PIC16F627A_05 Datasheet, PDF (70/180 Pages) Microchip Technology – Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F627A/628A/648A
FIGURE 11-1:
VREN
VOLTAGE REFERENCE BLOCK DIAGRAM
VDD
16 Stages
8R
R
R
R
R
VREF
Note: R is defined in Table 17-3.
16-1 Analog Mux
8R
VRR
VSS
VSS
VR3
(From VRCON<3:0>)
VR0
EXAMPLE 11-1: VOLTAGE REFERENCE
CONFIGURATION
MOVLW
MOVWF
BSF
MOVLW
MOVWF
MOVLW
MOVWF
BCF
CALL
0x02
;4 Inputs Muxed
CMCON
;to 2 comps.
STATUS,RP0 ;go to Bank 1
0x07
;RA3-RA0 are
TRISA
;outputs
0xA6
;enable VREF
VRCON
;low range set VR<3:0>=6
STATUS,RP0 ;go to Bank 0
DELAY10
;10μs delay
11.2 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 11-1) keep VREF from approaching VSS or VDD.
The Voltage Reference module is VDD derived and
therefore, the VREF output changes with fluctuations in
VDD. The tested absolute accuracy of the Voltage Ref-
erence module can be found in Table 17-3.
11.3 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 module should be disabled.
11.4 Effects of a Reset
A device Reset disables the Voltage Reference module
by clearing 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.
11.5 Connection Considerations
The Voltage 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 module output onto
the RA2 pin with an input signal present will increase
current consumption. Connecting RA2 as a digital output
with VREF enabled will also increase current consump-
tion.
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 module output for external connec-
tions to VREF. Figure 11-2 shows an example buffering
technique.
DS40044D-page 68
© 2005 Microchip Technology Inc.