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-51TM instruction set. By combining
a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C2051 is a pow-
erful microcomputer which provides a highly flexible and cost effective solution to
many embedded control applications.
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
with 2K Bytes
Port 1 is an 8-bit bidirectional I/O port. Port pins P1.2 to
P1.7 provide internal pullups. P1.0 and P1.1 require exter-
nal pullups. 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 output
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 (I
Port 3 pins P3.0 to P3.5, P3.7 are seven bidirectional I/O
pins with internal pullups. P3.6 is hard-wired as an input to
the output of the on-chip comparator and is not accessible
as a general 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 pullups and can be used as
inputs. As inputs, Port 3 pins that are externally being
pulled low will source current (I
of the AT89C2051 as listed below:
gramming 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.
Input to the inverting oscillator amplifier and input to the
internal clock operating circuit.
Output from the inverting oscillator amplifier.
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 maxi-
mum voltage high and low time specifications must be
tion Register (SFR) space is shown in the table below.
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-
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.
member of Atmel's growing family of microcontrollers. It
contains 2K bytes of flash program memory. It is fully com-
patible with the MCS-51 architecture, and can be pro-
grammed using the MCS-51 instruction set. However,
there are a few considerations one must keep in mind when
utilizing certain instructions to program this device.
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 mem-
ory), whereas LJMP 900H would not.
LCALL, LJMP, ACALL, AJMP, SJMP, JMP @A+DPTR
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 (locations 00H to
7FFH for the 89C2051). Violating the physical space limits
may cause unknown program behavior.
these conditional branching instructions the same rule
above applies. Again, violating the memory boundaries
may cause erratic execution.
service routine address locations of the 80C51 family archi-
tecture have been preserved.
The AT89C2051 contains 128 bytes of internal data mem-
ory. 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 exter-
nal PROGRAM memory execution. Therefore, no MOVX
[...] instructions should be included in the program.
even if they are written in violation of the restrictions men-
tioned above. It is the responsibility of the controller user to
know the physical features and limitations of the device
being used and adjust the instructions used correspond-
grammed (U) or can be programmed (P) to obtain the addi-
tional features listed in the table below:
chip peripherals remain active. The mode is invoked by
software. The content of the on-chip RAM and all the spe-
cial functions registers remain unchanged during this
mode. The idle mode can be terminated by any enabled
interrupt or by a hardware reset.
used, or set to '1' if external pullups are used.
ware reset, the device normally resumes program execu-
tion, 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
instruction that invokes power down is the last instruction
executed. The on-chip RAM and Special Function Regis-
ters retain their values until the power down mode is termi-
nated. 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 V
active long enough to allow the oscillator to restart and sta-
used, or set to '1' if external pullups are used.