Following their success in the microprocessor market, Intel began manufacturing
microcontrollers in 1976 with the introduction of the 8048 family. This early
microcontroller contains 64 bytes of RAM, 1 kB of ROM, a simple 8-bit
microprocessor core, and an 8-bit timer/counter as its sole on-board peripheral.
(Subsequent variants, the 8049 and 8050, include double and four times the
memory of the 8048, respectively.) The microprocessor consists of a 12-bit
program counter, an 8-bit accumulator and ALU, and a 3-bit stack pointer.
The 8048 is a complete computer on a single chip and gained a certain amount
of fame in the 1980s when it was used as the standard keyboard controller
on the IBM PC because of its simplicity and low cost. The 8048 was
manufactured in a 40-pin DIP and could be expanded with external memory
and peripherals via an optional external address/data bus. However, when
operated as a nonexpanded single-chip computer, the pins that would
otherwise function as its bus were available for general I/O purposes a
practice that is fairly standard on microcontrollers.
Motivated by the popularity of the 8048, Intel introduced the 8051 microcontroller
in 1980, which is substantially more powerful and ?exible. The 8051s basic
architecture is shown in Fig. 6.3. It contains 128 bytes of RAM, 4 kB of ROM,
two 16-bit timer/counters, and a serial port. Registers within the microprocessor
are 8 bits wide except for the 16-bit data pointer (DPTR) and program counter
(PC). Memory is divided into mutually exclusive program and data sections that
each can be expanded up to 64 kB in size via an external bus. Expansion is
accomplished by borrowing pins from two of the four 8-bit I/O ports. Intel
manufactured several variants of the 8051. The 8052 doubled the amount
of on-chip memory to 256 bytes of RAM and 8 kB of ROM and added a
third timer. The 8031/8032 are 8051/8052 chips without on-board ROM.
The 8751/8752 are 8051/8052 devices with EPROM instead of mask ROM.
As time went by and the popularity of the 8051 family increased, other
companies licensed the core architecture and developed many variants with
differing mixes of memory and peripherals.
Ports 0 through 3 are each eight-bit bidirectional I/O structures that can be used
as either generalpurpose signals or as dedicated interface signals according to
the system con?guration. In a single-chip con?guration where all memory is
contained on board, the four ports may be assigned freely. Some peripheral
functions use these I/O pins, but if a speci?c function is not required, the pins
may be used in a generic manner. Port 3 is the default peripheral port where
pins are used for the serial ports transmit and receive, external interrupt
request inputs, counter increment inputs, and external bus expansion control
signals. Port 1 is a general-purpose port that is also assigned for additional
peripheral support signals when an 8051 variant contains additional peripheral
functions beyond what can be supported on port 3 alone.
In a multichip con?guration where memory and/or additional peripherals are
added externally, ports 0 and 2 are used for bus expansion. Port 0 implements
a multiplexed address/data bus where the 8051 ?rst drives the lower eight
address bits and then either drives write-data or samples read-data in a
conventional bidirectional data bus scheme. In this standard con?guration,
the lower ad- dress bits, A[7:0], are latched externally by a discrete logic
chip (generally a 74LS373 or similar), and the 8051 drives an address latch
enable (ALE) signal to control this latch as shown in Fig. 6.4. This multiplexed
address/data scheme saves precious pins on the microcontroller that can be
used for valuable I/O functions. Some applications may suf?ce with just an
eight-bit external address bus. For example, if the only expansion necessary
were a special purpose I/O device, 256 bytes would probably be more than
enough to communicate with the device. However, some applications
demand a fully functional 16-bit external address bus. In these situations,
port 2 is used to drive the upper address bits, A[15:8].
FIGURE 6.3 8051 overall architecture.
FIGURE 6.4 8051 system with external address latch.
The 8051s microprocessor is very capable for such an early microcontroller.
It includes integer multiply and divide instructions that utilize eight-bit operands
in the accumulator and B register, and it then places the result back into those
registers. The stack, which grows upward in memory, is restricted to on-board
RAM only (256 bytes at most), so only an eight-bit stack pointer is implemented
. Aside from the general-purpose accumulator and B registers, the 8051
instruction set can directly reference 8 byte-wide general-purpose registers,
numbered R0 through R7, that are mapped as 4 banks in the lower 32 bytes
of on-board RAM. The active register bank can be changed at any time by
modifying two bank-select bits in the status word. The map of on-board data
memory is shown in Table 6.2. At reset, register bank 0 is selected, and the
stack pointer is set to 0×07, meaning that the stack will actually begin at
location 0×08 when the ?rst byte is eventually pushed. Above the register
banks is a 16-byte (128-bit) region of memory that is bit addressable.
Microcontroller applications often involve reading status information, checking
certain bits to detect particular events, and then triggering other events. Using
single bits rather than whole bytes to store status information saves precious
memory in a microcontroller. Therefore, the 8051s bit manipulation
instructions can make ef?cient use of the chips resources from both instruction
execution and memory usage perspectives. The remainder of the lower
128-byte memory region contains 80 bytes of general-purpose memory.
The upper 128 bytes of data memory are split into two sections: special-function
registers and RAM. Special-function registers are present in all 8051 variants,
but their de?nitions change according to the speci?c mix of peripherals in each
variant. Some special-function registers are standard across all 8051 variants.
These registers are typically those that were implemented on the original
8051/8052 devices and include the accumulator and B registers; the stack
pointer; the data pointer; and serial port, timer, and I/O port control registers.
Each time a manufacturer adds an on-board peripheral to the 8051,
accompanying control registers are added into the special-function
memory region.
On variants that incorporate 256 bytes of on-board RAM, the upper 128 bytes
are also mapped into a parallel region alongside the special-function registers.
Access between RAM and special-function registers is controlled by the
addressing mode used in a given instruction. Special-function registers are
accessed with direct addressing only. Therefore, such an instruction must
follow the opcode with an eight-bit address. The upper 128 bytes of RAM
are accessed with indirect addressing only. Therefore, such an instruction
must reference one of the eight general-purpose registers (R0 through R7
in the currently selected bank) whose value is used to index into that portion
of RAM. The lower 128 bytes of RAM are accessible via both direct and
indirect addressing.
The 8051 is a good study in maximizing the capabilities of limited resources.
Access to external memory is supported through a variety of indirect and indexed
schemes that provide an option to the system designer of how extensive an
external bus is implemented. Indirect access to external data memory is
supported in both 8- and 16-bit address con?gurations. In the 8-bit mode,
R0 through R7 are used as memory pointers, and the resulting address is
driven only on I/O port 0, freeing port 2 for uses other than as an address bus.
The DPTR functions as a pointer into data memory in 16-bit mode, enabling
a full 64-kB indirect addressing range. Indexed access to external program
memory is supported by both the DPTR and the PC. Being program memory
(ROM), only reads are sup- ported. Both DPTR and PC can serve as index
base address registers, and the current value in the accumulator serves as an
offset to calculate a ?nal address of either DPTR+A or PC+A.
The 8051s external bus interface is asynchronous and regulated by four basic
control signals: ALE, program storage enable (PSEN*), read enable (RD*),
and write enable (WR*). Figure 6.5 shows the interaction of these four
control signals and the two bus ports: ports 0 and 2. Recall that ALE
causes an external latch to retain A[7:0] that is driven from port 0 during
the ?rst half of the access and prior to port 0 transitioning to a data bus
role. The timing delays noted are for a standard 12-MHz operating
frequency (the highest frequency supported by the basic 8051 devices,
although certain newer devices can operate at substantially faster frequencies).*
FIGURE 6.5 8051 bus interface timing.
Although the speci?c timing delays of program memory and data memory
reads are different, they exhibit the same basic sequence of events. (More
time is allowed for data reads than for instruction reads from program
memory.) Therefore, if the engineer properly accounts for the timing
variations by selecting memory and logic components that are fast
enough to satisfy the PSEN* and RD* timing speci?cations simultaneously,
program and data memory can actually be merged into a uni?ed memory
space external to the chip. Such uni?cation can be performed by generating
a general memory read enable, MRE*, that is the AND function of PSEN*
and RD*. In doing so, whenever either read enable is driven low by the
8051, MRE* will be low. This can bene?t some applications by turning
the 8051 into a more general-purpose computing device that can load a
program into its data memory and then execute that same program
from program memory. It also enables indexed addressing to operate
on data memory, which normally is restricted to indirect addressing as
discussed previously.
Timers such as those found in the 8051 are useful for either counting external
events or triggering low-frequency events themselves. Each timer can be
con?gured in two respects: whether it is a timer or counter, and how the
count logic functions. The selection of timer versus counter is a decision
between incrementing the count logic based on the microcontrollers
operating frequency or on an external event sensed via an input port pin.
The 8051s internal logic runs in a repetitive pattern of 12 clock cycles in
which 1 machine cycle consists of 12 clock cycles. Therefore, the count
logic increments once each machine cycle when in timer mode. When in
counter mode, a low-to-high transition (rising edge) on a designated input
pin causes the counter to increment. The counter can be con?gured to
generate an interrupt each time it rolls over from its maximum count value
back to its starting value. This interrupt can be used to either trigger a
periodic maintenance routine at regular intervals (timer mode) or to take
action once an external event has occurred a set number of times
(counter mode). If not con?gured to generate an interrupt, the software
can periodically poll the timer to see how many events have occurred
or how much time has elapsed.
The timers inherently possess two 8-bit count registers that can be con?gured
in a variety of ways as shown in Fig. 6.6. A timer can be con?gured as a
conventional 16-bit counter, as two 8-bit counters, as a single 8-bit counter
with a 5-bit prescaler, and as a single 8-bit counter with an 8-bit reload
value. The ?rst two modes mentioned are straightforward: the timers count
from 0 to either 65,535 (16-bit) or 255 (8-bit) before rolling over and
perhaps generating an interrupt. The third mode is similar, but the 8-bit
counter increments only once every 32 machine cycles. The 5-bit (25 = 32)
prescaler functions as a divider ahead of the main counter. Apparently, the
main reason for in- cluding this mode was to retain function compatibility
with the 8048s prescaled timer. The fourth mode is interesting, because
the 8-bit counter is reloaded with an arbitrary 8-bit value rather than 0 after
reaching its terminal count value (255). When operated in timer mode, this
feature enables the timer to synthesize a wide range of low-frequency periodic
events. One very useful periodic event is an RS-232 bit-rate generator. A
commonly observed 8051 operating frequency is 11.0592 MHz. When
this frequency is divided by 12, a count increment rate of 921.6 kHz is
obtained. Further dividing this frequency by divisors such as 96 or 384
yields the standard RS-232 bit rates 9.6 kbps and 2.4 kbps. A divisor
of 384 cannot be implemented in an 8-bit counter. Instead, a selectable ?16
or ?32 counter is present in the serial port logic that generates the ?nal serial
bit rate.
FIGURE 6.6 8051 timer con?gurations.
The 8051s on-board serial port implements basic synchronous or asynchronous
transmit and receive shift-register functionality but does not incorporate hardware
handshaking of the type used in RS-232 communications. Serial transmission is
initiated by writing the desired data to a transmit register. Incoming data is
placed into a receive register, and an interrupt can be triggered to invoke a
serial port ISR. The serial port can be con?gured in one of four modes, two of
which are higher-fre-quency ?xed bit rates, and two of which are
lower-frequency variable bit rates established by the rollover characteristics
of an on-board timer. Mode 0 implements a synchronous serial interface
where the receive data pin is actually bidirectional and a transmit clock
is emitted on the transmit data pin. This mode operates on 8-bit data
and a ?xed bit rate of 1/12 the operating frequency.
Mode 1 implements an asynchronous transmit/receive serial port where ten
bits are exchanged for every byte: a start bit, eight data bits, and a stop bit.
The bit-rate is variable according to a timer rollover rate. Mode 3 is very
similar to mode 1, with the added feature that a ninth data bit is added to
each byte. This extra data bit can be used for parity in an RS-232
con?guration or for another application-speci?c purpose. These two
modes can be used to implement an RS-232 serial port without hardware
handshaking. Software-assisted hardware handshaking could be added
using general I/O pins on the 8051. Mode 2 is identical to mode 3 except
for its ?xed bit rate at either 1/32 or 1/64 the operating frequency. Modes
1 and 3 can be made to operate at standard RS-232 bit rates from
19.2 kbps on downward with the aforementioned 11.0592 MHz operating
frequency. A selectable ?16 or ?32 counter within the serial port logic
combines with the timer rollover to achieve the desired serial bit rate.
Intels 8051 architecture has been designed into countless applications in which
a small, embed- ded computer is necessary to regulate a particular process.
The original 40-pin devices are still commonly used and found in distributors
warehouses, but a host of newer devices are popular as well. Some of these
variants are larger and more capable than the original and include more
I/O ports, on- board peripherals, and memory. Some variants have taken
the opposite direction and are available in much smaller packages
(e.g., 20 pins) with low power consumption for battery-powered applications.
There are even special versions of the 8051 that are radiation hardened for
space and military applications. Companies that manufacture 8051 variants
include Atmel, Maxim (formerly Dallas Semiconductor), and Philips. Atmel
manufactures a line of small, low-power 8051 products. Maxim offers a
selection of high-speed 8051 microcontrollers that run at up to 33 MHz
with a 4-cycle architecture, as compared to 12 in the original 8051. Philips
has a broad 8051 product line with a variety of peripherals to suit many
individual applications.
The mature ROM-less 8031/8032 members of the 8051 family can be
ordered through many mail order retail electronics outlets for only a few
dollars apiece. The equally mature 8751/8752 EPROM devices can also
be found from many of these same sources, though at a higher price as a
result of the expense of the ceramic DIP in which they are most often
found. More specialized 8051 variants may be available only through
manufacturers authorized distributors.
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