MCP79522 datasheet(30/58 Pages) MICROCHIP

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MCP7951X/MCP7952X

DS20002300D-page 30

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5.6

Digital Trimming

The MCP795XX features digital trimming to correct for

inaccuracies of the external crystal or clock source, up

to roughly ±259 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

temperature operating range.

Digital trimming consists of the MCP795XX periodically

adding or subtracting clock cycles, resulting in small

adjustments in the internal timing.

The adjustment occurs once per minute when

CRSTRIM = 0. The TRIMSIGN bit specifies whether to

add cycles or to subtract them. The TRIMVAL<7:0> bits

are used to specify by how many clock cycles to adjust.

Each step in the TRIMVAL<7:0> 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<7:0> to 0x00 disables digital

trimming.

Digital trimming also occurs while operating off the

backup supply.

OSCTRIM: OSCILLATOR DIGITAL TRIM REGISTER (ADDRESS 0x09)

R/W-0

TRIMVAL7

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 7-0

TRIMVAL<7:0>: Oscillator Trim Value bits

When CRSTRIM = 0:

11111111 = Add or subtract 510 clock cycles every minute

11111110 = Add or subtract 508 clock cycles every minute

•

00000010 = Add or subtract 4 clock cycles every minute

00000001 = Add or subtract 2 clock cycles every minute

00000000 = Disable digital trimming

When CRSTRIM = 1:

11111111 = Add or subtract 510 clock cycles every second

11111110 = Add or subtract 508 clock cycles every second

•

00000010 = Add or subtract 4 clock cycles every second

00000001 = Add or subtract 2 clock cycles every second

00000000 = Disable digital trimming

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