MCP79400_13 datasheet(15/42 Pages) MICROCHIP

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DS25009C-page 15

MCP7940X

5.2.2

CALIBRATION

The MCP7940X utilizes digital calibration to correct for

inaccuracies of the input clock source (either external

or crystal). Calibration is enabled by setting the value

of the Calibration register at address 08H. Calibration

is achieved by adding or subtracting a number of input

clock cycles per minute in order to achieve ppm level

adjustments in the internal timing function of the

MCP7940X.

The MSB of the Calibration register is the sign bit, with

a ‘1’ indicating subtraction and a ‘0’ indicating addition.

The remaining seven bits in the register indicate the

number of input clock cycles (multiplied by two) that

are subtracted or added per minute to the internal

timing function.

The internal timing function can be monitored using

the MFP open-drain output pin by setting bit [6]

(SQWE) and bits [2:0] (RS2, RS1, RS0) of the control

waveform is disabled when the MCP7940X is running

in VBAT mode. With the SQWE bit set to ‘1’, there are

two methods that can be used to observe the internal

timing function of the MCP7940X:

A. RS2 BIT SET TO ‘0’

With the RS2 bit set to ‘0’, the RS1 and RS0 bits

enable the following internal timing signals to be

output on the MFP pin:

The frequencies listed in the table presume an input

clock source of exactly 32.768 kHz. In terms of the

equivalent number of input clock cycles, the table

becomes:

With regards to the calibration function, the Calibration

clock signal when bits RS1 and RS0 are set to ‘11’.

The setting of the Calibration register to a non-zero

value (i.e., values other than 00H or 80H) enables the

calibration function which can be observed on the

MFP output pin. The calibration function can be

expressed in terms of the number of input clock cycles

added/subtracted from the internal timing function.

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 5-2:

RS1 AND RS0 WITH AND WITHOUT CALIBRATION

RS2

RS1

RS0

Output Signal

1 Hz

4.096 kHz

8.192 kHz

32.768 kHz

RS2

RS1

RS0

Output Signal

32768

Toutput

(32768 +/- (2 * CALREG)) Tinput

where:

Toutput

clock period of MFP output signal

Tinput

clock period of input signal

CALREG

decimal value of Calibration

determined by the MSB of

Calibration register.

Delay

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