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8051 C Programming
Chapter 8
The 8051 Microcontroller by
Scott Mc Kenzie
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The RS-232 Standard
Most widely used serial I/O interfacing standard Input and output voltage levels are not TTL
compatible
1 bit is represented by -3 to -25 V
0 bit is +3 to +25 V -3 to +3 is undefined
To connect RS232 to a microcontroller system must
use voltage converters such as MAX232 to convert
the TTL logic levels to the RS232 voltage levels, andvice versa
MAX232 IC chips are commonly referred to as line drivers
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The RS-232 DB-25 Connector
RS232 Connector DB-25
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The RS-232 DB-9 Connector
DB-9 9-Pin Connector
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The Line Driver- Max 232
Max-232 Converts from RS232 voltage levels to TTL voltage levels
Uses a +5 V power source
MAX232 has two sets of line drivers for transferring and
receiving data Line drivers used for TxD are called T1 and T2
Line drivers for RxD are designated as R1 and R2
T1 and R1 are used together for TxD and RxD of the
8051 Second set is left unused
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Inside Max-232
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Why C51? Offers the benefits of high-level, structured programming languages
such as C. Writing subroutines with ease
Sequence of operations is simple to trace, facilitating debugging
Finite number of structures with standardized terminology
Structures lend themselves easily to building subroutines.
The set of structures is complete that is all programs can be written using
three structures Statements
Assignment, call to a subroutine, etc
Loops The while/do statement, Repeat/Until statements
Choice If-then-else statement, the case statement, the goto statement
Structured programming results in increased programming productivity
Relieves the programmer of the hardware details
Easier to write especially for large and complex programs
Produces more readable program source codes.
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The downside of C51
Generally generates larger machine codes May execute more slowly
Require relatively large amount of memory
Programmer has less control and less ability to
directly interact with the hardware Some problems can be difficult to solve using only
the three structures discussed earlier
Nested structures can be difficult to follow
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C51 Data types
Data Type Bits Bytes Value Range
bit 1 - 0 to 1
signed char 8 1 -128 to +127
unsigned char 8 1 0 to 255
signed short 16 2 -32768 to +32767
unsigned short 16 2 0 to 65535
signed int 16 2 -32768 to +32767
unsigned int 16 2 0 to 65535
signed long 32 4 -2147483648 to +2147483647
unsigned long 32 4 0 to 4294967295float 32 4 +-1.175494E-38 to +-3.402823E+38
sbit 1 - 0 to 1
sfr and sfr16 8 & 16 1 & 2 0 to 255 & 0 to 65535
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C-51 Data types
Data type bitcan be used to declare variables that reside in the 8051s bit-addressable locations
bit flag = 0;
Declares a bitvariable called flag and initializes it to 0.
Data type sbit is similar to the bitdata type, except that it is normally used to
declare 1-bit variables that reside in SFRs, for example:sbit P = 0xD0;
Declares the sbitP and specifies that it refers to bit address D0H, which is
really the LSB of PSW SFR.
In case ofsbitdeclarations the assignment operator indicates what address
the sbit resides in, while in bitdeclarations it is used to specify the initial
value of the bitvariable. We can use a previously defined sfrvariable as
base address and assign oursbitvariable to refer to a certain bit within that
sfr.sfr PSW = 0xD0;
sbit P = PSW^0;
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REG51.H
REG51.H contains the declarations for: All the special function registers in 8051
All the individual flags, status and control bits in the bit
addressable SFRs
REG51.H is an important include in our C51programs.
It enables us to refer to the SFRs and all the flags and
control bit etc using their symbols i.e. PSW, ACC, B, SP,
TR0, TR1 etc.
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Memory TypesMemory Type Description (Size)
code Code memory (64 Kbytes). Objects
declared in this segment must be initialized
at compile time.
data Directly addressable internal data memory
(128 bytes)
idata Indirectly addressable internal datamemory (256 bytes)
bdata Bit-addressable internal data memory (16
bytes)
xdata External data memory (64 Kbytes)
pdata Paged external data memory (256 bytes)
MOVX A, @Ri, MOVX @Ri, A
char code errormsg[] = An error occurred;
signed int data num1;
bit bdata numbit;
unsigned int xdata num2;
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Accessing External Data and Code Memory
Space
13
The higher order address
byte comes from P2, the
lower order comes from Ri,so before using this
instruction you have to
initialize P2
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Memory Models
Memory Model DescriptionSmall Variables default to the internal data memory (data)
Compact Variables default to the first 256 bytes of external data
memory (pdata)
Large Variables default to external data memory (xdata)
If you wish all the variables to be assigned a default memory type without
having to specify them one by one then
Use memory models by choosing anyone from the above.
The default model is Small.Use the
#pragmadirective to explicitly select any other memory model
than the default Smallmodel.
#pragma small
#pragma compact
#pragma large
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C51 Functions
return_type function_name(arguments) [memory] [reentrant][interrupt] [using]
return_type: refers to data type of the return value.function_name: is any name that you wish to call thefunction as.
arguments: is the list of the type and number of inputvalues.
memory: refers to an explicit memory model (small,compact or large)
reentrant: refers to whether the function is reentrant(recursive)
interrupt: indicates that the function is actually an ISRusing: explicitly specifies which register bank to use
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Parameter Passing
Parameters to functions are passed through registersand memory
Passing through registers is the faster and default way
The registers used and their purpose are as follows.
Since there are only 8 registers in available, any extra
parameters may be passed using fixed memory locations.
Number ofArguments
Char/1-bytepointer
Int/2-bytepointer
Long/Float GenericPointer
1 R7 R6 & R7 R4-R7 R1-R3
2 R5 R4 & R5 R0-R3
3 R3 R2 & R3
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Returning Values
Unlike parameters output values must be returnedfrom functions via registers only.
Return Type Register Description
bit Carry Flag (C)
char/unsigned char/1-
byte pointer
R7
int/unsigned int/2-byte
pointer
R6 and R7 MSB in R6 and LSB in R7
long/unsigned long R4-R7 MSB in R4 and LSB in R7
float R4-R7 32-bit IEEE format
generic pointer R1-R3 Memory type in R3, MSB in R2,
LSB in R1
