GCBASIC documentation
Analog/Digital conversion

About Analog to Digital Conversion

The analog-to-digital converter (ADC or A/D) module support is implemented by GCBASIC to provide 8-bit, 10-bit, and 12-bit single-channel measurement mode and differential-channel measurement mode.

GCBASIC configures the analog-to-digital converter clock source, the programmed acquisition time, and the justification of the response byte, word, or integer (as defined in the GCBASIC method used).

Normal or Single-Channel Measurement Mode

Single-channel measurement mode is the default method for reading the ADC port. The positive input is attached to a suitable device (a light sensor or adjustable resistor), and the commands ReadAD, ReadAD10, and ReadAD12 return a byte, word, or word value respectively.

The A/D module on most microcontrollers only supports single-ended mode. Single-channel mode uses a single A/D port, and the returned value represents the difference between the voltage on the analog pin and a fixed negative reference, which is usually ground or Vss.

The syntax for single-ended A/D is Returned_Value = ReadAD(Port).

Example

  Print ReadAD10(AN3)          ' <<< reading a single channel in single-ended mode

Key line: ReadAD10(AN3) — reads channel AN3 against ground and returns a 10-bit value.

Differential-Channel Measurement Mode

Some devices in the Microchip PIC family also support differential analog-to-digital conversion. With differential conversion, the differential voltage between two channels is measured and converted to a digital value. The returned value can be either positive or negative, and is therefore stored as an integer.

When configured for differential-channel measurement mode, the positive channel is connected to the defined positive analog pin (ANx), and the negative channel is connected to the defined negative analog pin. These two pins are internally connected (within the microcontroller) to a unity-gain differential amplifier, and once the amplifier has completed the comparison, the result is returned as an integer.

The positive channel input is selected using the CHSx bits, and the negative channel input is selected using the CHSNx bits. GCBASIC manages these bits automatically — the programmer only needs to supply the correct analog pin designators in the ReadADx commands.

The 12-bit returned result is available in the ADRESH and ADRESL registers, which GCBASIC returns as an integer variable.

Some Microchip PIC microcontrollers have differential A/D modules and support differential mode as well as 12-bit A/D. With differential mode, the returned value can be either a positive or negative number that represents the voltage differential between the two A/D ports.

The syntax for differential A/D is ReadAD( PositiveANPort , NegativeANPort ). Note: if the negative port is omitted, ReadAD() performs a single-ended read on the positive AN port.

Example

  Print ReadAD12( AN3, AN4 )          ' <<< reading the voltage difference between two channels

Key line: ReadAD12( AN3, AN4 ) — returns the signed difference between AN3 (positive) and AN4 (negative) as a 12-bit integer.

Using a Voltage Reference

Voltage references come in many forms and offer different features across the PIC, AVR, and LGT microcontroller families. In the end, accuracy and stability are a voltage reference’s most important characteristics, since the reference’s main purpose is to provide a known output voltage. Any variation from this known value is an error. It is therefore useful to use the internal voltage reference provided within the microcontroller where available.

To use a voltage reference source for ADC operation, set the AD_REF_SOURCE constant to your chosen source. It defaults to the VCC pin, and therefore the constant is set by default to AD_REF_AVCC. The voltage reference is specific to the microcontroller, but the options are generally as follows:

Optimising GCBASIC Code

GCBASIC supports a wide range of A/D modules, and the supporting library addresses up to 34 channels. To reduce the size of the code produced, you can define which channels are specifically supported. See Analog/Digital Conversion Code Optimisation for more details.

For the latest Microchip PIC microcontrollers that support differential and 12-bit A/D, refer to Microchip MPLAB Code Configurator (MCC) or the microcontroller’s datasheet.

See Also: