Name:
calibadc10
Syntax:

CALIBADC10 wordvariable

Wordvariable - receives the adc reading.

Description:

Calibrate the microcontrollers internal ADC by measuring a fixed internal fixed voltage reference.

0.6V 20M, 28X1, 40X1
1.2V 28X2-3V, 28X2-3V
1.024V All other parts that support this command

 

Note that this command is not available on 28X2-5V/40X2-5V.

The reference voltage used by the PICAXE microcontrollers ADC reading (readadc/ readadc10) commands is the supply voltage. In the case of a battery powered system, this supply voltage can change over time (as the battery runs down), resulting in a varying ADC reading for the same voltage input.

The calibadc/calibadc10 commands can help overcome this issue by providing the ADC reading of a nominal internal reference. Therefore by periodically using the calibadc command you can mathematically calibrate/compensate the readadc command for changes in supply voltage.

calibadc can be considered as 'carry out a readadc on a fixed reference'.

Note that the voltage specified is a nominal voltage only and will vary with each part. Microchip datasheet AN1072 provides further details on how to software calibrate and use this advanced feature.

At clock speeds >= 32MHz it is necessary to use two consecutive 'calibadc' commands together to ensure the Vref has time to settle.   

Theory of operation

Calibadc10 will give a result (Nref) which will depend on the reference voltage (Vref) and the PICAXE power supply voltage (Vpsu) as follows -

     Nref = Vref * 1023 / Vpsu

This can be rearranged to determine the power supply voltage (Vpsu) from the calibadc10 result (Nref) -

     Vpsu = Vref * 1023 / Nref

If the Vref were 1.024 volts then this will be -

     Vpsu = 1.024 * 1023 / Nref

     Vpsu = 1047.552 / Nref

Rounded to the nearest whole number -

     Vpsu = 1048 / Nref

This will give the voltage in 1V units.

To obtain a voltage in millivolts (mV) -

     Vpsu = 1.024 * 1023 * 1000

     Vpsu = 1047552 / Nref

Which, accounting for PICAXE numerical constraints -

     Vpsu = 52378 / Nref * 20

Applies To:
All (except 08, 08M, 14M, 18, 18A, 18M, 18X, 28, 28A, 28X, 40X)
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Using 'calibadc10'

This program reads the 10-bit 'calibadc10' value into the word variable 'w1' and then reports what that reading is.

Code Example:
main:	calibadc10 w1	; read the adc reading
	debug		; display current value
	pause 500	; wait a while
	goto main	; loop back to start
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Determine the PICAXE supply voltage

This code demonstrates how to determine the PICAXE supply voltage. For example it may be that a vital peripheral chip must run at >=3.15 volts or else it will be unpredictable in function.

Code Example:
Symbol Nref = w2			; Word variable for the calibadc10 reading
Symbol Vpsu = w3			; Word variable for the supply voltage

Main:	calibadc10 Nref			; Take the reference reading
	Vpsu = 52378 / Nref * 2		; Work out supply voltage
	if Vpsu > 315 then		; Is greater than 3.15 volts
	  ; Battery is above 3.15 	; Yes
	  ; volts so it is OK.
	  gosub SafeToWork
	else				; No	
 	  ; Shut down this project 
	  ; safely now, because the
	  ; battery is going flat!
	  gosub Shutdown
	end if
	goto Main			; Continue

; Notes
;
; Calibadc10 will give a result (Nref) which will depend on the reference 
; voltage (Vref) and the PICAXE power supply voltage (Vpsu) as follows -
;
;     Nref = Vref * 1023 / Vpsu
;
; This can be rearranged to determine the power supply voltage (Vpsu) from
; the calibadc10 result (Nref) -
;
;     Vpsu = Vref * 1023 / Nref
;
; This will give Vpsu in units of volts (1000mV). To give Vpsu in units
; of 10mV we can multiply by 100 -
;
;     Vpsu = Vref * 1023 * 100 / Nref
;
; For a Vref of 1.024 volts this will be -
;
;     Vpsu = 1.024 * 1023 * 100 / Nref
;
;     Vpsu = 104755.2 / Nref
;
; Unfortunately 104755.2 is larger than the maximum value which can be 
; held within a 16-bit word value (65535), however we can divide 
; 104755.2 by 2 and later multiply the result by 2 and still have the
; same answer albeit with a little less accuracy -
;
;     Vpsu = 104755.2 / Nref
;
;     Vpsu = ( (104755.2/2) / Nref ) * 2
;
;     Vpsu = ( 52377.6 / Nref ) * 2
;
; Rounding to the nearest whole number as the PICAXE does not do decimals
; and taking into account the left to right nature of PICAXE calculations
; gives -
;
;   Vpsu = 52378 / Nref * 2
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Battery voltage percentage indicator

This will determine the percentage of a battery voltage available when the PICAXE is powered by that battery. 'Vpsu100' should be set to the battery mV which indicates 100%. 'Vpsu0' should be set to the battery mV which indicates 0%.

Code Example:
#picaxe 20X2
#terminal 9600

symbol Vpsu100 = 5000			; 5000mV = 100%
symbol Vpsu0   = 4000			; 4000mV = 0%

symbol Nref100 = 1047552 / Vpsu100	; 1024 * 1023 / Vpsu100
symbol Nref0   = 1047552 / Vpsu0	; 1024 * 1023 / Vpsu0

symbol NrefDif = Nref0 - Nref100

symbol Nref    = w0			; b1:b0
symbol Vpsu    = w1			; b3:b2
symbol percent = b4

do
	calibadc10 Nref
	percent = Nref min Nref100 - Nref100 * 100 / NrefDif max 100
	percent = 100 - percent
	Vpsu = 52378 / Nref * 2
	sertxd( #Vpsu, "0mV", tab, #percent, "%", cr, lf )
	pause 1000 
loop
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