ADC Bar Graph Indicator on LCD

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The program below displays a bar graph on the LCD. The graph is an indicator of the voltage applied to the ADC, as illustrated.

As can be seen, the first location on the top line of the display and the first location on the bottom line of the display together make up the bar graph indicator. Since each character has eight rows (5 * 8, when cursor position is used), the maximum number of bars is 16.

The ADC has eight output lines, resulting in 256 digital outputs representing the 0 to 5 V input voltage range. However, since the indicator only has 16 bars, the program below uses the top four bits from the ADC output.
Eight different bar patterns are stored in the LCD module's CGRAM, and these are used to make up the bar graph indicator. The patterns are shown opposite.

Of the four outputs from the ADC that are used, the bottom three (ranging from 000B to 111B - ie: 0 to 7) are used to select the appropriate bar pattern in CGRAM.

The MSB is used to determine on which display line the bar pattern should be placed. If the MSB is 0, put the pattern on the bottom line and put a space on the top line (ie: blank the top line).

If the MSB is 1, put the pattern on the top line and put pattern 7 from CGRAM (ie: all eight bars displayed) on the bottom line.

Once the program completes initialisation, it enters an endless loop. A timer interrupts every 200 us and the timer ISR initiates an ADC conversion. When the conversion is complete, the ADC causes an external interrupt. The external ISR then updates the LCD accordingly.

ADC Notes
LCD Module Notes
 
The simulator is of course not real time, therefore when running this program it is best to set the update frequency to 1000 (ie: 1000 instructions executed between each update of the simulator GUI).

<- get the source code
org 0				; reset vector
JMP main ; jump to the main program

org 3 ; external 0 interrupt vector
JMP ext0ISR ; jump to the external 0 ISR

org 0BH ; timer 0 interrupt vector
JMP timer0ISR ; jump to timer 0 ISR


org 30H
main:
SETB IT0 ; set external 0 interrupt as edge-activated
SETB EX0 ; enable external 0 interrupt
MOV TMOD, #2 ; set timer 0 as 8-bit auto-reload interval timer
MOV TH0, #-200 ; | put -200 into timer 0 high-byte - this reload value,
; | with system clock of 12 MHz, will result in a timer 0 overflow every 200 us
MOV TL0, #-200 ; | put the same value in the low byte to ensure the timer starts counting from
; | 56 (256 - 200) rather than 0
SETB TR0 ; start timer 0
SETB ET0 ; enable timer 0 interrupt



; initialise the display
; see instruction set for details


CLR P1.3 ; clear RS - indicates that instructions are being sent to the module

; function set
CLR P1.7 ; |
CLR P1.6 ; |
SETB P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear
; function set sent for first time - tells module to go into 4-bit mode
; Why is function set high nibble sent twice? See 4-bit operation on pages 39 and 42 of HD44780.pdf.

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E
; same function set high nibble sent a second time

SETB P1.7 ; low nibble set (only P1.7 needed to be changed)

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E
; function set low nibble sent

CALL delay ; wait for BF to clear


; entry mode set
; set to increment with no shift
CLR P1.7 ; |
CLR P1.6 ; |
CLR P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

SETB P1.6 ; |
SETB P1.5 ; |low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear


; display on/off control
; the display is turned on, the cursor is turned off and blinking is turned off
CLR P1.7 ; |
CLR P1.6 ; |
CLR P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

SETB P1.7 ; |
SETB P1.6 ; | low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear


; set CGRAM address
; set to character 0, row 0 (address 000 000B) - all 8 CGRAM locations will be used to store the bar graph patterns
CLR P1.7 ; |
SETB P1.6 ; |
CLR P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CLR P1.6 ; |low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear


; put first bar symbol in RAM
MOV 30H, #0 ; | Locations 30H to 37H in RAM will be used to store the bar graph patterns before sending them to CGRAM.
MOV 31H, #0 ; | Start by putting 0 into all except the last one (37H).
MOV 32H, #0 ; | R0 will be used to point to the last position. Then 11111B will be put into this location (using indirect
MOV 33H, #0 ; | addressing via R0). This means the first pattern is all rows 0, except the last row all 1.
MOV 34H, #0 ; | R0 will then be incremented, which means the next row (location 36H in RAM) gets 11111B. This results in
MOV 35H, #0 ; | the first 6 rows all 0 and the last two all 1. Repeating 8 times results in the last pattern containing
MOV 36H, #0 ; | 11111B in all rows.
MOV 38H, #0FFH ; end of data - sendBar subroutine checks for FFH to see if all 8 rows have been sent.

MOV R0, #37H ; point to highest bar (ie: 11111B) position


; send bar symbols to CGRAM
SETB P1.3 ; set RS - indicates that data is being sent to module
MOV R2, #8 ; 8 patterns need to be sent
again:
MOV @R0, #11111B ; R0 points to the next row (row location in RAM) where a bar (11111B) needs to be stored
DEC R0 ; decrement R0 to point to next row for next iteration
CALL sendPattern ; send the current pattern to CGRAM
DJNZ R2, again ; repeat for 8 patterns


SETB EA ; set the global interrupt enable bit
JMP $ ; do nothing



;-------------------------------------------------------------------------------



; updateBarGraph subroutine
updateBarGraph:
CLR A ; | The bar graph has a max 16 rows (2 characters, 8 rows in each).
MOV C, B.4 ; | Therefore, only the top 4 bits in B (outputs of ADC were transferred to B by ADC ISR) are used.
MOV ACC.0, C ; | Of these 4 bits, the bottom 3 are placed in A, while the MSB is left in C.
MOV C, B.5 ; | The bottom 3 bits (0 to 7) decide how many bars are displayed (ie; which of the patterns
MOV ACC.1, C ; | in CGRAM are displayed), while the MSB decides which position to display it - MSB = 1, display
MOV C, B.6 ; | on top line, MSB = 0, display on bottom line.
MOV ACC.2, C ; |
MOV C, B.7 ; |

JC topLine ; if C is set, the voltage to the ADC is greater than 2.5V, therefore display on top line

; do the following if voltage to ADC is less than 2.5V
CALL toBottom ; set DDRAM address to bottom line
CALL sendCharacter ; the value in A (ranging from 0 to 7) points to the appropriate bar symbol in CGRAM
CALL toTop ; set DDRAM address to top line
MOV A, #' ' ; since the voltage to the ADC is less than 2.5V, the top line must be cleared (space character is used)
CALL sendCharacter ; send space character to top line
RET


; do the following if voltage to ADC is greater than 2.5V
topLine:
CALL toTop ; set DDRAM address to top line
CALL sendCharacter ; the value in A (ranging from 0 to 7) points to the appropriate bar symbol in CGRAM
CALL toBottom ; set DDRAM address to bottom line
MOV A, #7 ; | since the voltage to the ADC is greater than 2.5V, the bottom line must get a full
; | bar graph symbol (location 7 in CGRAM)
CALL sendCharacter ; send full bar graph to bottom line
RET



; toTop subroutine - point to top row
; set DDRAM address to 0
toTop:
CLR P1.3 ; clear RS - indicates that instructions are being sent to the module

SETB P1.7 ; |
CLR P1.6 ; |
CLR P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CLR P1.7 ; low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear
RET



; toBottom subroutine - point to bottom row
; set DDRAM address to 40H
toBottom:
CLR P1.3 ; clear RS - indicates that instructions are being sent to the module

SETB P1.7 ; |
SETB P1.6 ; |
CLR P1.5 ; |
CLR P1.4 ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CLR P1.7 ; |
CLR P1.6 ; | low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear
RET



; sendPattern subroutine - send bar pattern symbol to CGRAM
sendPattern:
MOV R1, #30H ; pattern is stored in RAM starting at location 30H
loop:
MOV A, @R1 ; move one row of pattern to A
CJNE A, #0FFH, skip ; | check to see if A contains FFH - if yes
RET ; | then all rows of pattern have been sent, therefore return
skip:
CALL sendCharacter ; send the pattern to the lcd module
INC R1 ; increment R1 to point at next row in pattern
JMP loop ; repeat
RET


; sendCharacter subroutine

sendCharacter:
SETB P1.3 ; set RS - indicates that data is being sent to module

MOV C, ACC.7 ; |
MOV P1.7, C ; |
MOV C, ACC.6 ; |
MOV P1.6, C ; |
MOV C, ACC.5 ; |
MOV P1.5, C ; |
MOV C, ACC.4 ; |
MOV P1.4, C ; | high nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

MOV C, ACC.3 ; |
MOV P1.7, C ; |
MOV C, ACC.2 ; |
MOV P1.6, C ; |
MOV C, ACC.1 ; |
MOV P1.5, C ; |
MOV C, ACC.0 ; |
MOV P1.4, C ; | low nibble set

SETB P1.2 ; |
CLR P1.2 ; | negative edge on E

CALL delay ; wait for BF to clear


; delay subroutine - used to allow LCD module carry out internal operation.
delay:
MOV R3, #50
DJNZ R3, $
RET



; timer 0 ISR - simply starts an ADC conversion
timer0ISR:
CLR P3.6 ; clear ADC WR line
SETB P3.6 ; then set it - this results in the required positive edge to start a conversion
RETI ; return from interrupt


; external 0 ISR - responds to the ADC conversion complete interrupt
ext0ISR:
CLR P3.7 ; clear the ADC RD line - this enables the data lines
MOV B, P2 ; move ADC outputs to B
SETB P3.7 ; disable the ADC data lines by setting RD
CALL updateBarGraph ; update the bar graph using the new reading from the ADC
RETI ; return from interrupt
 
 
Home Prefer the
Original?
New
EdSim51SH
About the
Simulator

User's
Guide

Installation
Instructions
Examples Contact
Us
The Fundamental
Simulations
Beginner's Guide
to the 8051 - Online
Site Map

 
Copyright (c) 2007-2013 James Rogers