AT89C52

11

Programmable Clock Out

A 50% duty cycle clock can be programmed to come out on

P1.0, as shown in Figure 5. This pin, besides being a regu-

lar I/O pin, has two alter nate fu nctio n s. It can b e

programmed to input the external clock for Timer/Counter 2

or to output a 50% duty cycle clock ranging from 61 Hz to 4

MHz at a 16 MHz operating frequency.

To configure the Timer/Counter 2 as a clock generator, bit

C/T2 (T2CON.1) must be cleared and bit T2OE (T2MOD.1)

must be set. Bit TR2 (T2CON.2) starts and stops the timer.

The clock-out frequency depends on the oscillator fre-

quency and the reload value of Timer 2 capture registers

(RCAP2H, RCAP2L), as shown in the following equation.

In the clock-out mode, Timer 2 roll-overs will not generate

an interrupt. This behavior is similar to when Timer 2 is

used as a baud-rate generator. It is possible to use Timer 2

as a baud-rate generator and a clock generator simulta-

neously. Note, however, that the baud-rate and clock-out

frequencies cannot be determined independently from one

another since they both use RCAP2H and RCAP2L.

UART

The UART in the AT89C52 operates the same way as the

UART in the AT89C51.

Interrupts

The AT89C52 has a total of six interrupt vectors: two exter-

nal interrupts (INT0 and INT1), three timer interrupts

(Timers 0, 1, and 2), and the serial port interrupt. These

interrupts are all shown in Figure 6.

Each of these interrupt sources can be individually enabled

or disabled by setting or clearing a bit in Special Function

Register IE. IE also contains a global disable bit, EA, which

disables all interrupts at once.

Note that Table shows that bit position IE.6 is unimple-

mented. In the A T89 C5 1, bit positi on IE.5 is als o

unimplemented. User software should not write 1s to these

bit positions, since they may be used in future AT89

products.

Timer 2 interrupt is generated by the logical OR of bits TF2

and EXF2 in register T2CON. Neither of these flags is

cleared by hardware when the service routine is vectored

to. In fact, the service routine may have to determine

whether it was TF2 or EXF2 that generated the interrupt,

and that bit will have to be cleared in software.

The Timer 0 and Timer 1 flags, TF0 and TF1, are set at

S5P2 of the cycle in which the timers overflow. The values

are then polled by the circuitry in the next cycle. However,

the Timer 2 flag, TF2, is set at S2P2 and is polled in the

same cycle in which the timer overflows.

Figure 6. Interrupt Sources

Clock-Out Frequency

Oscillator Fequency

4

65536

RCAP2H RCAP2L

(,

)

–

[]

×

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

=

Table 5. Interrupt Enable (IE) Register

(MSB)

(LSB)

EA

–

ET2

ES

ET1

EX1

ET0

EX0

Enable Bit = 1 enables the interrupt.

Enable Bit = 0 disables the interrupt.

Symbol

Position

Function

EA

IE.7

Disables all interrupts. If EA = 0,

no interrupt is acknowledged. If

EA = 1, each interrupt source is

individually enabled or disabled

by setting or clearing its enable

bit.

–

IE.6

Reserved.

ET2

IE.5

Timer 2 interrupt enable bit.

ES

IE.4

Serial Port interrupt enable bit.

ET1

IE.3

Timer 1 interrupt enable bit.

EX1

IE.2

External interrupt 1 enable bit.

ET0

IE.1

Timer 0 interrupt enable bit.

EX0

IE.0

External interrupt 0 enable bit.

User software should never write 1s to unimplemented bits,

because they may be used in future AT89 products.

IE1

IE0

1

1

0

0

TF1

TF0

INT1

INT0

TI

RI

TF2

EXF2