PIC16F870/871
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of 1.6 µs.
10.2
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:
Table 10-1shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
• 2TOSC
• 8TOSC
• 32TOSC
• Internal RC oscillator
TABLE 10-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))
AD Clock Source (TAD)
ADCS1:ADCS0
Maximum Device Frequency
Max.
Operation
2TOSC
8TOSC
00
01
10
11
1.25 MHz
5 MHz
32TOSC
RC(1, 2, 3)
20 MHz
Note 1
Note 1: The RC source has a typical TAD time of 4 µs but can vary between 2-6 µs.
2: When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only recommended for sleep
operation.
3: For extended voltage devices (LC), please refer to the Electrical Specifications section.
10.3
Configuring Analog Port Pins
The ADCON1, and TRIS registers control the operation
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.
The A/D operation is independent of the state of the
CHS2:CHS0 bits and the TRIS bits.
Note 1: When reading the port register, any pin
configured as an analog input channel 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.
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 device
specifications.
DS30569A-page 84
Preliminary
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
