
1995 Microchip Technology Inc.
DS30390B-page 115
PIC16C7X
13.2
Selecting the A/D Conversion Clock
Applicable Devices
70 71 71A 72 73 73A 74 74A
The A/D conversion time per bit is defined as T
AD
. The
A/D conversion requires 9.5 T
AD
per 8-bit conversion.
The source of the A/D conversion clock is software
selected. The four possible options for T
AD
are:
2T
OSC
8T
OSC
32T
OSC
Internal RC oscillator
For correct A/D conversions, the A/D conversion clock
(T
AD
) must be selected to ensure a minimum T
AD
time
of:
2.0
μ
s for the PIC16C71
1.6
μ
s for all other PIC16C7X devices
Table 13-2 and Table 13-1 show the resultant T
AD
times derived from the device operating frequencies
and the A/D clock source selected.
13.3
Configuring Analog Port Pins
Applicable Devices
70 71 71A 72 73 73A 74 74A
The ADCON1, TRISA, and TRISE registers control the
operation of the A/D port pins. The port pins that are
desired as analog inputs must have their correspond-
ing TRIS bits set (input). If the TRIS bit is cleared (out-
put), the digital output level (V
OH
or V
OL
) 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, all pins
configured as 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.
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 13-1:
T
AD
vs. DEVICE OPERATING FREQUENCIES, PIC16C71
TABLE 13-2:
T
AD
vs. DEVICE OPERATING FREQUENCIES, PIC16C70/71A/72/73/73A/74/74A
AD Clock Source (T
AD
)
Device Frequency
Operation
2T
OSC
ADCS1:ADCS0
20 MHz
16 MHz
4 MHz
1 MHz
2.0
μ
s
8.0
μ
s
333.33 kHz
6
μ
s
00
100 ns
(2)
400 ns
(2)
1.6
μ
s
(2)
2 - 6
μ
s
(1,4)
125 ns
(2)
500 ns
(2)
2.0
μ
s
500 ns
(2)
2.0
μ
s
8.0
μ
s
8T
OSC
01
24
μ
s
(3)
96
μ
s
(3)
2 - 6
μ
s
(1)
32T
OSC
10
32.0
μ
s
(3)
2 - 6
μ
s
(1)
RC
Note 1: The RC source has a typical T
AD
time of 4
μ
s.
2: These values violate the minimum required T
AD
time.
3: For faster conversion times, the selection of another clock source is recommended.
4: While in RC mode, with device frequency above 1 MHz, conversion accuracy is out of specification.
11
2 - 6
μ
s
(1,4)
2 - 6
μ
s
(1,4)
AD Clock Source (T
AD
)
Device Frequency
Operation
2T
OSC
ADCS1:ADCS0
20 MHz
5 MHz
1.25 MHz
1.6
μ
s
6.4
μ
s
333.33 kHz
6
μ
s
00
100 ns
(2)
400 ns
(2)
1.6
μ
s
400 ns
(2)
1.6
μ
s
6.4
μ
s
8T
OSC
01
24
μ
s
(3)
96
μ
s
(3)
2 - 6
μ
s
(1)
32T
OSC
10
25.6
μ
s
(3)
2 - 6
μ
s
(1,4)
RC
Note 1: The RC source has a typical T
AD
time of 4
μ
s.
2: These values violate the minimum required T
AD
time.
3: For faster conversion times, the selection of another clock source is recommended.
4: While in RC mode, with device frequency above 1 MHz, conversion accuracy is out of specification.
11
2 - 6
μ
s
(1,4)
2 - 6
μ
s
(1,4)