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.
SFR Types
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.
SFR Descriptions
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.
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.
