DS1921 datasheet(22/44 Pages) DALLAS

22 / 44 page

DS1921H/Z

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Read Memory with CRC [A5h]

The Read Memory with CRC command is used to read memory data that cannot be packetized, such as

the register page and the data recorded by the device during a mission. The command works essentially

the same way as the simple Read Memory, except for the 16-bit CRC that the DS1921H/Z generates and

transmits following the last data byte of a memory page.

After having sent the command code of the Read Memory with CRC command, the bus master sends a 2-

byte address (TA1 = T7:T0, TA2 = T15:T8) that indicates a starting byte location. With the subsequent

read data time slots the master receives data from the DS1921H/Z starting at the initial address and

continuing until the end of a 32-byte page is reached. At that point the bus master will send 16 additional

read data time slots and receive an inverted 16-bit CRC. With subsequent read data time slots the master

will receive data starting at the beginning of the next page followed again by the inverted CRC for that

page. This sequence will continue until the bus master resets the device.

With the initial pass through the Read Memory with CRC flow, 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

contents of the data memory. Subsequent passes through the Read Memory with CRC flow will generate

a 16-bit CRC that is the result of clearing the CRC generator and then shifting in the contents of the data

memory page. After the 16-bit CRC of the last page is read, the bus master will receive logical 0s from

the DS1921H/Z and inverted CRC16s at page boundaries until a reset pulse is issued. The Read Memory

with CRC command sequence can be ended at any point by issuing a reset pulse.

Clear Memory [3Ch]

The Clear Memory command is used to clear the Sample Rate, Mission Start Delay, Mission Time

Stamp, and Mission Samples Counter in the register page and the Temperature Alarm Memory and the

Temperature Histogram Memory. These memory areas must be cleared for the device to be set up for

another mission. The Clear Memory command does not clear the datalog memory or the temperature and

timer alarm flags in the Status Register. The RTC oscillator must be on and have counted at least 1

second, before issuing the command. For the Clear Memory command to function the EMCLR bit in

Control Register must be set to 1, and the Clear Memory command must be issued with the very next

access to the device’s memory functions. Issuing any other memory function command will reset the

EMCLR bit. The Clear Memory process takes 500µs. When the command is completed the MEMCLR bit

in the Status Register will read 1 and the EMCLR bit will be 0.

Convert Temperature [44h]

If a mission is not in progress (MIP = 0) the Convert Temperature command can be issued to measure the

current temperature of the device. The result of the temperature conversion will be found at memory

address 211h in the register page. This command takes maximum 360ms to complete. During this time

the device remains fully accessible for memory/control and ROM function commands.

DS1921 Datasheet(PDF) 22 Page - Dallas Semiconductor