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DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
V PUP
t REH
t REH
V TH
V HY
0V
CASE A
t GL
CASE B
t GL
CASE C
Figure 14. Noise Suppression Scheme
POLYNOMIAL = X 16 + X 15 + X 2 + 1
X 0
1ST
STAGE
X 1
2ND
STAGE
X 2
3RD
STAGE
X 3
4TH
STAGE
X 4
5TH
STAGE
X 5
6TH
STAGE
X 6
7TH
STAGE
X 7
8TH
STAGE
9TH
STAGE
10TH
STAGE
11TH
STAGE
12TH
STAGE
13TH
STAGE
14TH
STAGE
15TH
STAGE
16TH
STAGE
X 8
X 9
X 10
X 11
X 12
X 13
X 14
X 15
X 16
CRC OUTPUT
INPUT DATA
Figure 15. CRC-16 Hardware Description and Polynomial
CRC Generation
The DS1922L/DS1922T use two types of CRCs. One
CRC is an 8-bit type and is stored in the most signifi-
cant byte of the 64-bit ROM. The bus master can com-
pute a CRC value from the first 56 bits of the 64-bit
ROM and compare it to the value stored within the
DS1922L/DS1922T to determine if the ROM data has
been received error-free. The equivalent polynomial
function of this CRC is X 8 + X 5 + X 4 + 1. This 8-bit CRC
is received in the true (noninverted) form, and it is com-
puted at the factory and lasered into the ROM.
The other CRC is a 16-bit type, generated according to
the standardized CRC-16 polynomial function X 16 + X 15
+ X 2 + 1. This CRC is used for error detection when
reading register pages or the data-log memory using
the Read Memory with CRC command and for fast veri-
fication of a data transfer when writing to or reading
from the scratchpad. In contrast to the 8-bit CRC, the
44
16-bit CRC is always communicated in the inverted
form. A CRC generator inside the DS1922L/DS1922T
(Figure 15) calculates a new 16-bit CRC as shown in
the command flowchart of Figure 9. The bus master
compares the CRC value read from the device to the
one it calculates from the data and decides whether to
continue with an operation or to reread the portion of
the data with the CRC error. With the initial pass
through the Read Memory with CRC flowchart, the 16-
bit CRC value is the result of shifting the command byte
into the cleared CRC generator, followed by the two
address bytes and the data bytes. The password is
excluded from the CRC calculation. Subsequent pass-
es through the Read Memory with CRC flowchart gen-
erate a 16-bit CRC that is the result of clearing the CRC
generator and then shifting in the data bytes.
With the Write Scratchpad command, the CRC is gener-
ated by first clearing the CRC generator and then shift-
Maxim Integrated
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相关PDF资料
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IBUTTON TEMP/HUMIDITY LOGGER F5
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