Datasheet
2012-2018 Microchip Technology Inc. DS20002300C-page 29
MCP7951X/MCP7952X
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.
REGISTER 5-17: OSCTRIM: OSCILLATOR DIGITAL TRIM REGISTER (ADDRESS 0x09)
R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 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