DS1923-F5#【PDF 44/55 页】IBUTTON TEMP/HUMIDITY LOGGER F5

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DS1923

iButton Hygrochron Temperature/Humidity

Logger with 8KB Data-Log Memory

V PUP

t REH

V TH

V HY

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 DS1923 uses two types of CRCs. One CRC is an

8-bit type and is stored in the most significant byte of

the 64-bit ROM. The bus master can compute a CRC

value from the first 56 bits of the 64-bit ROM and com-

pare it to the value stored within the DS1923 to deter-

mine 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 computed 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 detec-

tion when reading register pages or the data-log mem-

ory using the Read Memory with CRC command and

for fast verification 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 DS1923

(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 2

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-

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