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DS1921G
Thermochron iButton Device
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 dur-
ing a mission. The command works the same way as
the simple Read Memory command, except for the 16-
bit CRC that the DS1921G generates and transmits fol-
lowing 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 = T[7:0], TA2 = T[15:8]) that indi-
cates a starting byte location. With the subsequent
read-data time slots, the master receives data from the
DS1921G starting at the initial address and continues
until the end of a 32-byte page is reached. At that point
the bus master sends 16 additional read-data time slots
and receives an inverted 16-bit CRC. With subsequent
read-data time slots the master receives data starting at
the beginning of the next page followed again by the
inverted CRC for that page. This sequence continues
until the bus master resets the device.
With the initial pass through the Read Memory with
CRC command flow, the 16-bit CRC value is the result
of shifting the command byte into the cleared CRC gen-
erator followed by the two address bytes and the con-
tents of the data memory. Subsequent passes through
the Read Memory with CRC command flow generate a
16-bit CRC that is the result of clearing the CRC gener-
ator and then shifting in the contents of the data memo-
ry page. After the 16-bit CRC of the last page is read,
the bus master receives logical “0”s from the DS1921G
and inverted CRC-16s 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 Timestamp,
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 data-
log memory or the temperature and timer alarm flags in
the Status register. The RTC oscillator must be on and
have counted at least 1s before issuing the command.
For the Clear Memory command to function, the
EMCLR bit in the 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 resets the
EMCLR bit. The Clear Memory process takes 500μs.
Maxim Integrated
When the command is completed the MEMCLR bit in
the Status register reads 1 and the EMCLR bit is 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 tem-
perature conversion can be found at memory address
211h in the register page. This command takes maxi-
mum 90ms to complete. During this time the device
remains fully accessible for memory/control and ROM
function commands.
Mission Start and Logging Process
The DS1921G does not use a special command to start
a mission. Instead, a mission is started by writing a
nonzero value to the Sample Rate register using the
Copy Scratchpad command. As shown in Figure 11, a
new mission can only be started if the previous mission
has been stopped (MIP = 0), the memory is cleared
(MEMCLR = 1), and the mission is enabled ( EM = 0). If
the new sample rate is different from zero, the value is
copied to the Sample Rate register. At the same time
the MIP bit is set and the MEMCLR bit is cleared to indi-
cate that the device is on a mission. Next, the Mission
Start Delay Counter starts decrementing every minute
until it is down to 0. Now the DS1921G waits until the
next minute boundary and starts the logging process,
which as its first action copies the applicable RTC reg-
isters to the Mission Timestamp register.
Stop Mission
The DS1921G does not have a special command to
stop a mission. A mission can be stopped at any time
by writing to any address in the range of 0200h to
0213h or by writing the MIP bit of the Status register at
address 0214h to 0. Either approach involves the use of
the Copy Scratchpad command. There is no need for
the Mission Start Delay to expire before a mission can
be stopped (see Figure 11).
Memory Access Conflicts
While a mission is in progress, a temperature sample is
periodically taken and stored in the data-log, his-
togram, and potential alarm memory. This “internal
activity” has priority over a Read Memory command’s
or Read Memory with CRC command’s access to these
pages. If a conflict occurs, the data read may be
invalid, even if the CRC value matches the data. To
ensure that the data read is valid, it is recommended to
first read the SIP bit of the Status register. If the SIP bit
is set, delay reading the data-log, histogram, and alarm
memory until SIP is 0. The interference is more likely to
be seen with a high sample rate (one sample every
25
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功能描述:iButton RoHS:否 存储类型:SRAM 存储容量:512 B 组织: 工作电源电压:3 V to 5.25 V 接口类型:1-Wire 最大工作温度:+ 85 C 尺寸:17.35 mm x 5.89 mm 封装 / 箱体:F5 MicroCan 制造商:Maxim Integrated
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功能描述:iButton
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功能描述:iButton
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功能描述:iButton
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