Rainbow Electronics DS1822-PAR - Manual (Page 6)

DS1822-PAR
6 of 19
Data is written to bytes 2, 3, and 4 of the scratchpad using the Write Scratchpad [4Eh] command, and the
data must be transmitted to the DS1822-PAR starting with the least significant bit of byte 2. To verify
data integrity, the scratchpad can be read (using the Read Scratchpad [BEh] command) after the data is
written. When reading the scratchpad, data is transferred over the 1-wire bus starting with the least
significant bit of byte 0. To transfer the T
H
, T
L
and configuration data from the scratchpad to EEPROM,
the master must issue the Copy Scratchpad [48h] command.
Data in the EEPROM registers is retained when the device is powered down; at power-up the EEPROM
data is reloaded into the corresponding scratchpad locations. Data can also be reloaded from EEPROM
to the scratchpad at any time using the Recall E
2
[B8h] command. The master can issue “read time slots”
(see the 1-WIRE BUS SYSTEM section) following the Recall E
2
command and the DS1822-PAR will
indicate the status of the recall by transmitting 0 while the recall is in progress and 1 when the recall is
done.
CONFIGURATION REGISTER
Byte 4 of the scratchpad memory contains the configuration register, which is organized as illustrated in
Figure 7. The user can set the conversion resolution of the DS1822-PAR using the R0 and R1 bits in this
register as shown in Table 3. The power-up default of these bits is R0 = 1 and R1 = 1 (12-bit resolution).
Note that there is a direct tradeoff between resolution and conversion time. Bit 7 and bits 0-4 in the
configuration register are reserved for internal use by the device and cannot be overwritten; these bits will
return 1s when read.
CONFIGURATION REGISTER Figure 7
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
0 R1 R0 1 1 1 1 1
THERMOMETER RESOLUTION CONFIGURATION Table 3
R1 R0 Resolution Max Conversion Time
0 0 9-bit 93.75 ms (t
CONV
/8)
0 1 10-bit 187.5 ms (t
CONV
/4)
1 0 11-bit 375 ms (t
CONV
/2)
1 1 12-bit 750 ms (t
CONV
)
CRC GENERATION
CRC bytes are provided as part of the DS1822-PAR’s 64-bit ROM code and in the 9
th
byte of the
scratchpad memory. The ROM code CRC is calculated from the first 56 bits of the ROM code and is
contained in the most significant byte of the ROM. The scratchpad CRC is calculated from the data
stored in the scratchpad, and therefore it changes when the data in the scratchpad changes. The CRCs
provide the bus master with a method of data validation when data is read from the DS1822-PAR. To
verify that data has been read correctly, the bus master must re-calculate the CRC from the received data
and then compare this value to either the ROM code CRC (for ROM reads) or to the scratchpad CRC (for
scratchpad reads). If the calculated CRC matches the read CRC, the data has been received error free. The
comparison of CRC values and the decision to continue with an operation are determined entirely by the