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AN #107 - Logarithmic bar graph (LED and LCD) Print
Logarithmic bar graph (LED and LCD)

Application note written by Ger Langezaal

This program is written to create a logarithmic LED/LCD bar graph dB (VU) meter with peak-hold and drop-down. Scale range is 30dB in 3dB steps.
The smart part is that the log conversion is done with the analog comparator on a RC discharge curve.

Ger created two versions : one with an LCD display and one with LED's. The LED version is included here. The LCD version can be downloaded.

Download source code


'-------------------------------------------------------------------------------
' Filename : BG30dB_LED.bas
' Revision : 1.0
' Controller : AVR AT90S2313-10
' Author : Ger langezaal
' Compiler : BASCOM-AVR Rev. 1.11.6.2
' ROM image : 3EE hex (1006 dec)
'
'---[ Small program description ]-----------------------------------------------
'
' This program is written to create a logarithmic LED bar graph dB (VU) meter
' with peak-hold and drop-down. Scale range is 30dB in 3dB steps.
' Log conversion is done with the analog comparator on a RC discharge curve.
' C1 is charged each 4mS with PB0 as output on a Timer0 Interrupt.
' Then PB0 is set to AIN0 (analog comparator +input) Timer1 is reset
' and start counting, C1 will be discharged by R1.
' Timer1 is counting until AIN0 < AIN1.
' Analog Comparator Output = ACSR bit 5.
' Timer1 value is in T1 stored for calculation.
'
' Display mode is set with PD6 (pin 11).
' 1 = bar mode
' 0 = dot mode (for low current applications)

'---[ LED to AVR connections ]--------------------------------------------------
'
' AVR Resistor Cathode dB
' Port pin Ohm LED nr Scale
' PD5 9 >--[680]--> 11 +6
' PD4 8 >--[680]--> 10 +3
' PD3 7 >--[680]--> 9 0
' PD2 6 >--[680]--> 8 -3
' PD1 3 >--[680]--> 7 -6
' PD0 2 >--[680]--> 6 -9
' PB7 19 >--[680]--> 5 -12
' PB6 18 >--[680]--> 4 -15
' PB5 17 >--[680]--> 3 -18
' PB4 16 >--[680]--> 2 -21
' PB3 15 >--[680]--> 1 -24
' PB2 14 >--[680]--> 0 infinit
'
' All LED Anodes to +5 Volt
'
'---[ Analog comparator inputs ]------------------------------------------------
'
'
' Meter DC input >-------[ R2 ]-------> PB1 (AIN1 pin 13)
' |
' GND <---------||-----
' C2
'
' ---[ R1 ]---
' GND <----| |----> PB0 (AIN0 pin 12)
' -----||-----
' C1
' R1 = 10k 5%
' R2 = 10k
' C1 = 47nF 5%
' C2 = 47nF
'
'---[ DC input versus Timer1 and Led position ]---------------------------------
'
' Measured Timer1 values for calculation:
' DC input = 3500mV Timer1 = 192 ( +6dB)
' DC input = 312mV Timer1 = 1544 (-15dB) 21dB = factor 11.22
' 21dB = 1543 - 192 = 1351 Timer1 counts
' 3dB = 1351 / 7 = 193 Timer1 counts
'
' Input mv Timer1 LED pos dB scale
' 3500 192 11 +6
' 2477 385 10 +3
' 1753 578 9 0
' 1241 771 8 -3
' 879 965 7 -6
'  622 1158 6 -9
' 440 1351 5 -12
' 312 1544 4 -15
' 220 1737 3 -18
' 156 1930 2 -21
' 110 2123 1 -24
' <110 >2123 0  infinit
'
'---[ Compiler and hardware related statements ]--------------------------------

$regfile = "2313def.dat" 'register file for AT90S2313
$crystal = 10000000 '10MHz crystal

Ddrb = &B11111100 'PB0 and PB1 are analog inputs
Portb.0 = 0 'disable input pullup
Portb.1 = 0 'disable input pullup

Ddrd = &B10111111 'config PD6 as input
Portd = &B11111111 'output high and enable input pullup

Acsr.7 = 0 'enable analog comparator ACSR bit 7 = 0

'---[ Variables ]---------------------------------------------------------------

Dim Peak_pos As Byte
Dim Peak_hold As Byte
Dim Drop_hold As Byte
Dim Led_pos As Word
Dim Bar_pattern As Word
Dim T1 As Word

'---[ Constants ]---------------------------------------------------------------

Const Peak_hold_time = 200
Const Drop_down_time = 40
Const Led_max = 11 'Led 0 - 11
Const T1_fs = 192 'full scale Timer1 value
Const T1_step = 193 '3dB step Timer1 value
Const T1_range = T1_step * Led_max + T1_fs 'Calculate Timer1 range

Displ_mod
Alias Pind.6 'display mode 1 = Dot 0 = Bar
Ac_out
Alias Acsr.5 'analog comparator output = ACSR bit 5

'---[ Timer Configuration ]-----------------------------------------------------

Config Timer0 = Timer , Prescale = 256 'On Interrupt Timer
Config Timer1 = Timer , Prescale = 8 'R/C Timer
Config Watchdog = 1024 'reset after 1 Sec no reset watchdog
Enable Interrupts
Enable Timer0
Enable Timer1
On Timer0 Dc_input_sample 'on overflow timer0 jump to label
Timer0 = 0
Timer1 = 0
Start Timer0
Start Timer1

'---[ Main program loop ]-------------------------------------------------------

Do
 Bar_pattern
= &HFFFF 'set all bits

 
If Displ_mod = 1 Then 'bar display mode
 Bar_pattern
= Lookup(led_pos , Bar_mode)
 
Else 'dot display mode
 Bar_pattern
.led_pos = 0 'reset dot bit
 
End If

 Bar_pattern
.peak_pos = 0 'reset peak bit

 
Portd.5 = Bar_pattern.11 'led 11 = bit 11
 
Portd.4 = Bar_pattern.10
 
Portd.3 = Bar_pattern.9
 
Portd.2 = Bar_pattern.8
 
Portd.1 = Bar_pattern.7
 
Portd.0 = Bar_pattern.6
 
Portb.7 = Bar_pattern.5
 
Portb.6 = Bar_pattern.4
 
Portb.5 = Bar_pattern.3
 
Portb.4 = Bar_pattern.2
 
Portb.3 = Bar_pattern.1
 
Portb.2 = Bar_pattern.0 'led 0 = bit 0
Loop

'-------------------------------------------------------------------------------
End
'-------------------------------------------------------------------------------

'---[ Interrupt Service Routine on Timer0 overflow ]---------------------------

Dc_input_sample
:
 
Timer0 = 100 'preset Timer0 for sample rate of 4mS

 
Ddrb.0 = 1 'set AIN0 as PB0 output
 
Portb.0 = 1 'set PB0 high to charge C1
 
Waitus 200 'wait for charge complete
 
Ddrb.0 = 0 'reset PB0 as AIN0 analog input
 
Portb.0 = 0 'disable AIN0 pullup

 
Timer1 = 0 'clear R/C Timer1
 
Bitwait Ac_out , Reset 'wait for AIN0 < AIN1
 T1
= Timer1 'read Timer1 value

 
If T1 < T1_fs Then 'check T1 low limit
 T1
= T1_fs 'T1 clipping
 
Elseif T1 > T1_range Then 'check T1 high limit
 T1
= T1_range 'T1 clipping
 
End If

 Led_pos
= T1_range - T1 'calculate led position
 Led_pos
= Led_pos T1_step 'led_pos = 0 - 11

 
If Led_pos >= Peak_pos Then 'new peak value
 Peak_pos
= Led_pos 'store peak value
 Peak_hold
= Peak_hold_time  'preset peak hold timer
 
Else
 
If Peak_hold > 0 Then
 
Decr Peak_hold
 
Else
 
If Drop_hold > 0 Then
 
Decr Drop_hold
 
Else
 Drop_hold
= Drop_down_time 'preset drop down timer
 
If Peak_pos > 0 Then Decr Peak_pos 'drop down one position
 
End If
 
End If
 
End If
 
Reset Watchdog
Return

'---[ Led Bar pattern lookup data ]---------------------------------------------

' Led 11......... 0
Bar_mode
:
Data &B1111111111111110% 'Word constants must end with the %-sign
Data &B1111111111111100%
Data &B1111111111111000%
Data &B1111111111110000%
Data &B1111111111100000%
Data &B1111111111000000%
Data &B1111111110000000%
Data &B1111111100000000%
Data &B1111111000000000%
Data &B1111110000000000%
Data &B1111100000000000%
Data &B1111000000000000%

'-------------------------------------------------------------------------------