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 ad-
dressing access SFR space.
For example, the following direct addressing instruction
accesses the SFR at location 0A0H (which is P2).
MOV 0A0H, #data
Instructions that use indirect addressing access the upper
128 bytes of RAM. For example, the following indirect ad-
dressing instruction, where R0 contains 0A0H, accesses
the data byte at address 0A0H, rather than P2 (whose ad-
dress is 0A0H).
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.
Special Function Registers
A map of the on-chip memory area called the Special
Function 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 capture 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.
Table 2. T2CON—Timer/Counter 2 Control Register
T2CON Address = 0C8H
Reset Value = 0000 0000B
Bit Addressable
TF2
EXF2
RCLK
TCLK
EXEN2
TR2
C/T2
CP/RL2
Bit
7
6
5
4
3
2
1
0
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.
AT89C52
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