dsPIC30F Family Reference Manual
DS70065C-page 18-18 © 2004 Microchip Technology Inc.
18.11.3.4 Synchronizing A/D Operations to Internal or External Events
The modes where an external event trigger pulse ends sampling and starts conversion
(SSRC = 001, 010, 011) may be used in combination with auto sampling (ASAM = 1) to cause
the A/D to synchronize the sample conversion events to the trigger pulse source. For example,
in Figure 18-8 where SSRC = 010 and ASAM = 1, the A/D will always end sampling and start
conversions synchronously with the timer compare trigger event. The A/D will have a sample
conversion rate that corresponds to the timer comparison event rate.
Figure 18-7: Manual Sample Start, Conversion Trigger Based Conversion Start
Figure 18-8: Auto-Sample Start, Conversion Trigger Based Conversion Start
18.11.3.5 Sample Time Considerations for Automatic Sampling/Conversion Sequences
Different sample/conversion sequences provide different available sampling times for the S/H
channel to acquire the analog signal. The user must ensure the sampling time exceeds the
sampling requirements, as outlined in Section 18.15 “A/D Sampling Requirements”.
Assuming that the module is set for automatic sampling and an external trigger pulse is used as
the conversion trigger, the sampling interval is a portion of the trigger pulse interval.
The sampling time is the trigger pulse period, less the time required to complete the conversion.
Equation 18-3: Available Sampling Time, Sequential Sampling
Conversion Trigger
ADCLK
SAMP
ADCBUF0
TSAMP
TCONV
BSF ADCON1,SAMP
Instruction Execution
Conversion Trigger
ADCLK
SAMP
ADCBUF0
T
SAMP
T
CONV
BSF ADCON1,ASAM
Instruction Execution
T
CONV
T
SAMP
ADCBUF1
DONE
Reset by
software
TSMP = Trigger Pulse Interval (TSEQ) – Conversion Time (TCONV)
T
SMP =TSEQ – TCONV
Note:T
SEQ is the trigger pulse interval time.
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