Week 6 and 7

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Week 6 and 7

• Logical Operations

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Logical Instructions

• Logical instructions perform Boolean operations
  such as AND and OR operations on byte values
• In order to adhere to convention, a byte with its
  bit locations is below.
• Note that bit 0 is the least significant bit
  (LSB) and bit 7 is the most significant bit (MSB)

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Logical Instructions ANL

• Logical AND can be used to set selective bits to
  a zero and leave other bits unchanged
• All 8 bits of each operand involved
• Consider the instruction
• ANL A,data
• Which ANDs a constant value with the contents of
  the accumulator and stores the result in the
  accumulator
• For example, if A contains 5AH, then the
  instruction ANL A,06H will produce
  the result 02H in A
• ANDing is a good way to mask off certain bits

01011010 00000110 00000010
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Logical Instructions ORL

• The logical OR instruction operates in a similar
  manner
• Instead of masking off bits, it can be used to
  set selective bits in a byte that are clear
  beforehand
• Consider the instruction
• ORL A,data
• Which ORs a constant value with the contents of
  the accumulator and stores the result in the
  accumulator
• For example, if A contains 5AH, then the
  instruction ORL A,06H will produce
  the result 5EH in A
• Note that this example will set bits 1 and 2 to a
  one irregardless of their previous value. All
  other bits are unaltered.

01011010 00000110 01011110
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Logical Instructions XRL

• The logical XOR instruction operates by
  performing exclusive operations on a bit by bit
  basis
• it can be used to toggle (invert)
• selective bits in a byte
• Consider the instruction
• XRL A,data
• Which XORs a constant value with the contents of
  the accumulator and stores the result in the
  accumulator
• For example, if A contains 5AH, then the
  instruction XRL A,06H will produce
  the result 5CH in A
• Note that this example inverts bits 1 and 2. All
  other bits are unaltered.

01011010 00000110 01011100
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Logical Instructions Full details for ANL, ORL
and XRL

• Just like the arithmetic instructions, there are
  a number of variations, They are
• ANL A,source ORL A,source XRL A,source
• ANL A,data ORL A,data XRL A,data
• ANL direct,A ORL direct,A XRL direct,A
• ANL direct,data ORL direct,data XRL
  direct,data
• Where source can be any of Rn, direct or _at_Ri

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Logical Instructions Rotate instructions

• As well as the boolean type instructions, it is
  common for processors to support shift and/or
  rotate instructions
• Perform power of 2 mul/div quickly
• Shift instructions most useful
• Extract bits from value
• Can be used for some serial I/O applications
• The 8051 supports rotate instructions
• These rotate instructions shift the contents of
  the accumulator to the left or right, one bit a
  time
• All rotate instructions operate on A and store
  the rotate values in A

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Logical Instructions RL A

• The instruction is called the Rotate Left (RL)
• RL A
• And has the effect of rotating the bits to the
  left, with the MSB fedback into the LSB.
• Example
• Before 11101100
• After 11011001

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Logical Instructions RLC A

• Rotate Left through Carry flag (RLC)
• RLC A
• And has the effect of rotating the bits to the
  left through the carry bit, with the carry bit
  fedback into the LSB.
• Example
• Before 1 01101100
• After 0 11011001

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Logical Instructions Other Rotate instructions

• Can also rotate right
• RR A
• RRC A
• Same ideas as before, except rotation is right

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Example

• Given A, extract bits 3 and 4 as a 2 bit value in
  A
• ANL A,18H
• RR A
• RR A
• RR A

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Example

• Given a value in memory location 22H, determine
  the number of ones in the stored value.

MOV R1,0 MOV R0,8 CLR C Loop MOV
A,22H RRC A MOV 22H,A MOV A,0 ADDC
A,R1 MOV R1,A DJNZ R0,loop
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SFRs

• Special Function Registers (SFRs) are 8 bit
  registers that have a special purpose
• One such purpose is that certain SFRs are used to
  control/interact with hardware components
• An example is the use of an SFR to control an I/O
  pin that turns on a DC motor
• Another example is the use of an SFR to read a
  switch input
• There are many different SFRs.
• The objective initially is to understand how to
  access an SFR and then to see some simple
  examples

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Data Memory (Revisted)

• Data memory is shown here
• Remember that the upper 128 bytes are accessible
  only through indirect addressing (_at_Ri)
• The lower 128 bytes can use both direct and
  indirect addressing
• In effect, so far we have only seen 128 bytes
  (7-bit address) that are accessible using direct
  addressing???
• Well, there are more!!!

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SFRs

• From the programmers model view, SFRs appear as
  memery locations with addresses in the range 80H
  to FFH
• Even though there are already 128 locations with
  same addresses, this is still possible
• Reason SFRs are accessed using only direct
  addressing, while the upper 128 bytes of data
  memory is accessible using only indirect
  addressing.

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SFRs
This appears confusing. Why is it like this??

• Given R0 with the value 82H, consider the
  following two instructions
• MOV A,82H
• MOV A,_at_R0
• The first instruction accesses address 82H in the
  SFR section
• The second instruction accesses address 82H in
  the upper 128 bytes of data memory

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SFRs

• It is not possible to use these locations for
  general purpose storage
• Each of the possible 128 SFRs has a particular
  function
• One particular SFR at address 90H is used to
  control I/O port P1
• I/O port P1 refers to 8 I/O pins and their
  associated output section
• Each one of the eight I/O pins can be
  individually controlled through the SFR at
  address 90H
• Microcontrollers have I/O ports on-chip whereas
  microprocessors require additional external H/W
  for I/O ports

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Example

• 
  
• Author R. Conway
• Date Feb 2000
• File led.a51
• Hardware Any 8051 based Microcontroller
• Description Switches the port pin P1.4 on
  and off with
• 200mSec period _at_ 50 duty
  cycle. The pin could be
• used to turn on and off a LED
• Vcc
• 8051 -
• ---- 270R
• _
• \ /
• P1.4 --gto---(/)---
• / \// LED

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_________________________________________________
___________________
MAIN PROGRAM MOV
90H,10H MOV A,10H main XRL 90H,A
toggle (complement) bit 4 of port 1 JMP main
_______________________________________
_____________________________
Example
EXCITING THIS IS OUR FIRST REAL OUTPUT FROM THE
8051!!!

• This program will cause the external pin called
  P1.4 to toggle high and low at a very high speed
  (for the 8051!).
• With this pin controlling an LED, it is not
  possible to see it go on and off at such a high
  speed
• Connect a scope probe to the pin on the 8051 and
  see the waveform

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Example Output
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Example

• Can we make the LED go on and off at a slower
  speed?
• Yes By introducing a delay between toggling pin
  4 of port 1.
• Use a software delay

• MOV 90H,10H
• MOV A,10H
• main XRL 90H,A
• delay 100ms
• MOV R7,200 200500us100ms
• dy1 MOV R6,250 2502us500us
• dy2 DJNZ R6,dy2 here for 500us
• DJNZ R7,dy1 repeat 200 times
  (100ms delay)
• JMP main

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Example Output
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Example More Readable code
MOV P1,10H MOV A,10H main XRL P1,A
delay 100ms MOV R7,200
200500us100ms dy1 MOV R6,250
2502us500us dy2 DJNZ R6,dy2 here for
500us DJNZ R7,dy1 repeat 200 times
(100ms delay) JMP main
P1 equ 90H
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Example Using Simulator
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SFRs

• SFRs are located at addresses in the range 80H to
  FFH. However, not all addresses are used.
• Two views

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Exercises

• Detail using instructions how to
• Zero the 4 least significant bits in R1, leaving
  the 4 most significant bits unchanged
• Set bits 0,1 and 3 of A to a one, leaving
  remaining bits unchanged
• Set bits 0 and 3 of A to a one, bits 2 and 3 to a
  zero and remaining bits unchanged
• Invert all the bits of A
• Jump to the location labelled go, if bits 7 or 3
  of A are 1.
• Jump to the location labelled go, if bits 7 and 3
  of A are 1.

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Exercises

1. How can the SFR section use the same addresses as
   the upper 128 bytes of RAM?
2. Can the SFR section be used for general purpose
   storage?
3. Give an example of 4 special purpose registers
4. List their addresses
5. List their symbolic names
6. Detail a complete program that toggles port 2.1
   and 2.4 (at same time) as fast as possible

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Exercises

• Detail a complete program that turns port 2.0 on
  for 100ms, then turns port 2.0 off and port 2.1
  on for 100ms, then turns port 2.1 off and port
  2.2 on for 100ms and so on up to port 2.7 and
  then restarts back at port 2.0. Develop a
  flowchart solution first!