Datasheet

DS1920

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OPERATION—ALARM SIGNALING

After the DS1920 has performed a temperature conversion, the temperature value is compared to the

trigger values stored in TH and TL. Since these registers are 8 bits only, the 0.5°C bit is ignored for

comparison. The most significant bit of TH or TL directly corresponds to the sign bit of the 16-bit

temperature register. If the result of a temperature measurement is higher than TH or lower than TL, an

alarm flag inside the device is set. This flag is updated with every temperature measurement. As long as

the alarm flag is set, the DS1920 will respond to the alarm search command. This allows many DS1920s

to be connected in parallel doing simultaneous temperature measurements. If somewhere the temperature

exceeds the limits, the alarming device(s) can be identified and read immediately without having to read

nonalarming devices.

64-BIT LASERED ROM

Each DS1920 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code

(DS1920 code is 10h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first

56 bits. (See Figure 4.) The 64-bit ROM and ROM Function Control section allow the DS1920 to operate

as a 1-Wire device and follow the 1-Wire protocol detailed in the 1-Wire Bus System section. The

memory and control functions of the DS1920 are not accessible until the ROM function protocol has been

satisfied. This protocol is described in the ROM function protocol flowchart (Figure 5). The 1-Wire bus

master must first provide one of five ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search

ROM, 4) Skip ROM, or 5) Alarm Search. After a ROM function sequence has been successfully

executed, the functions specific to the DS1920 are accessible and the bus master may then provide any

one of the five memory and control function commands.

CRC GENERATION

The DS1920 has an 8-bit CRC 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 DS1920 to determine if the ROM data has been received error-free by the bus master. The equivalent

polynomial function of this CRC is:

CRC = X

+ X

+ 1

The DS1920 also generates an 8-bit CRC value using the same polynomial function shown above and

provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is

used for data transfer validation, the bus master must calculate a CRC value using the polynomial

function given above and compare the calculated value to either the 8-bit CRC value stored in the 64-bit

ROM portion of the DS1920 (for ROM reads) or the 8-bit CRC value computed within the DS1920

(which is read as a 9th byte when the scratchpad is read). The comparison of CRC values and decision to

continue with an operation are determined entirely by the bus master. There is no circuitry inside the

DS1920 that prevents a command sequence from proceeding if the CRC stored in or calculated by the

DS1920 does not match the value generated by the bus master.

The 1-Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR

gates as shown in Figure 6. Additional information about the Maxim 1-Wire Cyclic Redundancy Check is

available in the Book of iButton Standards.

The shift register bits are first initialized to 0. For the ROM section, starting with the least significant bit

of the family code, 1 bit at a time is shifted in. After the 8th bit of the family code has been entered, then

the serial number is entered. After the 48th bit of the serial number has been entered, the shift register

contains the CRC value. Shifting in the 8 bits of CRC should return the shift register to all 0s.