AT89C52

5

Special Function Registers

A map of the on-chip memory area called the Special Func-

tion Register (SFR) space is shown in Table 1.

Note that not all of the addresses are occupied, and unoc-

cupied addresses may not be implemented on the chip.

Read accesses to these addresses will in general return

random data, and write accesses will have an indetermi-

nate effect.

User software should not write 1s to these unlisted loca-

tions, since they may be used in future products to invoke

new features. In that case, the reset or inactive values of

the new bits will always be 0.

Timer 2 Registers Control and status bits are contained in

registers T2CON (shown in Table 2) and T2MOD (shown in

Table 4) for Timer 2. The register pair (RCAP2H, RCAP2L)

are the Capture/Reload registers for Timer 2 in 16-bit cap-

ture mode or 16-bit auto-reload mode.

Interrupt Registers The individual interrupt enable bits are

in the IE register. Two priorities can be set for each of the

six interrupt sources in the IP register.r

Data Memory

The AT89C52 implements 256 bytes of on-chip RAM. The

upper 128 bytes occupy a parallel address space to the

Special Function Registers. That means the upper 128

bytes have the same addresses as the SFR space but are

physically separate from SFR space.

When an instruction accesses an internal location above

address 7FH, the address mode used in the instruction

specifies whether the CPU accesses the upper 128 bytes

of RAM or the SFR space. Instructions that use direct

addressing access SFR space.

For example, the following direct addressing instruction

accesses the SFR at location 0A0H (which is P2).

MOV 0A0H, #data

Table 2. T2CON – Timer/Counter 2 Control Registe

T2CON Address = 0C8H

Reset Value = 0000 0000B

Bit Addressable

Bit

TF2

EXF2

RCLK

TCLK

EXEN2

TR2

C/T2

CP/RL2

76543210

Symbol

Function

TF2

Timer 2 overflow flag set by a Timer 2 overflow and must be cleared by software. TF2 will not be set when either

RCLK = 1 or TCLK = 1.

EXF2

Timer 2 external flag set when either a capture or reload is caused by a negative transition on T2EX and

EXEN2 = 1. When Timer 2 interrupt is enabled, EXF2 = 1 will cause the CPU to vector to the Timer 2 interrupt

routine. EXF2 must be cleared by software. EXF2 does not cause an interrupt in up/down counter mode

(DCEN = 1).

RCLK

Receive clock enable. When set, causes the serial port to use Timer 2 overflow pulses for its receive clock in serial

port Modes 1 and 3. RCLK = 0 causes Timer 1 overflow to be used for the receive clock.

TCLK

Transmit clock enable. When set, causes the serial port to use Timer 2 overflow pulses for its transmit clock in serial

port Modes 1 and 3. TCLK = 0 causes Timer 1 overflows to be used for the transmit clock.

EXEN2

Timer 2 external enable. When set, allows a capture or reload to occur as a result of a negative transition on T2EX

if Timer 2 is not being used to clock the serial port. EXEN2 = 0 causes Timer 2 to ignore events at T2EX.

TR2

Start/Stop control for Timer 2. TR2 = 1 starts the timer.

C/T2

Timer or counter select for Timer 2. C/T2 = 0 for timer function. C/T2 = 1 for external event counter (falling edge

triggered).

CP/RL2

Capture/Reload select. CP/RL2 = 1 causes captures to occur on negative transitions at T2EX if EXEN2 = 1. CP/RL2

= 0 causes automatic reloads to occur when Timer 2 overflows or negative transitions occur at T2EX when EXEN2

= 1. When either RCLK or TCLK = 1, this bit is ignored and the timer is forced to auto-reload on Timer 2 overflow.