User manual

Robotics experiment with PIC microcontroller l 61

5.3 ADC configuration

For using the ADC module of PIC16F887 microcontroller the following functions

must be considered:

Port configuration

Channel selection

ADC voltage reference selection

ADC conversion clock source

Results formatting

5.4.1 Port configuration

The ADC can be used to convert both analog and digital signals. When convert-

ing analog signals, the I/O pin should be configured for analog by setting the associated

TRIS and ANSEL bits.

5.4.2 Channel selection

The CHS bits of the ADCON0 register determine which channel is connected to the

sample and hold circuit. When changing channels, a delay is required before starting the

next conversion.

5.4.3 ADC Voltage reference

The VCFG bits of the ADCON0 register provide independent control of the positive

and negative voltage references. The positive voltage reference can be either Vdd or

an external voltage source. Likewise, the negative voltage reference can be either Vss or

an external voltage source.

For the RBX-877V2.0 Robot Controller board will select the positive reference to

+5V and negative reference at ground or Vss.

5.4.4 Conversion Clock

The source of the conversion clock is software selectable via the ADCS bits of the

ADCON0 register. There are four possible clock options:

OSC

/2 : for 20MHz clock, T

= 100ns

OSC

/8 : for 20MHz clock, T

= 400ns

OSC

/32 : for 20MHz clock, T

= 1.6µs

(dedicated internal oscillator) : T

= 2 to 6µs

The time to complete one bit conversion is defined as T

. One full 10-bit conver-

sion requires 11 T

periods.