The 8051 is a flexible microcontroller with a relatively
large number of modes of operations. Your program may
inspect and/or change the operating mode of the 8051 by
manipulating the values of the 8051's Special Function
Registers (SFRs).
SFRs are accessed as if they were normal Internal RAM.
The only difference is that Internal RAM is from address 00h
through 7Fh whereas SFR registers exist in the address range
of 80h through FFh.
Each SFR has an address (80h through FFh) and a name. The
following chart provides a graphical presentation of the
8051's SFRs, their names, and their address.
As you can see, although the address range of 80h through
FFh offer 128 possible addresses, there are only 21 SFRs in
a standard 8051. All other addresses in the SFR range (80h
through FFh) are considered invalid. Writing to or reading
from these registers may produce undefined values or
behavior.
Programming Tip: It is
recommended that you not read or write to SFR addresses
that have not been assigned to an SFR. Doing so may
provoke undefined behavior and may cause your program to
be incompatible with other 8051-derivatives that use the
given SFR for some other purpose.
As mentioned in the chart itself, the SFRs that have a
blue background are SFRs related to the I/O ports. The 8051
has four I/O ports of 8 bits, for a total of 32 I/O lines.
Whether a given I/O line is high or low and the value read
from the line are controlled by the SFRs in green.
The SFRs with yellow backgrouns are SFRs which in some
way control the operation or the configuration of some
aspect of the 8051. For example, TCON controls the
timers, SCON controls the serial port.
The remaining SFRs, with green backgrounds, are "other
SFRs." These SFRs can be thought of as auxillary SFRs in the
sense that they don't directly configure the 8051 but
obviously the 8051 cannot operate without them. For example,
once the serial port has been configured using SCON,
the program may read or write to the serial port using the
SBUF register.
Programming Tip: The SFRs
whose names appear in red in the chart above are SFRs that
may be accessed via bit operations (i.e., using the
SETB and CLR instructions). The other SFRs
cannot be accessed using bit operations. As you can see,
all SFRs that whose addresses are divisible by 8 can be
accessed with bit operations.
This section will endeavor to quickly overview each of
the standard SFRs found in the above SFR chart map. It is
not the intention of this section to fully explain the
functionality of each SFR--this information will be covered
in separate chapters of the tutorial. This section is to
just give you a general idea of what each SFR does.
P0 (Port 0, Address 80h,
Bit-Addressable): This is input/output port 0.
Each bit of this SFR corresponds to one of the pins on the
microcontroller. For example, bit 0 of port 0 is pin P0.0,
bit 7 is pin P0.7. Writing a value of 1 to a bit of this SFR
will send a high level on the corresponding I/O pin whereas
a value of 0 will bring it to a low level.
Programming Tip: While the
8051 has four I/O port (P0, P1, P2, and P3), if your
hardware uses external RAM or external code memory (i.e.,
your program is stored in an external ROM or EPROM chip or
if you are using external RAM chips) you may not use P0 or
P2. This is because the 8051 uses ports P0 and P2 to
address the external memory. Thus if you are using
external RAM or code memory you may only use ports P1 and
P3 for your own use.
SP (Stack Pointer,
Address 81h): This is the stack pointer of the
microcontroller. This SFR indicates where the next value to
be taken from the stack will be read from in Internal RAM.
If you push a value onto the stack, the value will be
written to the address of SP + 1. That is to say, if SP
holds the value 07h, a PUSH instruction will push the value
onto the stack at address 08h. This SFR is modified by all
instructions which modify the stack, such as PUSH, POP,
LCALL, RET, RETI, and whenever interrupts are provoked by
the microcontroller.
Programming Tip: The SP
SFR, on startup, is initialized to 07h. This means the
stack will start at 08h and start expanding upward in
internal RAM. Since alternate register banks 1, 2, and 3
as well as the user bit variables occupy internal RAM from
addresses 08h through 2Fh, it is necessary to initialize
SP in your program to some other value if you will be
using the alternate register banks and/or bit memory. It's
not a bad idea to initialize SP to 2Fh as the first
instruction of every one of your programs unless you are
100% sure you will not be using the register banks and bit
variables.
DPL/DPH (Data Pointer Low/High,
Addresses 82h/83h): The SFRs DPL and DPH work
together to represent a 16-bit value called the Data
Pointer. The data pointer is used in operations
regarding external RAM and some instructions involving code
memory. Since it is an unsigned two-byte integer value, it
can represent values from 0000h to FFFFh (0 through 65,535
decimal).
Programming Tip: DPTR is
really DPH and DPL taken together as a 16-bit value. In
reality, you almost always have to deal with DPTR one byte
at a time. For example, to push DPTR onto the stack you
must first push DPL and then DPH. You can't simply plush
DPTR onto the stack. Additionally, there is an instruction
to "increment DPTR." When you execute this instruction,
the two bytes are operated upon as a 16-bit value.
However, there is no instruction that decrements DPTR. If
you wish to decrement the value of DPTR, you must write
your own code to do so.
PCON (Power Control, Addresses
87h): The Power Control SFR is used to control
the 8051's power control modes. Certain operation modes of
the 8051 allow the 8051 to go into a type of "sleep" mode
which requires much less power. These modes of operation are
controlled through PCON. Additionally, one of the bits in
PCON is used to double the effective baud rate of the 8051's
serial port.
TCON (Timer Control, Addresses
88h, Bit-Addressable): The Timer Control SFR is
used to configure and modify the way in which the 8051's two
timers operate. This SFR controls whether each of the two
timers is running or stopped and contains a flag to indicate
that each timer has overflowed. Additionally, some non-timer
related bits are located in the TCON SFR. These bits are
used to configure the way in which the external interrupts
are activated and also contain the external interrupt flags
which are set when an external interrupt has occured.
TMOD (Timer Mode, Addresses
89h): The Timer Mode SFR is used to configure the
mode of operation of each of the two timers. Using this SFR
your program may configure each timer to be a 16-bit timer,
an 8-bit autoreload timer, a 13-bit timer, or two separate
timers. Additionally, you may configure the timers to only
count when an external pin is activated or to count "events"
that are indicated on an external pin.
TL0/TH0 (Timer 0 Low/High,
Addresses 8Ah/8Ch): These two SFRs, taken
together, represent timer 0. Their exact behavior depends on
how the timer is configured in the TMOD SFR; however, these
timers always count up. What is configurable is how and when
they increment in value.
TL1/TH1 (Timer 1 Low/High,
Addresses 8Bh/8Dh): These two SFRs, taken
together, represent timer 1. Their exact behavior depends on
how the timer is configured in the TMOD SFR; however, these
timers always count up. What is configurable is how and when
they increment in value.
P1 (Port 1, Address 90h,
Bit-Addressable): This is input/output port 1.
Each bit of this SFR corresponds to one of the pins on the
microcontroller. For example, bit 0 of port 1 is pin P1.0,
bit 7 is pin P1.7. Writing a value of 1 to a bit of this SFR
will send a high level on the corresponding I/O pin whereas
a value of 0 will bring it to a low level.
SCON (Serial Control, Addresses
98h, Bit-Addressable): The Serial Control SFR is
used to configure the behavior of the 8051's on-board serial
port. This SFR controls the baud rate of the serial port,
whether the serial port is activated to receive data, and
also contains flags that are set when a byte is successfully
sent or received.
Programming Tip: To use
the 8051's on-board serial port, it is generally necessary
to initialize the following SFRs: SCON, TCON, and TMOD.
This is because SCON controls the serial port. However, in
most cases the program will wish to use one of the timers
to establish the serial port's baud rate. In this case, it
is necessary to configure timer 1 by initializing TCON and
TMOD.
SBUF (Serial Control, Addresses
99h): The Serial Buffer SFR is used to send and
receive data via the on-board serial port. Any value written
to SBUF will be sent out the serial port's TXD pin.
Likewise, any value which the 8051 receives via the serial
port's RXD pin will be delivered to the user program via
SBUF. In other words, SBUF serves as the output port when
written to and as an input port when read from.
P2 (Port 2, Address A0h,
Bit-Addressable): This is input/output port 2.
Each bit of this SFR corresponds to one of the pins on the
microcontroller. For example, bit 0 of port 2 is pin P2.0,
bit 7 is pin P2.7. Writing a value of 1 to a bit of this SFR
will send a high level on the corresponding I/O pin whereas
a value of 0 will bring it to a low level.
Programming Tip: While the
8051 has four I/O port (P0, P1, P2, and P3), if your
hardware uses external RAM or external code memory (i.e.,
your program is stored in an external ROM or EPROM chip or
if you are using external RAM chips) you may not use P0 or
P2. This is because the 8051 uses ports P0 and P2 to
address the external memory. Thus if you are using
external RAM or code memory you may only use ports P1 and
P3 for your own use.
IE (Interrupt Enable, Addresses
A8h): The Interrupt Enable SFR is used to enable
and disable specific interrupts. The low 7 bits of the SFR
are used to enable/disable the specific interrupts, where as
the highest bit is used to enable or disable ALL interrupts.
Thus, if the high bit of IE is 0 all interrupts are disabled
regardless of whether an individual interrupt is enabled by
setting a lower bit.
P3 (Port 3, Address B0h,
Bit-Addressable): This is input/output port 3.
Each bit of this SFR corresponds to one of the pins on the
microcontroller. For example, bit 0 of port 3 is pin P3.0,
bit 7 is pin P3.7. Writing a value of 1 to a bit of this SFR
will send a high level on the corresponding I/O pin whereas
a value of 0 will bring it to a low level.
IP (Interrupt Priority,
Addresses B8h, Bit-Addressable): The Interrupt
Priority SFR is used to specify the relative priority of
each interrupt. On the 8051, an interrupt may either be of
low (0) priority or high (1) priority. An interrupt may only
interrupt interrupts of lower priority. For example, if we
configure the 8051 so that all interrupts are of low
priority except the serial interrupt, the serial interrupt
will always be able to interrupt the system, even if another
interrupt is currently executing. However, if a serial
interrupt is executing no other interrupt will be able to
interrupt the serial interrupt routine since the serial
interrupt routine has the highest priority.
PSW (Program Status Word,
Addresses D0h, Bit-Addressable): The Program
Status Word is used to store a number of important bits that
are set and cleared by 8051 instructions. The PSW SFR
contains the carry flag, the auxiliary carry flag, the
overflow flag, and the parity flag. Additionally, the PSW
register contains the register bank select flags which are
used to select which of the "R" register banks are currently
selected.
Programming Tip: If you
write an interrupt handler routine, it is a very good idea
to always save the PSW SFR on the stack and restore
it when your interrupt is complete. Many 8051 instructions
modify the bits of PSW. If your interrupt routine does not
guarantee that PSW is the same upon exit as it was upon
entry, your program is bound to behave rather erradically
and unpredictably--and it will be tricky to debug since
the behavior will tend not to make any sense.
ACC (Accumulator,
Addresses E0h, Bit-Addressable): The Accumulator
is one of the most-used SFRs on the 8051 since it is
involved in so many instructions. The Accumulator resides as
an SFR at E0h, which means the instruction MOV A,#20h
is really the same as MOV E0h,#20h. However, it is a
good idea to use the first method since it only requires two
bytes whereas the second option requires three bytes.
B (B Register, Addresses F0h,
Bit-Addressable): The "B" register is used in two
instructions: the multiply and divide operations. The B
register is also commonly used by programmers as an
auxiliary register to temporarily store values.
The chart above is a summary of all the SFRs that exist
in a standard 8051. All derivative microcontrollers of the
8051 must support these basic SFRs in order to maintain
compatability with the underlying MSCS51 standard.
A common practice when semiconductor firms wish to
develop a new 8051 derivative is to add additional SFRs to
support new functions that exist in the new chip.
For example, the Dallas Semiconductor DS80C320 is upwards
compatible with the 8051. This means that any program that
runs on a standard 8051 should run without modification on
the DS80C320. This means that all the SFRs defined above
also apply to the Dallas component.
However, since the DS80C320 provides many new features
that the standard 8051 does not, there must be some way to
control and configure these new features. This is
accomplished by adding additional SFRs to those listed here.
For example, since the DS80C320 supports two serial ports
(as opposed to just one on the 8051), the SFRs SBUF2 and
SCON2 have been added. In addition to all the SFRs listed
above, the DS80C320 also recognizes these two new SFRs as
valid and uses their values to determine the mode of
operation of the secondary serial port. Obviously, these new
SFRs have been assigned to SFR addresses that were unused in
the original 8051. In this manner, new 8051 derivative chips
may be developed which will run existing 8051 programs.
Programming Tip: If you
write a program that utilizes new SFRs that are specific
to a given derivative chip and not included in the above
SFR list, your program will not run properly on a standard
8051 where that SFR does not exist. Thus, only use
non-standard SFRs if you are sure that your program wil
only have to run on that specific microcontroller.
Likewise, if you write code that uses non-standard SFRs
and subsequently share it with a third-party, be sure to
let that party know that your code is using non-standard
SFRs to save them the headache of realizing that due to
strange behavior at run-time.
