Microchip Technology DM183037 Data Sheet
PIC18F97J94 FAMILY
DS30575A-page 456
2012 Microchip Technology Inc.
22.2
A/D Terminology and Conversion
Sequence
Sequence
Sample time is the time that the A/D module's S/H
amplifier is connected to the analog input pin. The sam-
ple time may be started and ended automatically by the
A/D Converter's hardware or under direct program con-
trol. There is a minimum sample time to ensure that the
S/H amplifier will give sufficient accuracy for the A/D
conversion.
The conversion trigger ends the sampling time and
begins an A/D conversion or a repeating sequence.
The conversion trigger sources can be taken from a
variety of hardware sources or can be controlled
directly in software. One of the conversion trigger
options is an auto-conversion, which uses a counter
and the A/D clock to set the time between auto-conver-
sions. The Auto-Sample mode and auto-conversion
trigger can be used together to provide continuous,
amplifier is connected to the analog input pin. The sam-
ple time may be started and ended automatically by the
A/D Converter's hardware or under direct program con-
trol. There is a minimum sample time to ensure that the
S/H amplifier will give sufficient accuracy for the A/D
conversion.
The conversion trigger ends the sampling time and
begins an A/D conversion or a repeating sequence.
The conversion trigger sources can be taken from a
variety of hardware sources or can be controlled
directly in software. One of the conversion trigger
options is an auto-conversion, which uses a counter
and the A/D clock to set the time between auto-conver-
sions. The Auto-Sample mode and auto-conversion
trigger can be used together to provide continuous,
automatic conversions without software intervention.
When automatic sampling is used, an extended sam-
pling interval is extended between the time the sam-
pling ends and the conversion starts.
Conversion time is the time required for the A/D
Converter to convert the voltage held by the S/H ampli-
fier. An A/D conversion requires one A/D clock cycle
(TAD) to convert each bit of the result, plus two addi-
tional clock cycles, or a total of 14 TAD cycles for a
12-bit conversion. When the conversion is complete,
the result is loaded into one of the A/D result buffers.
The S/H can be reconnected to the input pin and a CPU
interrupt may be generated. The sum of the sample
time(s) and the A/D conversion time provides the total
A/D sequence time.
When automatic sampling is used, an extended sam-
pling interval is extended between the time the sam-
pling ends and the conversion starts.
Conversion time is the time required for the A/D
Converter to convert the voltage held by the S/H ampli-
fier. An A/D conversion requires one A/D clock cycle
(TAD) to convert each bit of the result, plus two addi-
tional clock cycles, or a total of 14 TAD cycles for a
12-bit conversion. When the conversion is complete,
the result is loaded into one of the A/D result buffers.
The S/H can be reconnected to the input pin and a CPU
interrupt may be generated. The sum of the sample
time(s) and the A/D conversion time provides the total
A/D sequence time.
shows the basic
conversion sequence and the relationship between
intervals.
intervals.
FIGURE 22-2:
A/D SAMPLE/CONVERT SEQUENCE
Sample Time
A/D Conversion Time
Total A/D Sequence Time
S/H amplifier is connected to
the analog input pin for sampling.
the analog input pin for sampling.
Input disconnected; S/H amplifier holds signal.
Conversion trigger starts A/D conversion.
Conversion trigger starts A/D conversion.
Conversion complete, result is loaded
into A/D Buffer register.
Interrupt is generated (optional).
into A/D Buffer register.
Interrupt is generated (optional).
Extended Sampling Time
(1)
Sampling ends (manual or
automatic trigger).
automatic trigger).
Total A/D Sample Time
Note 1: In Automatic Sampling modes, Extended Sampling Time is added to the sequence when the value for the
Auto-Sampling Time is greater than 0. Otherwise, sampling ends and conversion starts whenever the SAMP bit is
cleared.
cleared.