DS1925 datasheet(37/46 Pages) MAXIM

37 / 46 page

The sum of tRL + δ (rise time) on one side and the internal

timing generator of the DS1925 on the other side define

the master sampling window (tMSRMIN to tMSRMAX) in

which the master must perform a read from the data line.

For most reliable communication, tRL should be as short

as permissible and the master should read close to but no

later than tMSRMAX. After reading from the data line, the

master must wait until tSLOT is expired. This guarantees

sufficient recovery time tREC for the DS1925 to get ready

for the next time slot.

CRC Generation

With the DS1925 there are two different types of CRCs

(cyclic redundancy checks). 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 compare it to the value

stored within the DS1925 to determine if the ROM data

has been received error-free. The equivalent polynomial

function of this CRC is X8 + X5 + X4 + 1. This 8-bit CRC

is received in the true (noninverted) form. 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 x16 + x15 +

x2 + 1. This CRC is used for error detection when read-

ing register pages or the data-log memory using the XPC

Read Memory 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 DS1925 (Figure 14) calculates a new 16-bit

CRC as shown in 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 XPC Read Memory

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 pass-

word is excluded from the CRC calculation. Subsequent

passes through the XPC Read Memory flowchart gener-

ate 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 shifting

in the command code, the target addresses TA1 and TA2,

and all the data bytes. The DS1925 transmits this CRC

only if the data bytes written to the scratchpad include

scratchpad ending offset 11111b. The data can start at

any location within the scratchpad.

With the Read Scratchpad command, the CRC is gener-

ated by first clearing the CRC generator and then shifting

in the command code, the target addresses TA1 and

TA2, the E/S byte, and the scratchpad data starting at the

target address. The DS1925 transmits this CRC only if

the reading continues through the end of the scratchpad,

regardless of the actual ending offset. For more informa-

tion on generating CRC values, refer to Application Note

27: Understanding and Using Cyclic Redundancy Checks

with Maxim 1-Wire and iButton Products.

Figure 14. CRC-16 Hardware Description and Polynomial

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 37

X11

12th

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16th

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X12

X13

X14

X15

X16

CRC

OUTPUT

INPUT DATA

9th

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X10

1st

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2nd

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POLYNOMIAL = X16 + X15 + X2 + 1

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