MCP7940N-ESLASHMS datasheet(29/58 Pages) MICROCHIP

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DS20005010J-page 29

MCP7940N

5.6

Digital Trimming

The MCP7940N features digital trimming to correct for

inaccuracies of the external crystal or clock source, up

to roughly ±129 PPM when CRSTRIM = 0. In addition

to compensating for intrinsic inaccuracies in the clock,

this feature can also be used to correct for error due to

temperature variation. This can enable the user to

achieve high levels of accuracy across a wide tempera-

ture operating range.

Digital trimming consists of the MCP7940N periodically

adding or subtracting clock cycles, resulting in small

adjustments in the internal timing. The adjustment

occurs once per minute when CRSTRIM = 0. The SIGN

bit specifies whether to add cycles or to subtract them.

The TRIMVAL<6:0> bits are used to specify by how

many clock cycles to adjust. Each step in the

TRIMVAL<6:0> value equates to adding or subtracting

two clock pulses to or from the 32.768 kHz clock signal.

This results in a correction of roughly 1.017 PPM per

step when CRSTRIM = 0. Setting TRIMVAL<6:0> to

0x00 disables digital trimming.

Digital trimming also occurs while operating off the

backup supply.

OSCTRIM: OSCILLATOR DIGITAL TRIM REGISTER (ADDRESS 0x08)

R/W-0

SIGN

TRIMVAL6

TRIMVAL5

TRIMVAL4

TRIMVAL3

TRIMVAL2

TRIMVAL1

TRIMVAL0

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 clear

x = Bit is unknown

bit 7

SIGN: Trim Sign bit

1 = Add clocks to correct for slow time

0 = Subtract clocks to correct for fast time

bit 6-0

TRIMVAL<6:0>: Oscillator Trim Value bits

When CRSTRIM = 0:

1111111 = Add or subtract 254 clock cycles every minute

1111110 = Add or subtract 252 clock cycles every minute

•

0000010 = Add or subtract 4 clock cycles every minute

0000001 = Add or subtract 2 clock cycles every minute

0000000 = Disable digital trimming

When CRSTRIM = 1:

1111111 = Add or subtract 254 clock cycles 128 times per second

1111110 = Add or subtract 252 clock cycles 128 times per second

•

0000010 = Add or subtract 4 clock cycles 128 times per second

0000001 = Add or subtract 2 clock cycles 128 times per second

0000000 = Disable digital trimming

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