PIC16F631/677/685/687/689/690
9.1.3
ADC VOLTAGE REFERENCE
9.1
ADC Configuration
The VCFG bit of the ADCON0 register provides control
of the positive voltage reference. The positive voltage
reference can be either VDD or an external voltage
source. The negative voltage reference is always
connected to the ground reference.
When configuring and using the ADC the following
functions must be considered:
• Port configuration
• Channel selection
• ADC voltage reference selection
• ADC conversion clock source
• Interrupt control
9.1.4
CONVERSION CLOCK
The source of the conversion clock is software
selectable via the ADCS bits of the ADCON1 register.
There are seven possible clock options:
• Results formatting
9.1.1
PORT CONFIGURATION
• FOSC/2
The ADC can be used to convert both analog and digital
signals. When converting analog signals, the I/O pin
should be configured for analog by setting the associated
TRIS and ANSEL bits. See the corresponding port
section for more information.
• FOSC/4
• FOSC/8
• FOSC/16
• FOSC/32
• FOSC/64
Note:
Analog voltages on any pin that is defined
as a digital input may cause the input
buffer to conduct excess current.
• FRC (dedicated internal oscillator)
The time to complete one bit conversion is defined as
TAD. One full 10-bit conversion requires 11 TAD periods
as shown in Figure 9-2.
9.1.2
CHANNEL SELECTION
For correct conversion, the appropriate TAD specification
must be met. See A/D conversion requirements in
Section 17.0 “Electrical Specifications” for more
information. Table 9-1 gives examples of appropriate
ADC clock selections.
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. Refer to Section 9.2
“ADC Operation” for more information.
Note:
Unless using the FRC, any changes in the
system clock frequency will change the
ADC clock frequency, which may
adversely affect the ADC result.
DS41262D-page 108
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
