Datasheet

 1997 Microchip Technology Inc. DS30444E - page 87
PIC16C9XX
12.8 Use of the CCP Trigger
An A/D conversion can be started by the “special event
trigger” of the CCP1 module. This requires that the
CCP1M3:CCP1M0 bits (CCP1CON<3:0>) be pro-
grammed as 1011 and 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).
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.
12.9 Connection Considerations
If the input voltage exceeds the rail values (VSS or VDD)
by greater than 0.2V, then the accuracy of the conver-
sion is out of specification.
An external RC filter is sometimes added for anti-alias-
ing of the input signal. The R component should be
selected to ensure that the total source impedance is
kept under the 10 kΩ recommended specification. Any
external components connected (via hi-impedance) to
an analog input pin (capacitor, zener diode, etc.) should
have very little leakage current at the pin.
12.10 Transfer Function
The ideal transfer function of the A/D converter is as fol-
lows: the first transition occurs when the analog input
voltage (V
AIN) is Analog VREF / 256 (Figure 12-5).
FIGURE 12-5: A/D TRANSFER FUNCTION
Digital code output
FFh
FEh
04h
03h
02h
01h
00h
0.5 LSb
1 LSb 2 LSb 3 LSb 4 LSb
255 LSb 256 LSb
(full scale)
Analog input voltage