PIC16C62X
EXAMPLE 8-1: VOLTAGE REFERENCE
CONFIGURATION
8.4
Effects of a Reset
A device reset disables the Voltage Reference by clear-
ing bit VREN (VRCON<7>). This reset also disconnects
the reference from the RA2 pin by clearing bit VROE
(VRCON<6>) and selects the high voltage range by
clearing bit VRR (VRCON<5>). The VREF value select
bits, VRCON<3:0>, are also cleared.
MOVLW
MOVWF
BSF
0x02
; 4 Inputs Muxed
; to 2 comps.
; go to Bank 1
; RA3-RA0 are
; outputs
CMCON
STATUS,RP0
0x07
MOVLW
MOVWF
MOVLW
MOVWF
TRISA
0xA6
; enable VREF
; low range
8.5
Connection Considerations
VRCON
; set VR<3:0>=6
; go to Bank 0
; 10µs delay
TheVoltage Reference Module operates independently
of the comparator module. The output of the reference
generator may be connected to the RA2 pin if the
TRISA<2> bit is set and the VROE bit, VRCON<6>, is
set. Enabling the Voltage Reference output onto the
RA2 pin with an input signal present will increase cur-
rent consumption. Connecting RA2 as a digital output
with VREF enabled will also increase current consump-
tion.
BCF
STATUS,RP0
DELAY10
CALL
8.2
Voltage Reference Accuracy/Error
The full range of VSS to VDD cannot be realized due to
the construction of the module. The transistors on the
top and bottom of the resistor ladder network
(Figure 8-2) keep VREF from approaching VSS or VDD.
The Voltage Reference is VDD derived and therefore,
the VREF output changes with fluctuations in VDD. The
tested absolute accuracy of the Voltage Reference can
be found in Table 12-3.
The RA2 pin can be used as a simple D/A output with
limited drive capability. Due to the limited drive
capability, a buffer must be used in conjunction with the
Voltage Reference output for external connections to
VREF. Figure 8-3 shows an example buffering
technique.
8.3
Operation During Sleep
When the device wakes up from sleep through an
interrupt or a Watchdog Timer time-out, the contents of
the VRCON register are not affected. To minimize
current consumption in SLEEP mode, the Voltage
Reference should be disabled.
FIGURE 8-3: VOLTAGE REFERENCE OUTPUT BUFFER EXAMPLE
(1)
RA2
R
VREF
Module
+
–
•
VREF Output
•
Voltage
Reference
Output
Impedance
Note 1: R is dependent upon the Voltage Reference Configuration VRCON<3:0> and VRCON<5>.
TABLE 8-1:
REGISTERS ASSOCIATED WITH VOLTAGE REFERENCE
Value On
All Other
Resets
Value On
POR
Address Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
9Fh
1Fh
85h
VRCON
VREN
VROE VRR
—
—
VR3
CIS
VR2
CM2
VR1
CM1
VR0
CM0
000- 0000 000- 0000
00-- 0000 00-- 0000
CMCON C2OUT C1OUT
TRISA
—
—
—
—
TRISA4 TRISA3 TRISA2 TRISA1 TRISA0 ---1 1111 ---1 1111
Note: - = Unimplemented, read as "0"
DS30235G-page 44
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
