PIC16C71X
7.2
Selecting the A/D Conversion Clock
7.3
Configuring Analog Port Pins
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 9.5TAD per 8-bit conversion.
The source of the A/D conversion clock is software
selectable. The four possible options for TAD are:
The ADCON1 and TRISA registers control the opera-
tion of the A/D port pins. The port pins that are desired
as analog inputs must have their corresponding TRIS
bits set (input). If the TRIS bit is cleared (output), the
digital output level (VOH or VOL) will be converted.
• 2TOSC
The A/D operation is independent of the state of the
CHS2:CHS0 bits and the TRIS bits.
• 8TOSC
• 32TOSC
• Internal RC oscillator
Note 1: When reading the port register, all pins
configured as analog input channels will
read as cleared (a low level). Pins config-
ured as digital inputs, will convert an ana-
log input. Analog levels on a digitally
configured input will not affect the conver-
sion accuracy.
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of:
2.0 µs for the PIC16C71
1.6 µs for all other PIC16C71X devices
Note 2: Analog levels on any pin that is defined as
a digital input (including the AN7:AN0
pins), may cause the input buffer to con-
sume current that is out of the devices
specification.
Table 7-1 and Table 7-2 and show the resultant TAD
times derived from the device operating frequencies
and the A/D clock source selected.
TABLE 7-1:
TAD vs. DEVICE OPERATING FREQUENCIES, PIC16C71
AD Clock Source (TAD)
Device Frequency
Operation
2TOSC
ADCS1:ADCS0
20 MHz
16 MHz
4 MHz
1 MHz
2.0 µs
8.0 µs
333.33 kHz
(2)
(2)
(2)
00
01
10
11
6 µs
100 ns
125 ns
500 ns
(2)
(2)
(3)
8TOSC
2.0 µs
400 ns
500 ns
24 µs
(2)
(3)
(3)
32TOSC
2.0 µs
8.0 µs
1.6 µs
32.0 µs
2 - 6 µs
96 µs
(5)
(1,4)
(1,4)
(1,4)
(1)
(1)
RC
2 - 6 µs
2 - 6 µs
2 - 6 µs
2 - 6 µs
Legend: Shaded cells are outside of recommended range.
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 device frequency is greater than 1 MHz, the RC A/D conversion clock source is recommended for
sleep operation only.
5: For extended voltage devices (LC), please refer to Electrical Specifications section.
TABLE 7-2:
TAD vs. DEVICE OPERATING FREQUENCIES, PIC16C710/711, PIC16C715
Device Frequency
5 MHz 1.25 MHz
AD Clock Source (TAD)
Operation
2TOSC
ADCS1:ADCS0
20 MHz
333.33 kHz
(2)
(2)
00
01
10
11
1.6 µs
6.4 µs
6 µs
100 ns
400 ns
(2)
(3)
8TOSC
1.6 µs
400 ns
24 µs
(3)
(3)
32TOSC
1.6 µs
6.4 µs
25.6 µs
96 µs
(5)
(1,4)
(1,4)
(1,4)
(1)
RC
2 - 6 µs
2 - 6 µs
2 - 6 µs
2 - 6 µs
Legend: Shaded cells are outside of recommended range.
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 device frequency is greater than 1 MHz, the RC A/D conversion clock source is recommended for
sleep operation only.
5: For extended voltage devices (LC), please refer to Electrical Specifications section.
1997 Microchip Technology Inc.
DS30272A-page 41
MICROCHIP [ MICROCHIP ]
