GCBASIC documentation
Analog/Digital conversion

Syntax:

For a normal (also called a Single Channel) read, use:

    user_variable = ReadAD12( ANX )

For a Differential Channel read, use the following, where ANpX is the positive port and ANnY is the negative port:

    user_variable = ReadAD12( ANpX , ANnY )

To force a 12-bit AD channel to always respond with a value in the range [0 to 4095], use:

    user_variable = ReadAD12( ANX , TRUE )

Command Availability:

When using ReadAD12( ANX ), the returned value is a 12-bit number [0-4095]. The receiving variable can be a word, integer, or long.

When using ReadAD12( ANpX , ANnY ), the returned value is an integer, since negative values can be returned.

ReadAD12 is a function that reads the built-in analog-to-digital converter (ADC) that most microcontroller chips include. The port is specified as AN0, AN1, AN2, and so on, up to the number of analog inputs available on the chip in use. Those familiar with Atmel AVR microcontrollers can also refer to the ports as ADC0, ADC1, and so on — refer to the chip’s datasheet to find the number of ports available. (Note: it is perfectly acceptable to use ANx on AVR, or ADCx on a Microchip PIC.)

Other functions with similar behaviour are ReadAD and ReadAD10 — see their Help pages for the specific usage of each.

AD_DELAY controls the acquisition delay: the time the ADC’s internal holding capacitor is given to charge to the input voltage before the reading is taken. The default value is 20 us. If a reading looks noisy or consistently off — especially from a high-impedance sensor, such as a bare voltage divider without a buffer — increasing this delay is often the fix, since the capacitor needs more time to settle. Change it with:

    #define AD_DELAY 4 10us

ADSPEED controls the source of the clock for the ADC module; it varies from one microcontroller to another. InternalClock is a Microchip PIC-only option that drives the ADC from an internal RC oscillator. The default value is 128.

    'default value
    #define ADSPEED MEDIUMSPEED

    'pre-defined constants
    #define HIGHSPEED 255
    #define MEDIUMSPEED 128
    #define LOWSPEED 0

AD_ACQUISITION_TIME_SELECT_BITS also controls the acquisition time select bits. Acquisition time is the duration the ADC’s charge-holding capacitor stays connected to the AD channel, from the moment the read starts until conversion begins — the same underlying delay that AD_DELAY adjusts, exposed here as the raw bit pattern for microcontrollers whose datasheet documents it this way.

The default value of AD_ACQUISITION_TIME_SELECT_BITS is 0b100 (decimal 4), which sets all three ACQT bits. To change it:

    'change the default value
    #define AD_ACQUISITION_TIME_SELECT_BITS 0b001    'this will only set ACQT bit 0; ACQT bits 1 and 2 will be cleared

Example 1 - Read 12-bit ADC

    #chip 16F1788, 8

    'Set the input pin direction
    Dir PORTA.0 In

    'Print 255 readings
    For CurrentAddress = 0 to 255

        'Take a reading and show it
        Print str(ReadAD12(AN0))          ' <<< the ReadAD12 instruction

        'Wait 10 minutes before getting another reading
        Wait 10 min
    Next

Key line: Print str(ReadAD12(AN0)) — reads channel AN0 and returns whatever resolution the chip’s ADC module natively supports (8, 10, or 12 bits).

Example 2 - Force a 12-bit value to be returned

    #chip 16F1788, 8

    'Set the input pin direction
    Dir PORTA.0 In

    'Print 255 readings
    For CurrentAddress = 0 to 255

        'Take a reading and show it
        Print str(ReadAD12(AN0), TRUE)          ' <<< forcing a 12-bit result regardless of the chip's native ADC resolution
        'Wait 10 minutes before getting another reading
        Wait 10 min
    Next

Key line: ReadAD12(AN0), TRUE — the TRUE argument guarantees a value in the range [0-4095] even on a chip whose ADC natively returns 8 or 10 bits.

Example 3 - Differential reading

This example uses the differential capabilities of the ADC and writes the output to a serial terminal. The output value will be in the range [-4095 to 4095]. AN0 and AN2 are used for the differential reading.

    #chip 16F1789, 8

    'USART settings
    #define USART_BAUD_RATE 9600  'Initializes USART port with 9600 baud
    #define USART_TX_BLOCKING     'wait for tx register to be empty
    wait 100 ms

    'Set the input pin direction
    Dir PORTA.0 In
    Dir PORTA.2 In

    'Loop to take readings until the EEPROM is full
    For CurrentAddress = 0 to 255

        'Take a reading and log it
        HSerPrint ReadAD12( AN0, AN2 )          ' <<< the differential ReadAD12 instruction
        HserPrintCRLF
        'Wait 10 minutes before getting another reading
        Wait 10 min

    Next

Key line: ReadAD12( AN0, AN2 ) — reads the voltage difference between AN0 (positive) and AN2 (negative) as a signed integer, rather than an absolute voltage on a single pin.

See Also: