MCP79411 datasheet(15/38 Pages) MICROCHIP

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 2011 Microchip Technology Inc.

Preliminary

DS22266B-page 15

MCP7941X

In the case where the MSB of the Calibration register

is set to ‘1’, the MFP output waveforms that appear

when bits RS1 and RS0 are set to ‘01’ or ‘10’ are not

as responsive to the setting of the Calibration register.

For

example,

when

outputting

the

4.096

kHz

waveform (RS1, RS0 set to ‘01’), the output waveform

is generated using only eight input clock cycles.

Consequently, attempting to subtract more than eight

input clock cycles from this output does not have a

meaningful effect on the resulting waveform. Any

effect on the output will appear as a modification in

both the frequency and duty cycle of the waveform

appearing on the MFP output pin.

B.RS2 BIT SET TO ‘1’

With the RS2 bit set to ‘1’, the following internal timing

signal is output on the MFP pin:

The frequency listed in the table presumes an input

clock source of exactly 32.768 kHz. In terms of the

equivalent number of input clock cycles, the table

becomes:

Unlike

the

method

previously

described,

the

calibration setting is continuously applied and affects

every cycle of the output waveform. This results in the

modulation of the frequency of the output waveform

based upon the setting of the Calibration register.

Using this setting, the calibration function can be

expressed as:

Since the calibration is done every cycle, the frequency

of the output MFP waveform is constant.

4.2.4

MFP

Pin 7 is a multi-function pin and supports the following

functions:

• Use of the OUT bit in the Control register for

single bit I/O

• Alarm Outputs – Available in VBAT mode

• FOUT mode – driven from a FOSC divider – Not

available in VBAT mode

The internal control logic for the MFP is connected to

the switched internal supply bus, this allows operation

in VBAT mode. The Alarm Output is the only mode that

operates in VBAT mode, other modes are suspended.

4.2.5

VBAT

If the VBAT feature is not being used, the VBAT pin

should be connected to GND. A low-value series

resistor is recommended between the external battery

and the VBAT pin.

The VBAT point is defined as 1.5V typical. When VDD

falls below 1.5V the system will continue to operate

the RTCC and SRAM using the VBAT supply. The

following conditions apply:

4.2.6

CRYSTAL SPECS

The MCP7941X has been designed to operate with a

standard 32 kHz crystal. Devices with a specified load

capacitance of either 12pF or 6pF can be used. The

end user should fully validate the chosen crystal across

all the expected design parameters of the system to

ensure correct operation.

The following crystals have been tested and shown to

work with the MCP7941X:

• CM200S 12pF surface mount crystals from

Citizen

• ECS-.327 12pF surface mount crystals from ECS

INC

• CFS206 12pF leaded crystals from Citizen

This is not a definitive list and all crystals should be

tested in the target application across all temperature,

voltage and other significant environmental conditions.

RS2

RS1

RS0

Output Signal

64.0 Hz

RS2

RS1

RS0

Output Signal

512

Toutput

(2 * (256 +/- (2 * CALREG))) Tinput

where:

Toutput

clock period of MFP output signal

Tinput

clock period of input signal

CALREG

decimal value of the Calibration

determined by the MSB of the

Calibration register.

TABLE 4-2:

Supply

Condition

Read/Write

Access

Powered

VCC < VTRIP, VCC < VBAT

VBAT

VCC > VTRIP, VCC < VBAT

Yes

VCC

VCC > VTRIP, VCC > VBAT

Yes

VCC

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