bit EXF2 in T2CON to be set. The EXF2 bit,

like TF2, can generate an interrupt. The capture mode is

illustrated in Figure 1.

Auto-Reload (Up or Down Counter)

Timer 2 can be programmed to count up or down when

configured in its 16 bit auto-reload mode. This feature is

invoked by the DCEN (Down Counter Enable) bit located

in the SFR T2MOD (see Table 4). Upon reset, the DCEN

bit is set to 0 so that timer 2 will default to count up. When

DCEN is set, Timer 2 can count up or down, depending on

the value of the T2EX pin.

(continued)

MOV @R0, #data

Note that stack operations are examples of indirect ad-

dressing, so the upper 128-bytes of data RAM are avail-

able as stack space.

Data Memory (Continued)

cillator frequency. To ensure that a given level is sampled

at least once before it changes, the level should be held

for at least one full machine cycle.

1-6

AT48801

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