Ok, then there's another idea: You're setting A0 to digital input while
reading the analog voltage. It's usually better to set analog inputs to
"analog", as they then won't be loaded by the digital input circuitry. I doubt
that this is the real problem, as a "measurement" which always reads 838, not
+/-1, is very suspicious. What's the value when shorting the two pins?
Greets,
Kiste
Am Freitag, 31. Januar 2020, 12:43:51 MEZ hat Gilles BARTHELEMY
<[email protected]> Folgendes geschrieben:
Thank you for your answer.Yes, obviously anything multiplied by zero equal
zero...
Anyway, I tried several values for IN_STRAY_CAP_TO_GND, with no result.
Displaying the mesured value (VAL) always shows 838, no matter what capacitor I
test (or even with no capacitor at all).A0 tied to ground shows 0, and tied to
VCC shows 1023, which seems correct. So appearently, measuring works. Maybe
it's impossible to do with a 16F877A, but I doubt. Anyway, using the link you
gave, I ordered some 18F45K22. Wait and see.For my application, I must take the
measure this way, because the goal is to get a wire length "on the fly" when a
default is detected on an analog input.
A+Gilles
Le vendredi 31 janvier 2020 07:13:05 UTC+1, Kiste a écrit :
const word IN_STRAY_CAP_TO_GND = 0 --initially this was 30.00
const word IN_EXTRA_CAP_TO_GND = 0
const word IN_CAP_TO_GND = IN_STRAY_CAP_TO_GND + IN_EXTRA_CAP_TO_GND
[...]
-- Calculate and print result
capacitance = (val * IN_CAP_TO_GND) / (MAX_ADC_VALUE - val)
IN_CAP_TO_GND is zero, therefore capacitance is always zero, no matter all the
other values.
Using the not-so-well defined stray capacitance in a capacitive voltage divider
is of course possible, but there are PICs which have far better peripherals
built in for capatitance measurement. Take a look at
https://www.microchip.com/ maps/Microcontroller.aspx
For "CPU Type" select "8-bit PIC MCU", then further down try "CTMU" (Charge
Time Measurement Unit) or "Capacitive Voltage Divider".
Greets,Kiste
Am Donnerstag, 30. Januar 2020, 23:30:47 MEZ hat Gilles BARTHELEMY
<[email protected]> Folgendes geschrieben:
Hello,
I found this code for Arduino. It needs no additional part to measure a
capacitor value, just plug it between two pins of the microcontroller.
const int OUT_PIN = A2;
const int IN_PIN = A0;
//Capacitance between IN_PIN and Ground
//Stray capacitance is always present. Extra capacitance can be added to
//allow higher capacitance to be measured.
const float IN_STRAY_CAP_TO_GND = 24.48; //initially this was 30.00
const float IN_EXTRA_CAP_TO_GND = 0.0;
const float IN_CAP_TO_GND = IN_STRAY_CAP_TO_GND + IN_EXTRA_CAP_TO_GND;
const int MAX_ADC_VALUE = 1023;
void setup()
{
pinMode(OUT_PIN, OUTPUT);
//digitalWrite(OUT_PIN, LOW); //This is the default state for outputs
pinMode(IN_PIN, OUTPUT);
//digitalWrite(IN_PIN, LOW);
Serial.begin(115200);
}
void loop()
{
//Capacitor under test between OUT_PIN and IN_PIN
//Rising high edge on OUT_PIN
pinMode(IN_PIN, INPUT);
digitalWrite(OUT_PIN, HIGH);
int val = analogRead(IN_PIN);
//Clear everything for next measurement
digitalWrite(OUT_PIN, LOW);
pinMode(IN_PIN, OUTPUT);
//Calculate and print result
float capacitance = (float)val * IN_CAP_TO_GND / (float)(MAX_ADC_VALUE - val);
Serial.print(F("Capacitance Value = "));
Serial.print(capacitance, 3);
Serial.print(F(" pF ("));
Serial.print(val);
Serial.println(F(") "));
while (millis() % 500 != 0)
;
}I translated it to Jal, I just adapted the pins to use, and converted
floating values to integer, since I don't need them for my applicationInclude
16f877a
Pragma Target CLOCK 20_000_000
Pragma Target OSC hs
Pragma Target WDT disabled
Pragma Target BROWNOUT enabled
Pragma Target PWRTE disabled
Pragma Target LVP disabled
Pragma Target CPD disabled
Pragma Target WRT disabled
Pragma Target DEBUG disabled
Pragma Target CP disabled
-- include delay
include print
include format
-- Désactive les convertisseurs A/D
enable_digital_io()
--
-- Mesure en haute résolution
const bit ADC_HIGH_RESOLUTION = high
-- On va utiliser un seul canal analogique
const byte ADC_NCHANNEL = 1
-- Pas de tension de référence externe
const byte ADC_NVREF = ADC_NO_EXT_VREF
include adc
adc_init()
-- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~ %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~
%~%~%~%~%~%~%~%~%
-- % Traitement du port RS232
-- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~ %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~
%~%~%~%~%~%~%~%~%
-- -- Définition des valeurs du port série
const USART_HW_Serial = TRUE -- TRUE = RS232, FALSE = SPI
const Serial_HW_Baudrate = 9600
include Serial_Hardware
Serial_HW_Init
-- some aliases so it is easy to change from serial hw to serial sw.
alias serial_write is serial_hw_write
alias serial_read is serial_hw_read
alias serial_data is serial_hw_data
alias serial_data_available is serial_hw_data_available
-- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~ %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~
%~%~%~%~%~%~%~%~%
-- % FIN Traitement du port RS232
-- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~ %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~
%~%~%~%~%~%~%~%~%
print_cls_ansi(serial_data)
print_string(serial_data," Debut programme\r\n")
print_crlf(serial_data)
alias OUT_PIN is pin_B7
alias OUT_PIN_DIRECTION is pin_B7_direction
alias IN_PIN is pin_A0
alias IN_PIN_DIRECTION is pin_A0_direction
--Capacitance between IN_PIN and Ground
--Stray capacitance is always present. Extra capacitance can be added to
--allow higher capacitance to be measured.
const word IN_STRAY_CAP_TO_GND = 0 --initially this was 30.00
const word IN_EXTRA_CAP_TO_GND = 0
const word IN_CAP_TO_GND = IN_STRAY_CAP_TO_GND + IN_EXTRA_CAP_TO_GND
const word MAX_ADC_VALUE = 1024
OUT_PIN_DIRECTION = OUTPUT
-- OUT_PIN = LOW --This is the default state for outputs
IN_PIN_DIRECTION = OUTPUT
-- IN_PIN = LOW
var word val = 0
var dword capacitance = 0
forever loop
-- Capacitor under test between OUT_PIN and IN_PIN
-- Rising high edge on OUT_PIN
IN_PIN_DIRECTION = INPUT
OUT_PIN = HIGH
val = adc_read_high_res(0)
OUT_PIN = LOW
-- Clear everything for next measurement
IN_PIN_DIRECTION = OUTPUT
-- Calculate and print result
capacitance = (val * IN_CAP_TO_GND) / (MAX_ADC_VALUE - val)
print_string(serial_data, "Capacitance Value = ")
print_dword_dec(serial_data, capacitance)
print_string(serial_data, " pF (")
print_word_dec(serial_data, val)
print_string(serial_data, ")\r\n")
_usec_delay (1_000_000)
end loopUnfortunately, it doesn't work at all, ADC doesn't see any change with
several values of capacitors, I can't figure out why... I obviously miss
something, but what?Any idea will be welcomed!
Link to the original project:https://wordpress.codewrite.
co.uk/pic/2014/01/21/cap- meter-with-arduino-uno/
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