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Semiconductor Group
46
On-Chip Peripheral Components
Table 7-2
Read-Modify-Write Instructions
lt is not obvious that the last three instructions in this list are read-modify-write instructions, but they
are. The reason is that they read the port byte, all 8 bits, modify the addressed bit, then write the
complete byte back to the latch.
The reason why read-modify-write instructions are directed to the latch rather than the pin is to avoid
a possible misinterpretation of the voltage level at the pin. For example, a port bit might be used to
drive the base of a transistor. When a "1" is written to the bit, the transistor is turned on. lf the CPU
then reads the same port bit at the pin rather than the latch, it will read the base voltage of the
transistor (approx. 0.7 V, i.e. a logic low level !) and interpret it as "0". For example, when modifying
a port bit by a SETB or CLR instruction, another bit in this port with the above mentioned
configuration might be changed if the value read from the pin were written back to the latch.
However, reading the latch rather than the pin will return the correct value of "1".
Instruction
Function
ANL
Logic AND; e.g. ANL P1, A
ORL
Logic OR; e.g. ORL P2, A
XRL
Logic exclusive OR; e.g. XRL P3, A
JBC
Jump if bit is set and clear bit; e.g. JBC P1.1, LABEL
CPL
Complement bit; e.g. CPL P3.0
INC
Increment byte; e.g. INC P4
DEC
Decrement byte; e.g. DEC P5
DJNZ
Decrement and jump if not zero; e.g. DJNZ P3, LABEL
MOV Px.y, C
Move carry bit to bit y of port x
CLR Px.y
Clear bit y of port x
SETB Px.y
Set bit y of port x
*
Summary of Contents for SAB 80515 Series
Page 9: ...Semiconductor Group 9 Introduction Figure 1 2 Block Diagram ...
Page 12: ...Semiconductor Group 12 Fundamental Structure Figure 2 1 Detailed Block Diagram ...
Page 18: ...Semiconductor Group 18 Central Processing Unit Figure 3 1 Fetch Execute Sequence ...
Page 73: ...Semiconductor Group 73 On Chip Peripheral Components Figure 7 25 A D Converter Block Diagram ...
Page 83: ...Semiconductor Group 83 On Chip Peripheral Components Figure 7 33 a Timer 2 Block Diagram ...
Page 215: ...Device Specifications Semiconductor Group 215 ...
Page 217: ...Device Specifications Semiconductor Group 217 Pin Configuration P LCC 68 ...
Page 219: ...Device Specifications Semiconductor Group 219 Logic Symbol ...
Page 226: ...Device Specifications Semiconductor Group 226 Figure 1 Block Diagram ...
Page 229: ...Device Specifications Semiconductor Group 229 Figure 2 Memory Address Spaces ...
Page 239: ...Device Specifications Semiconductor Group 239 Figure 4 Block Diagram of the A D Converter ...
Page 241: ...Device Specifications Semiconductor Group 241 Figure 5 Interrupt Request Sources ...
Page 242: ...Device Specifications Semiconductor Group 242 Figure 6 Interrupt Priority Level Structure ...