MCP7940MIP datasheet(13/38 Pages) MICROCHIP

13 / 38 page

 2012 Microchip Technology Inc.

Preliminary

DS22292A-page 13

MCP7940M

With bits RS1 and RS0 set to ‘00’, the calibration

function can be expressed as:

Since the calibration is done once per minute (i.e.,

when the internal minute counter is incremented), only

one cycle in sixty of the MFP output waveform is

affected by the calibration setting. Also note that the

duty cycle of the MFP output waveform will not

necessarily be at 50% when the calibration setting is

applied.

With bits RS1 and RS0 set to ‘01’ or ‘10’, the

calibration function can not be expressed in terms of

the input clock period. In the case where the MSB of

the Calibration register is set to ‘0’, the waveform

appearing at the MFP output pin will be “delayed”,

once per minute, by twice the number of input clock

cycles defined in the Calibration register. The MFP

waveform will appear as:

FIGURE 4-2:

RS1 AND RS0 WITH AND WITHOUT CALIBRATION

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.

Toutput

(32768 +/- (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.

Delay

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.

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