PIC17C7XX Datasheet, PDF (183/328 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers with 10-bit A/D

PIC17C7XX
16.2 Selecting the A/D Conversion Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires a minimum 12TAD per 10-bit
conversion. The source of the A/D conversion clock is
software selected. The four possible options for TAD
are:
• 8TOSC
• 32TOSC
• 64TOSC
• Internal RC oscillator
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 16-1 and Table 16-2 show the resultant TAD
times derived from the device operating frequencies
and the A/D clock source selected. These times are for
standard voltage range devices.
TABLE 16-1: TAD vs. DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))
AD Clock Source (TAD)
Device Frequency
Operation
ADCS1:ADCS0
33 MHz
20 MHz
5 MHz
1.25 MHz 333.33 kHz
8TOSC
00
242 ns⁽²⁾
400 ns⁽²⁾
1.6 µs
6.4 µs
24 µs
32TOSC
01
970 ns⁽²⁾
1.6 µs
6.4 µs
25.6 µs⁽³⁾
96 µs⁽³⁾
64TOSC
10
1.94 µs
3.2 µs
12.8 µs⁽³⁾
51.2 µs⁽³⁾
192 µs⁽³⁾
RC
11
2 - 6 µs^{(1, 4)}
2 - 6 µs^{(1, 4)} 2 - 6 µs^{(1, 4)} 2 - 6 µs^{(1, 4)}
2 - 6 µs⁽¹⁾
Legend: Shaded cells are are outside of recommended ranges.
Note 1: The RC source has a typical TAD time of 4 µs.
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 frequencies is greater than 1 MHz, the RC A/D conversion clock source is only recom-
mended for sleep operation.
TABLE 16-2: TAD vs. DEVICE OPERATING FREQUENCIES (EXTENDED VOLTAGE DEVICES (LC))
AD Clock Source (TAD)
Device Frequency
Operation
ADCS1:ADCS0
8 MHz
4 MHz
2 MHz
1 MHz
333.33 kHz
8TOSC
00
1.0 µs⁽²⁾
2.0 µs⁽²⁾
4 µs
8 µs
24 µs
32TOSC
01
4.0 µs
8 µs
16 µs
32 µs⁽³⁾
96 µs⁽³⁾
64TOSC
10
8.0 µs
16 µs
32 µs⁽³⁾
64 µs⁽³⁾
192 µs⁽³⁾
RC
11
3 - 9 µs^{(1, 4)}
3 - 9 µs^{(1, 4)} 3 - 9 µs^{(1, 4)} 3 - 9 µs⁽¹⁾
3 - 9 µs⁽¹⁾
Legend: Shaded cells are are outside of recommended ranges.
Note 1: The RC source has a typical TAD time of 6 µs.
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 frequencies is greater than 1 MHz, the RC A/D conversion clock source is only recom-
mended for sleep operation.
© 1998 Microchip Technology Inc.
DS30289A-page 183