The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of
Flash Programmable and Erasable Read Only Memory (PEROM) and 128 bytes RAM. The device is
manufactured using Atmel’s high density nonvolatile memory technology and is compatible with the
industry standard MCS-51™ instruction set and pinout. The chip combines a versatile 8-bit CPU with
Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly
flexible and cost effective solution to many embedded control applications.
• Compatible with MCS-51™ Products
• 4K Bytes of In-System Reprogrammable Flash Memory
• Endurance: 1,000 Write/Erase Cycles
• Fully Static Operation: 0 Hz to 24 MHz
• Three-Level Program Memory Lock
• 128 x 8-Bit Internal RAM
• 32 Programmable I/O Lines
• Two 16-Bit Timer/Counters
• Six Interrupt Sources
• Programmable Serial Channel
• Low Power Idle and Power Down Modes
The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM, 32
I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial
port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for
operation down to zero frequency and supports two software selectable power saving modes. The Idle
Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to
continue functioning. The Power Down Mode saves the RAM contents but freezes the oscillator
disabling all other chip functions until the next hardware reset.
VCC Supply voltage.
Port 0 is an 8-bit open drain bidirectional I/O port. As an output port each pin can sink eight TTL
inputs. When is are written to port 0 pins, the pins can be used as high impedance inputs.
Port 0 may also be configured to be the multiplexed loworder address/data bus during accesses to
external program and data memory. In this mode P0 has internal pullups.
Port 0 also receives the code bytes during Flash programming, and outputs the code bytes during
program verification. External pullups are required during program verification.
Port 1 is an 8-bit bidirectional I/O port with internal pullups. The Port 1 output buffers can
sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the internal
pullups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will source
current (IIL) because of the internal pullups.
Port 1 also receives the low-order address bytes during Flash programming and verification.
Port 2 is an 8-bit bidirectional I/O port with internal pullups. The Port 2 output buffers can
sink/source four TTL inputs. When 1s are written to Port 2 pins they are pulled high by the internal
pullups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source
current (IIL) because of the internal pullups.
Port 2 emits the high-order address byte during fetches from external program memory and during
accesses to external data memory that use 16-bit addresses (MOVX @ DPTR). In this application it
uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit
addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special Function Register.
Port 2 also receives the high-order address bits and some control signals during Flash programming
Port 3 is an 8-bit bidirectional I/O port with internal pullups. The Port 3 output buffers can
sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the internal
pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source
current (IIL) because of the pullups.
Port 3 also serves the functions of various special features of the AT89C51 as listed below:
Port 3 also receives
some control signals for
Flash programming and
Reset input. A high on
this pin for two machine
cycles while the oscillator is running resets the device.
Port pin alternate functions
P3.0 rxd (serial input port)
P3.1 txd (serial output port)
P3.2 ^int0 (external interrupt0)
P3.3 ^int1 (external interrupt1)
P3.4 t0 (timer0 external input)
P3.5 t1 (timer1 external input)
P3.6 ^WR (external data memory write strobe)
P3.7 ^rd (external data memory read strobe)
Address Latch Enable output pulse for latching the low byte of the address during accesses to
external memory. This pin is also the program pulse input (PROG) during Flash programming.
In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be
used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during
each access to external Data Memory.
If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set,
ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high.
Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.
Program Store Enable is the read strobe to external program memory.
When the AT89C51 is executing code from external program memory, PSEN is activated twice
each machine cycle, except that two PSEN activations are skipped during each access to external data
External Access Enable. EA must be strapped to GND in order to enable the device to fetch code
from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit
1 is programmed, EA will be internally latched on reset.
EA should be strapped to VCC for internal program executions.
This pin also receives the 12-volt programming enable voltage(VPP) during Flash programming,
for parts that require 12-volt VPP.
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
Output from the inverting oscillator amplifier.
XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier which can be
configured for use as an on-chip oscillator, as shown in Figure 1. Either a quartz crystal or ceramic
resonator may be used. To drive the device from an external clock source, XTAL2 should be left
unconnected while XTAL1 is driven as shown in Figure 2. There are no requirements on the duty cycle
of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two
flip-flop, but minimum and maximum voltage high and low time specifications must be observed.
In idle mode, the CPU puts itself to sleep while all the onchip peripherals remain active. The mode
is invoked by software. The content of the on-chip RAM and all the special functions registers remain
unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a
It should be noted that when idle is terminated by a hard ware reset, the device normally resumes
program execution, from where it left off, up to two machine cycles before the internal reset algorithm
takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins
is not inhibited. To eliminate the possibility of an unexpected write to a port pin when Idle is terminated
by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or
to external memory.
Status of External Pins During Idle and Power Down Modes
mode Program memory ALE ^psen Port0 Port1 Port2 Port3
idle internal 1
data data data Data
Idle External 1 1 float Data data Data
Power down Internal 0 0 Data Data Data Data
Power down External 0 0 float data Data data
Power Down Mode
In the power down mode the oscillator is stopped, and the instruction that invokes power down is
the last instruction executed. The on-chip RAM and Special Function Registers retain their values until
the power down mode is terminated. The only exit from power down is a hardware reset. Reset
redefines the SFRs but does not change the on-chip RAM. The reset should not be activated before
VCC is restored to its normal operating level and must be held active long enough to allow the oscillator
to restart and stabilize.
Program Memory Lock Bits
On the chip are three lock bits which can be left unprogrammed (U) or can be programmed (P) to
obtain the additional features listed in the table below:
Lock Bit Protection Modes
Program lock bits Protection type
Lb1 Lb2 Lb3
1 U U U No program lock features
2 P U U Movc instructions executed from external program memory are
disable from fetching code bytes from internal memory, ^ea is
sampled and latched on reset, and further programming of the flash
3 P P U Same as mode 2, also verify is disable.
4 P P P Same as mode 3, also external execution is disabled.
When lock bit 1 is programmed, the logic level at the EA pin is sampled and latched during reset.
If the device is powered up without a reset, the latch initializes to a random value, and holds that value
until reset is activated. It is necessary that the latched value of EA be in agreement with the current logic
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