Rainbow Electronics DS1920 - Manual (Page 8)

DS1920
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1-WIRE CRC CODE Figure 6
MEMORY
The DS1920’s memory is organized as shown in Figure 7. The memory consists of a scratchpad and 2
bytes of EEPROM which store the high and low temperature triggers TH and TL. The scratchpad helps
insure data integrity when communicating over the 1-Wire bus. Data is first written to the scratchpad
where it can be read back. After the data has been verified, a copy scratchpad command will transfer the
data to the EEPROM. This process insures data integrity when modifying the memory.
The scratchpad is organized as 8 bytes of memory. The first 2 bytes contain the measured temperature
information. The 3
rd
and 4
th
bytes are volatile copies of TH and TL and are refreshed with every power-on
reset. The next 2 bytes are not used; upon reading back, however, they will appear as all logic 1s. The 7
th
and 8
th
bytes are count registers, which may be used in obtaining higher temperature resolution (see
“Operation-Measuring Temperature” section).
There is a 9
th
byte which may be read with a Read Scratchpad command. This byte is a cyclic redundancy
check (CRC) over all of the 8 previous bytes. This CRC is implemented as described in the section titled
“CRC Generation.”
DS1920 MEMORY MAP Figure 7
SCRATCHPAD BYTE EEPROM
TEMPERATURE LSB 0
TEMPERATURE MSB 1
TH/USER BYTE 1 2 TH/USER BYTE 1
TL/USER BYTE 2 3 TL/USER BYTE 2
RESERVED 4
RESERVED 5
COUNT REMAIN 6
COUNT PER °C 7
CRC 8
1-WIRE BUS SYSTEM
The 1-Wire bus is a system which has a single bus master and one or more slaves. The DS1920 behaves
as a slave. The discussion of this bus system is broken down into three topics: hardware configuration,
transaction sequence, and 1-Wire signaling (signal types and timing).