PIC16C717-I/SS View Datasheet(PDF) - Microchip Technology
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PIC16C717-I/SS Datasheet PDF : 200 Pages
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PIC16C717/770/771
11.3 Selecting the A/D Conversion Clock
The A/D conversion cycle requires 13TAD: 1 TAD for set-
tling time, and 12 TAD for conversion. The source of the
A/D conversion clock is software selected. If neither the
internal VRH nor VRL are used for the A/D converter,
the four possible options for TAD are:
• 2 TOSC
• 8 TOSC
• 32 TOSC
• A/D RC oscillator
If the VRH or VRL are used for the A/D converter refer-
ence, then the TAD requirement is automatically
increased by a factor of 8.
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of 1.6 µs. Table 11-1 shows the resultant TAD times
derived from the device operating frequencies and the
A/D clock source selected.
The ADIF bit is set on the rising edge of the 14th TAD.
The GO/DONE bit is cleared on the falling edge of the
14th TAD.
TABLE 11-1: TAD vs. DEVICE OPERATING FREQUENCIES
A/D Reference
Source
A/D Clock Source (TAD)
Device Frequency
Operation ADCS<1:0>
20 MHz
5 MHz
4 MHz
1.25 MHz
2 TOSC
External VREF or
Analog Supply
8 TOSC
32 TOSC
A/D RC
Internal VRH or 16 TOSC
VRL
64 TOSC
256 TOSC
A/D RC
00
100 ns(2)
400 ns(2)
500 ns(2)
1.6 µs
01
800 ns(2)
1.6 µs
2.0 µs
6.4 µs
10
1.6 µs
6.4 µs
8.0 µs(3)
24 µs(3)
11
2 - 6 µs(1,4)
2 - 6 µs(1,4)
2 - 6 µs(1,4)
2 - 6 µs(1,4)
00
800 ns(2)
3.2 µs(2)
4 µs(2)
12.8 µs
01
6.4 µs(2)
12.8 µs
16 µs
51.2 µs
10
12.8 µs
51.2 µs
64 µs(3)
192 µs(3)
11
16 - 48 µs(4,5) 16 - 48 µs(4,5) 16 - 48 µs(4,5) 16 - 48 µs(4,5)
Legend: Shaded cells are outside of recommended range.
Note 1: The A/D RC source has a typical TAD time of 4 µs for VDD > 3.0V.
2: These values violate the minimum required TAD time.
3: For faster conversion times, the selection of another clock source is recommended.
4: When the device frequency is greater than 1 MHz, the A/D RC clock source is only recommended if the conversion will be
performed during sleep.
5: The resource has a typical TAD time of 32 µs for VDD > 3.0V.
DS41120A-page 118
Advanced Information
© 1999 Microchip Technology Inc.
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