Datasheet
DS1922L/DS1922T
Temperature Logger iButton with 8KB
Data-Log Memory
44 ______________________________________________________________________________________
V
PUP
V
TH
V
HY
0V
t
REH
t
GL
t
REH
t
GL
CASE A CASE CCASE B
Figure 14. Noise Suppression Scheme
1ST
STAGE
2ND
STAGE
3RD
STAGE
4TH
STAGE
7TH
STAGE
8TH
STAGE
6TH
STAGE
5TH
STAGE
X
0
X
1
X
2
X
3
X
4
POLYNOMIAL = X
16
+ X
15
+ X
2
+ 1
INPUT DATA
CRC OUTPUT
X
5
X
6
11TH
STAGE
12TH
STAGE
15TH
STAGE
14TH
STAGE
13TH
STAGE
X
11
X
12
9TH
STAGE
10TH
STAGE
X
9
X
10
X
13
X
14
X
7
16TH
STAGE
X
16
X
15
X
8
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
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-