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PIC18F2455/2550/4455/4550

FIGURE 23-1: COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM

CVRSS = 1

VREF+

VDD

 

 

 

 

 

 

 

 

 

 

 

CVRSS = 0

 

 

 

8R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CVREN

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

16 Steps

R

R

R

CVRR

 

 

 

 

 

 

8R

 

 

 

 

 

 

VREF-

 

CVRSS = 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CVR3:CVR0

-1 MUX

 

CVREF

 

16-to

 

 

CVRSS = 0

23.2Voltage Reference Accuracy/Error

The full range of voltage reference cannot be realized due to the construction of the module. The transistors on the top and bottom of the resistor ladder network (Figure 23-1) keep CVREF from approaching the reference source rails. The voltage reference is derived from the reference source; therefore, the CVREF output changes with fluctuations in that source. The tested absolute accuracy of the voltage reference can be found in Section 28.0 “Electrical Characteristics”.

23.3Operation During Sleep

When the device wakes up from Sleep through an interrupt or a Watchdog Timer time-out, the contents of the CVRCON register are not affected. To minimize current consumption in Sleep mode, the voltage reference should be disabled.

23.4Effects of a Reset

A device Reset disables the voltage reference by clearing bit, CVREN (CVRCON<7>). This Reset also disconnects the reference from the RA2 pin by clearing bit, CVROE (CVRCON<6>) and selects the high-voltage range by clearing bit, CVRR (CVRCON<5>). The CVR value select bits are also cleared.

23.5Connection 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 and the CVROE bit are both set. Enabling the voltage reference output onto RA2 when it is configured as a digital input will increase current consumption. Connecting RA2 as a digital output with CVRSS enabled will also increase current consumption.

The RA2 pin can be used as a simple D/A output with limited drive capability. Due to the limited current drive capability, a buffer must be used on the voltage reference output for external connections to VREF. Figure 23-2 shows an example buffering technique.

DS39632D-page 276

Preliminary

2007 Microchip Technology Inc.

PIC18F2455/2550/4455/4550

FIGURE 23-2: COMPARATOR VOLTAGE REFERENCE OUTPUT BUFFER EXAMPLE

PIC18FXXXX

 

CVREF

R(1)

 

Module

 

 

 

 

Voltage

RA2

 

Reference

 

 

Output

 

 

Impedance

 

+

CVREF Output

Note 1: R is dependent upon the voltage reference configuration bits, CVRCON<5> and CVRCON<3:0>.

TABLE 23-1:

REGISTERS ASSOCIATED WITH COMPARATOR VOLTAGE REFERENCE

 

 

 

 

 

 

 

 

 

 

 

Reset

Name

 

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Values

 

 

 

 

 

 

 

 

 

 

on page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CVRCON

 

CVREN

CVROE

CVRR

CVRSS

CVR3

CVR2

CVR1

CVR0

53

 

 

 

 

 

 

 

 

 

 

 

CMCON

 

C2OUT

C1OUT

C2INV

C1INV

CIS

CM2

CM1

CM0

53

TRISA

 

TRISA6(1)

TRISA5

TRISA4

TRISA3

TRISA2

TRISA1

TRISA0

54

Legend: Shaded cells are not used with the comparator voltage reference.

Note 1: PORTA<6> and its direction and latch bits are individually configured as port pins based on various oscillator modes. When disabled, these bits read as ‘0’.

2007 Microchip Technology Inc.

Preliminary

DS39632D-page 277

PIC18F2455/2550/4455/4550

NOTES:

DS39632D-page 278

Preliminary

2007 Microchip Technology Inc.

PIC18F2455/2550/4455/4550

24.0HIGH/LOW-VOLTAGE DETECT (HLVD)

PIC18F2455/2550/4455/4550 devices have a High/Low-Voltage Detect module (HLVD). This is a programmable circuit that allows the user to specify both a device voltage trip point and the direction of change from that point. If the device experiences an excursion past the trip point in that direction, an interrupt flag is set. If the interrupt is enabled, the program execution will branch to the interrupt vector address and the software can then respond to the interrupt.

The High/Low-Voltage Detect Control register (Register 24-1) completely controls the operation of the HLVD module. This allows the circuitry to be “turned off” by the user under software control which minimizes the current consumption for the device.

The block diagram for the HLVD module is shown in Figure 24-1.

REGISTER 24-1: HLVDCON: HIGH/LOW-VOLTAGE DETECT CONTROL REGISTER

R/W-0

 

U-0

R-0

R/W-0

R/W-0

R/W-1

R/W-0

R/W-1

 

 

 

 

 

 

 

 

 

 

 

 

VDIRMAG

 

IRVST

 

HLVDEN

HLVDL3(1)

 

HLVDL2(1)

 

HLVDL1(1)

HLVDL0(1)

bit 7

 

 

 

 

 

 

 

 

 

 

bit 0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Legend:

 

 

 

 

 

 

 

 

 

 

 

R = Readable bit

 

W = Writable bit

 

U = Unimplemented bit, read as ‘0’

 

-n = Value at POR

‘1’ = Bit is set

 

‘0’ = Bit is cleared

x = Bit is unknown

 

 

 

 

 

 

bit 7

VDIRMAG: Voltage Direction Magnitude Select bit

 

 

 

 

 

1

= Event occurs when voltage equals or exceeds trip point (HLVDL3:HLDVL0)

 

 

0

= Event occurs when voltage equals or falls below trip point (HLVDL3:HLVDL0)

 

bit 6

Unimplemented: Read as ‘0

 

 

 

 

 

 

 

bit 5

IRVST: Internal Reference Voltage Stable Flag bit

 

 

 

 

 

1

= Indicates that the voltage detect logic will generate the interrupt flag at the specified voltage range

 

0

= Indicates that the voltage detect logic will not generate the interrupt flag at the specified voltage

 

 

range and the HLVD interrupt should not be enabled

 

 

 

 

bit 4

HLVDEN: High/Low-Voltage Detect Power Enable bit

 

 

 

 

 

1

= HLVD enabled

 

 

 

 

 

 

 

 

0

= HLVD disabled

 

 

 

 

 

 

 

bit 3-0

HLVDL3:HLVDL0: Voltage Detection Limit bits(1)

 

 

 

 

 

1111 = External analog input is used (input comes from the HLVDIN pin)

 

 

 

1110 = Maximum setting

 

 

 

 

 

 

 

.

.

.

0000 = Minimum setting

Note 1: See Table 28-6 in Section 28.0 “Electrical Characteristics” for specifications.

2007 Microchip Technology Inc.

Preliminary

DS39632D-page 279

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