PIC16C63A/65B/73B/74B
12.2 Selecting the A/D Conversion
Clock
12.5 A/D Operation During SLEEP
The A/D module can operate during SLEEP mode. This
requires that the A/D clock source be set to RC
(ADCS1:ADCS0 = 11). When the RC clock source is
selected, the A/D module waits one instruction cycle
before starting the conversion. This allows the SLEEP
instruction to be executed, which eliminates all digital
switching noise from the conversion. When the conver-
sion is completed, the GO/DONE bit will be cleared,
and the result loaded into the ADRES register. If the
A/D interrupt is enabled, the device will wake-up from
SLEEP. If the A/D interrupt is not enabled, the A/D mod-
ule will then be turned off, although the ADON bit will
remain set.
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 9.5 TAD per 8-bit conversion.
The source of the A/D conversion clock is software
selectable. The four possible options for TAD are:
• 2 TOSC
• 8 TOSC
• 32 TOSC
• Internal RC oscillator (2 - 6 µS)
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
(parameter #130).
When the A/D clock source is another clock option (not
RC), a SLEEPinstruction will cause the present conver-
sion to be aborted and the A/D module to be turned off,
though the ADON bit will remain set.
12.3 Configuring Analog Port Pins
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 (VOH or VOL) will be
converted.
Turning off the A/D places the A/D module in its lowest
current consumption state.
Note: For the A/D module to operate in SLEEP,
the A/D clock source must be set to RC
(ADCS1:ADCS0 = 11). To perform an A/D
conversion in SLEEP, ensure the SLEEP
instruction immediately follows the instruc-
tion that sets the GO/DONE bit.
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 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.
12.6 Effects of a RESET
A device RESET forces all registers to their RESET
state. The A/D module is disabled and any conversion
in progress is aborted. All pins with analog functions
are configured as analog inputs.
2: Analog levels on any pin that is defined as
a digital input, but not as an analog input,
may cause the input buffer to consume
current that is out of the devices specifi-
cation.
The ADRES register will contain unknown data after a
Power-on Reset.
12.7 Use of the CCP Trigger
3: The TRISE register is not provided on the
An A/D conversion can be started by the “special event
trigger” of the CCP2 module. This requires that the
CCP2M3:CCP2M0 bits (CCP2CON<3:0>) be pro-
grammed as 1011and that the A/D module is enabled
(ADON bit is set). When the trigger occurs, the
GO/DONE bit will be set, starting the A/D conversion,
and the Timer1 counter will be reset to zero. Timer1 is
reset to automatically repeat the A/D acquisition period
with minimal software overhead (moving the ADRES to
the desired location). The appropriate analog input
channel must be selected and the minimum acquisition
done before the “special event trigger” sets the
GO/DONE bit (starts a conversion).
PIC16C73B.
12.4 A/D Conversions
Note: The GO/DONE bit should NOT be set in
the same instruction that turns on the A/D.
Clearing the GO/DONE bit during a conversion will
abort the current conversion. The ADRES register will
NOT be updated with the partially completed A/D con-
version sample. That is, the ADRES register will con-
tinue to contain the value of the last completed
conversion (or the last value written to the ADRES reg-
ister). After the A/D conversion is aborted, a 2 TAD wait
is required before the next acquisition is started. After
this 2 TAD wait, an acquisition is automatically started
on the selected channel. The GO/DONE bit can then
be set to start another conversion.
If the A/D module is not enabled (ADON is cleared),
then the “special event trigger” will be ignored by the
A/D module, but will still reset the Timer1 counter.
2000 Microchip Technology Inc.
DS30605C-page 83
MICROCHIP [ MICROCHIP ]
