PIC14000-04 Microchip Technology, PIC14000-04 Datasheet - Page 61

• Manufacturer Part Number: PIC14000-04
• Description: 28-Pin Programmable Mixed Signal Controller
• Manufacturer: Microchip Technology

8.6
Four configuration bits (ADCON1<7:4>) are used to
control a programmable current source for generating
the ramp voltage to the A/D comparator. It allows com-
pensation for full-scale input voltage, clock frequency
and CDAC capacitor tolerance variations. The current
values range from 0 to 33.75 A (nominal) in 2.25 A
increments. The intermediate values of the current
source are as follows:
TABLE 8-2:
1996 Microchip Technology Inc.
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
ADCON1<7:4>
Programmable Current Source
0
1
0
0
0
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
PROGRAMMABLE CURRENT
SOURCE SELECTION
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
OFF - all current
sources disabled
2.25 A
4.5 A
6.75 A
9 A
11.25 A
13.5 A
15.75 A
18 A
20.25 A
22.5 A
24.75 A
27 A
29.25 A
31.5 A
33.75 A
Current Source
Output
Preliminary
The programmable current source output is tied to the
CDAC pin and is used to charge an external capacitor
to generate the ramp voltage for the A/D comparator.
(Refer to Figure 8-1.) This capacitor should have a low
voltage-coefficient as found in teflon, polypropylene, or
polystyrene capacitors, for optimum results. The
capacitor must be discharged at the beginning of each
conversion cycle by asserting ADRST (ADCON0<1>)
for at least 200
Asserting ADRST disables the current sources inter-
nally. Current flow begins when ADRST is cleared.
s to allow a complete discharge.
PIC14000
DS40122B-page 61