chap8 - Chapter 8 Single-bit Instructions and Programming 1...

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Chapter 8
Single-bit Instructions and
Programming

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Objective
•
8051 has a powerful function ： 8051 allows
programmer to access a single bit (not a byte) on
the RAM or register. This type of accessing
method is called bit-addressable.
•
The bit-addressing mode can only provide on
specific area of the RAM or some specified
registers.
•
8051 provides a set of bit-addressable commands
for this operation.
•
This chapter is to understand how to use these bit-
addressable commands.

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Sections
8.1 Single-bit Instruction Programming
8.2 Single-bit Operations with CY
8.3 Reading Input Pins vs. Port Latch

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Section 8.1
Single-bit Instruction Programming

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Bit-addressability (1/2)
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In most microprocessors, data is accessed in byte-
sized chunks.
•
However, in many applications we need to change
one bit, for example, to turn on or off a device.
•
The bit-addressability of the 8051
–
The ability to access data in single bit instead of the
whole byte.
–
Make 8051 become one of the most powerful 8-bit
microprocessors.

Bit-addressability (2/2)
•
Which portions of the microprocessor, I/O ports,
registers, RAM, or ROM are bit-addressable?
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I/O Ports and Bit-addressability
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We can access either the entire 8 bits or any single
bit without altering the rest.
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Example:
SETB P2.3 ;set pin 3 of P2 to high
(1)
CLR P1.0 ;clear pin 0 of P1(0)
–
Remember that D0 is the LSB and D7 is the MSB.

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Example 8-1 (a-1)
Write the following program.
(a) Create a square wave of 50% duty cycle on bit 0 of port 1.
in
8051
P1.0

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Example 8-1 (a-2)
(a) Create a square wave of 50% duty cycle on bit 0 of port 1.
HERE: SETB P1.0 ;set to high bit 0 of port 1
LCALL DELAY ;call the delay subroutine
CLR P1.0 ;P1.0=0
LCALL DELAY
SJMP HERE
Another way to write the above program is:
HERE: CPL P1.0 ;complement bit 0 of port 1
LCALL DELAY ;call the delay subroutine
SJMP HERE

Example 8-1 (b-1)
Write the following programs.
(b) Create a square wave of 66% duty cycle on bit 3 of port 1.
in
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Example 8-1 (b-2)
(b) Create a square wave of 66% duty cycle on bit 3 of port 1.
BACK:SETB P1.3 ;set port 1 bit 3 high
LCALL DELAY
LACLL DELAY
CLR P1.3 ;clear bit 2 of port 1
LACLL DELAY
SJMP BACK

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Bit Address of I/O Ports
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Each port has its byte address.
–
Ex: P0 has address 80H.