• Compatible with MCS®-51Products
• 2K Bytes of Reprogrammable Flash Memory
– Endurance: 10,000 Write/Erase Cycles
• 2.7V to 6V Operating Range
• Fully Static Operation: 0 Hz to 24 MHz
• Two-level Program Memory Lock
• 128 x 8-bit Internal RAM
• 15 Programmable I/O Lines 8-bit
• Two 16-bit Timer/Counters
• Six Interrupt Sources Microcontroller
• Programmable Serial UART Channel
• Direct LED Drive Outputs with 2K Bytes
• On-chip Analog Comparator
• Low-power Idle and Power-down Modes Flash
• Green (Pb/Halide-free) Packaging Option
1. Description AT89C2051
The AT89C2051 is a low-voltage, high-performance CMOS 8-bit microcomputer with
2K bytes of Flash programmable and erasable read-only memory (PEROM). The
device is manufactured using Atmel’s high-density nonvolatile memory technology
and is compatible with the industry-standard MCS-51 instruction set. By combining a
versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C2051 is a power-
ful microcomputer which provides a highly-flexible and cost-effective solution to many
embedded control applications.
The AT89C2051 provides the following standard features: 2K bytes of Flash, 128
bytes of RAM, 15 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt
architecture, a full duplex serial port, a precision analog comparator, on-chip oscillator
and clock circuitry. In addition, the AT89C2051 is designed with static logic for opera-
tion 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
2. Pin Configuration
2.1 20-lead PDIP/SOIC
RST/VPP 1 20 VCC
(RXD) P3.0 2 19 P1.7
(TXD) P3.1 3 18 P1.6
XTAL2 4 17 P1.5
XTAL1 5 16 P1.4
(INT0) P3.2 6 15 P1.3
(INT1) P3.3 7 14 P1.2
(TO) P3.4 8 13 P1.1 (AIN1)
(T1) P3.5 9 12 P1.0 (AIN0)
GND 10 11 P3.7
3. Block Diagram
4. Pin Description
4.3 Port 1
The Port 1 is an 8-bit bi-directional I/O port. Port pins P1.2 to P1.7 provide internal pull-ups. P1.0
and P1.1 require external pull-ups. P1.0 and P1.1 also serve as the positive input (AIN0) and the
negative input (AIN1), respectively, of the on-chip precision analog comparator. The Port 1 out-
put buffers can sink 20 mA and can drive LED displays directly. When 1s are written to Port 1
pins, they can be used as inputs. When pins P1.2 to P1.7 are used as inputs and are externally
pulled low, they will source current (IIL) because of the internal pull-ups.
Port 1 also receives code data during Flash programming and verification.
4.4 Port 3
Port 3 pins P3.0 to P3.5, P3.7 are seven bi-directional I/O pins with internal pull-ups. P3.6 is
hard-wired as an input to the output of the on-chip comparator and is not accessible as a gen-
eral-purpose I/O pin. The Port 3 output buffers can sink 20 mA. When 1s are written to Port 3
pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 3
pins that are externally being pulled low will source current (IIL) because of the pull-ups.
Port 3 also serves the functions of various special features of the AT89C2051 as listed below:
Port Pin Alternate Functions
P3.0 RXD (serial input port)
P3.1 TXD (serial output port)
P3.2 INT0 (external interrupt 0)
P3.3 INT1 (external interrupt 1)
P3.4 T0 (timer 0 external input)
P3.5 T1 (timer 1 external input)
Port 3 also receives some control signals for Flash programming and verification.
Reset input. All I/O pins are reset to 1s as soon as RST goes high. Holding the RST pin high for
two machine cycles while the oscillator is running resets the device.
Each machine cycle takes 12 oscillator or clock cycles.
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
Output from the inverting oscillator amplifier.
5. Oscillator Characteristics
The 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 5-1. 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 5-2. There are no require-
ments 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.
Figure 5-1. Oscillator Connections
Note: C1, C2 = 30 pF ± 10 pF for Crystals
= 40 pF ± 10 pF for Ceramic Resonators
Figure 5-2. External Clock Drive Configuration
6. Special Function Registers
A map of the on-chip memory area called the Special Function Register (SFR) space is shown in
the table below.
Note that not all of the addresses are occupied, and unoccupied addresses may not be imple-
mented on the chip. Read accesses to these addresses will in general return random data, and
write accesses will have an indeterminate effect.
User software should not write 1s to these unlisted locations, 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.
Table 6-1. AT89C2051 SFR Map and Reset Values
0F0H B 0F7H
0E0H ACC 0E7H
0D0H PSW 0D7H
0B8H IP 0BFH
0B0H P3 0B7H
0A8H IE 0AFH
98H SCON SBUF 9FH
90H P1 97H
88H TCON TMOD TL0 TL1 TH0 TH1 8FH
00000000 00000000 00000000 00000000 00000000 00000000
80H SP DPL DPH PCON 87H
00000111 00000000 00000000 0XXX0000
7. Restrictions on Certain Instructions
The AT89C2051 and is an economical and cost-effective member of Atmel’s growing family of
microcontrollers. It contains 2K bytes of Flash program memory. It is fully compatible with the
MCS-51 architecture, and can be programmed using the MCS-51 instruction set. However,
there are a few considerations one must keep in mind when utilizing certain instructions to pro-
gram this device.
All the instructions related to jumping or branching should be restricted such that the destination
address falls within the physical program memory space of the device, which is 2K for the
AT89C2051. This should be the responsibility of the software programmer. For example, LJMP
7E0H would be a valid instruction for the AT89C2051 (with 2K of memory), whereas LJMP 900H
7.1 Branching Instructions
LCALL, LJMP, ACALL, AJMP, SJMP, JMP @A+DPTR – These unconditional branching
instructions will execute correctly as long as the programmer keeps in mind that the destination
branching address must fall within the physical boundaries of the program memory size (loca-
tions 00H to 7FFH for the 89C2051). Violating the physical space limits may cause unknown
CJNE [...], DJNZ [...], JB, JNB, JC, JNC, JBC, JZ, JNZ – With these conditional branching
instructions the same rule above applies. Again, violating the memory boundaries may cause
For applications involving interrupts the normal interrupt service routine address locations of the
80C51 family architecture have been preserved.
7.2 MOVX-related Instructions, Data Memory
The AT89C2051 contains 128 bytes of internal data memory. Thus, in the AT89C2051 the stack
depth is limited to 128 bytes, the amount of available RAM. External DATA memory access is
not supported in this device, nor is external PROGRAM memory execution. Therefore, no MOVX
[...] instructions should be included in the program.
A typical 80C51 assembler will still assemble instructions, even if they are written in violation of
the restrictions mentioned above. It is the responsibility of the controller user to know the physi-
cal features and limitations of the device being used and adjust the instructions used
8. Program Memory Lock Bits
On the chip are two lock bits which can be left unprogrammed (U) or can be programmed (P) to
obtain the additional features listed in the Table 8-1.
Table 8-1. Lock Bit Protection Modes(1)
Program Lock Bits
LB1 LB2 Protection Type
1 U U No program lock features
2 P U Further programming of the Flash is disabled
3 P P Same as mode 2, also verify is disabled
Note: 1. The Lock Bits can only be erased with the Chip Erase operation.
9. Idle Mode
In idle mode, the CPU puts itself to sleep while all the on-chip peripherals remain active. The
mode is invoked by software. The content of the on-chip RAM and all the special functions regis-
ters remain unchanged during this mode. The idle mode can be terminated by any enabled
interrupt or by a hardware reset.
The P1.0 and P1.1 should be set to “0” if no external pull-ups are used, or set to “1” if
external pull-ups are used.
It should be noted that when idle is terminated by a hardware 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.
10. 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.
The P1.0 and P1.1 should be set to “0” if no external pull-ups are used, or set to “1” if
external pull-ups are used.
11. Programming The Flash
The AT89C2051 is shipped with the 2K bytes of on-chip PEROM code memory array in the
erased state (i.e., contents = FFH) and ready to be programmed. The code memory array is pro-
grammed one byte at a time. Once the array is programmed, to re-program any non-blank byte,
the entire memory array needs to be erased electrically.
Internal Address Counter: The AT89C2051 contains an internal PEROM address counter
which is always reset to 000H on the rising edge of RST and is advanced by applying a positive
going pulse to pin XTAL1.
Programming Algorithm: To program the AT89C2051, the following sequence is
1. Power-up sequence:
Apply power between VCC and GND pins
Set RST and XTAL1 to GND
2. Set pin RST to “H”
Set pin P3.2 to “H”
3. Apply the appropriate combination of “H” or “L” logic
levels to pins P3.3, P3.4, P3.5, P3.7 to select one of the programming operations
shown in the PEROM Programming Modes table.
To Program and Verify the Array:
4. Apply data for Code byte at location 000H to P1.0 to P1.7.
5. Raise RST to 12V to enable programming.
6. Pulse P3.2 once to program a byte in the PEROM array or the lock bits. The byte-write
cycle is self-timed and typically takes 1.2 ms.
7. To verify the programmed data, lower RST from 12V to logic “H” level and set pins P3.3
to P3.7 to the appropriate levels. Output data can be read at the port P1 pins.
8. To program a byte at the next address location, pulse XTAL1 pin once to advance the
internal address counter. Apply new data to the port P1 pins.
9. Repeat steps 6 through 8, changing data and advancing the address counter for the
entire 2K bytes array or until the end of the object file is reached.
10. Power-off sequence:
set XTAL1 to “L”
set RST to “L”
Turn VCC power off
Data Polling: The AT89C2051 features Data Polling to indicate the end of a write cycle. During
a write cycle, an attempted read of the last byte written will result in the complement of the writ-
ten data on P1.7. Once the write cycle has been completed, true data is valid on all outputs, and
the next cycle may begin. Data Polling may begin any time after a write cycle has been initiated.
Ready/Busy: The Progress of byte programming can also be monitored by the RDY/BSY output
signal. Pin P3.1 is pulled low after P3.2 goes High during programming to indicate BUSY. P3.1 is
pulled High again when programming is done to indicate READY.
Program Verify: If lock bits LB1 and LB2 have not been programmed code data can be read
back via the data lines for verification:
1. Reset the internal address counter to 000H by bringing RST from “L” to “H”.
2. Apply the appropriate control signals for Read Code data and read the output data at
the port P1 pins.
3. Pulse pin XTAL1 once to advance the internal address counter.
4. Read the next code data byte at the port P1 pins.
5. Repeat steps 3 and 4 until the entire array is read.
The lock bits cannot be verified directly. Verification of the lock bits is achieved by observing that
their features are enabled.
Chip Erase: The entire PEROM array (2K bytes) and the two Lock Bits are erased electrically
by using the proper combination of control signals and by holding P3.2 low for 10 ms. The code
array is written with all “1”s in the Chip Erase operation and must be executed before any non-
blank memory byte can be re-programmed.
Reading the Signature Bytes: The signature bytes are read by the same procedure as a nor-
mal verification of locations 000H, 001H, and 002H, except that P3.5 and P3.7 must be pulled to
a logic low. The values returned are as follows.
(000H) = 1EH indicates manufactured by Atmel
(001H) = 21H indicates 89C2051
12. Programming Interface
Every code byte in the Flash array can be written and the entire array can be erased by using
the appropriate combination of control signals. The write operation cycle is self-timed and once
initiated, will automatically time itself to completion.
Most major worldwide programming vendors offer support for the Atmel AT89 microcontroller
series. Please contact your local programming vendor for the appropriate software revision.
13. Flash Programming Modes
Mode RST/VPP P3.2/PROG P3.3 P3.4 P3.5 P3.7
Write Code Data(1)(3) 12V L H H H
Read Code Data(1) H H L L H H
Bit - 1 12V H H H H
Bit - 2 12V H H L L
Chip Erase 12V (2) H L L L
Read Signature Byte H H L L L L
Notes: 1. The internal PEROM address counter is reset to 000H on the rising edge of RST and is advanced by a positive pulse at
2. Chip Erase requires a 10 ms PROG pulse.
3. P3.1 is pulled Low during programming to indicate RDY/BSY.
Figure 13-1. Programming the Flash Memory
Figure 13-2. Verifying the Flash Memory
14. Flash Programming and Verification Characteristics
TA = 0°C to 70°C, VCC = 5.0 ± 10%
Symbol Parameter Min Max Units
VPP Programming Enable Voltage 11.5 12.5 V
IPP Programming Enable Current 250 µA
tDVGL Data Setup to PROG Low 1.0 µs
tGHDX Data Hold after PROG 1.0 µs
tEHSH P3.4 (ENABLE) High to VPP 1.0 µs
tSHGL VPP Setup to PROG Low 10 µs
tGHSL VPP Hold after PROG 10 µs
tGLGH PROG Width 1 110 µs
tELQV ENABLE Low to Data Valid 1.0 µs
tEHQZ Data Float after ENABLE 0 1.0 µs
tGHBL PROG High to BUSY Low 50 ns
tWC Byte Write Cycle Time 2.0 ms
tBHIH RDY/BSY\ to Increment Clock Delay 1.0 µs
tIHIL Increment Clock High 200 ns
Note: 1. Only used in 12-volt programming mode.
15. Flash Programming and Verification Waveforms
16. Absolute Maximum Ratings*
Operating Temperature ................................. -55°C to +125°C *NOTICE: Stresses beyond those listed under “Absolute
Maximum Ratings” may cause permanent dam-
Storage Temperature ..................................... -65°C to +150°C age to the device. This is a stress rating only and
functional operation of the device at these or any
Voltage on Any Pin other conditions beyond those indicated in the
with Respect to Ground .....................................-1.0V to +7.0V operational sections of this specification is not
implied. Exposure to absolute maximum rating
Maximum Operating Voltage ............................................ 6.6V conditions for extended periods may affect device
DC Output Current...................................................... 25.0 mA
17. DC Characteristics
TA = -40°C to 85°C, VCC = 2.7V to 6.0V (unless otherwise noted)
Symbol Parameter Condition Min Max Units
VIL Input Low-voltage -0.5 0.2 VCC - 0.1 V
VIH Input High-voltage (Except XTAL1, RST) 0.2 VCC + 0.9 VCC + 0.5 V
VIH1 Input High-voltage (XTAL1, RST) 0.7 VCC VCC + 0.5 V
VOL Output Low-voltage(1) IOL = 20 mA, VCC = 5V 0.5 V
(Ports 1, 3) IOL = 10 mA, VCC = 2.7V
IOH = -80 μA, VCC = 5V ± 10% 2.4 V
VOH Output High-voltage IOH = -30 μA 0.75 VCC V
(Ports 1, 3)
IOH = -12 μA 0.9 VCC V
IIL Logical 0 Input Current VIN = 0.45V -50 µA
(Ports 1, 3)
ITL Logical 1 to 0 Transition Current VIN = 2V, VCC = 5V ± 10% -750 µA
(Ports 1, 3)
ILI Input Leakage Current 0 < VIN < VCC ±10 µA
(Port P1.0, P1.1)
VOS Comparator Input Offset Voltage VCC = 5V 20 mV
VCM Comparator Input Common 0 VCC V
RRST Reset Pull-down Resistor 50 300 kΩ
CIO Pin Capacitance Test Freq. = 1 MHz, TA = 25°C 10 pF
Active Mode, 12 MHz, VCC = 6V/3V 15/5.5 mA
Power Supply Current Idle Mode, 12 MHz, VCC = 6V/3V
P1.0 & P1.1 = 0V or VCC 5/1 mA
Power-down Mode(2) VCC = 6V, P1.0 & P1.1 = 0V or VCC 100 µA
VCC = 3V, P1.0 & P1.1 = 0V or VCC 20 µA
Notes: 1. Under steady state (non-transient) conditions, IOL must be externally limited as follows:
Maximum IOL per port pin: 20 mA
Maximum total IOL for all output pins: 80 mA
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater
than the listed test conditions.
2. Minimum VCC for Power-down is 2V.
18. External Clock Drive Waveforms
19. External Clock Drive
VCC = 2.7V to 6.0V VCC = 4.0V to 6.0V
Symbol Parameter Min Max Min Max Units
1/tCLCL Oscillator Frequency 0 12 0 24 MHz
tCLCL Clock Period 83.3 41.6 ns
tCHCX High Time 30 15 ns
tCLCX Low Time 30 15 ns
tCLCH Rise Time 20 20 ns
tCHCL Fall Time 20 20 ns
20. Serial Port Timing: Shift Register Mode Test Conditions
VCC = 5.0V ± 20%; Load Capacitance = 80 pF
12 MHz Osc Variable Oscillator
Symbol Parameter Min Max Min Max Units
tXLXL Serial Port Clock Cycle Time 1.0 12 tCLCL µs
tQVXH Output Data Setup to Clock Rising Edge 700 10 tCLCL-133 ns
tXHQX Output Data Hold after Clock Rising Edge 50 2 tCLCL-117 ns
tXHDX Input Data Hold after Clock Rising Edge 0 0 ns
tXHDV Clock Rising Edge to Input Data Valid 700 10 tCLCL-133 ns
21. Shift Register Mode Timing Waveforms
22. AC Testing Input/Output Waveforms(1)
Note: 1. AC Inputs during testing are driven at VCC - 0.5V for a logic 1 and 0.45V for a logic 0. Timing measurements are made at VIH
min. for a logic 1 and VIL max. for a logic 0.
23. Float Waveforms(1)
Note: 1. For timing purposes, a port pin is no longer floating when a 100 mV change from load voltage occurs. A port pin begins to
float when 100 mV change from the loaded VOH/VOL level occurs.
24. ICC (Active Mode) Measurements
TYPICAL ICC - ACTIVE (85°C)
0 6 12 18 24
25. ICC (Idle Mode) Measurements
TYPICAL ICC - IDLE (85°C)
0 3 6 9 12
26. ICC (Power Down Mode) Measurements
TYPICAL ICC vs. VOLTAGE- POWER DOWN (85°C)
3.0V 4.0V 5.0V 6.0V
Notes: 1. XTAL1 tied to GND
2. P.1.0 and P1.1 = VCC or GND
3. Lock bits programmed
27. Ordering Information
27.1 Green Package Option (Pb/Halide-free)
(MHz) Supply Ordering Code Package Operation Range
12 2.7V to 6.0V AT89C2051-12PU 20P3 Industrial
AT89C2051-12SU 20S (-40° C to 85° C)
24 4.0V to 6.0V AT89C2051-24PU 20P3 Industrial
AT89C2051-24SU 20S (-40° C to 85° C)
20P3 20-lead, 0.300” Wide, Plastic Dual In-line Package (PDIP)
20S 20-lead, 0.300” Wide, Plastic Gull Wing Small Outline (SOIC)
28. Package Information
28.1 20P3 – PDIP
C (Unit of Measure = mm)
eC SYMBOL MIN NOM MAX NOTE
eB A – – 5.334
A1 0.381 – –
D 24.892 – 26.924 Note 2
E 7.620 – 8.255
E1 6.096 – 7.112 Note 2
B 0.356 – 0.559
Notes: 1. This package conforms to JEDEC reference MS-001, Variation AD. B1 1.270 – 1.551
2. Dimensions D and E1 do not include mold Flash or Protrusion. L 2.921 – 3.810
Mold Flash or Protrusion shall not exceed 0.25 mm (0.010"). C 0.203 – 0.356
eB – – 10.922
eC 0.000 – 1.524
e 2.540 TYP
TITLE DRAWING NO. REV.
2325 Orchard Parkway 20P3, 20-lead (0.300"/7.62 mm Wide) Plastic Dual
R San Jose, CA 95131 Inline Package (PDIP) 20P3 D
28.2 20S – SOIC
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