DS1922T Maxim, DS1922T Datasheet - Page 13

• Manufacturer Part Number: DS1922T
• Description: The DS1922L/T temperature logger iButtons® are rugged, self-sufficient systems that measure temperature and record the result in a protected memory section
• Manufacturer: Maxim

The DS1922L is an ideal device to monitor for extended
periods of time the temperature of any object it is
attached to or shipped with, such as fresh produce,
medical drugs and supplies, and for use in refrigerators
and freezers. With its shifted temperature range, the
DS1922T is suited to monitor processes that require
temperatures close to the boiling point of water, such
as pasteurization of food items. Note that the initial seal-
ing level of the DS1922L/DS1922T achieves the equiva-
lent of IP56. Aging and use conditions can degrade the
integrity of the seal over time, so for applications with
significant exposure to liquids, sprays, or other similar
environments, it is recommended to place the
DS1922L/DS1922T in the DS9107 iButton capsule. The
DS9107 provides a watertight enclosure that has been
rated to IP68 (refer to Application Note 4126:
Understanding the IP (Ingress Protection) Ratings of
iButton Data Loggers and Capsules ). Software for
setup and data retrieval through the 1-Wire interface is
available for free download from the iButton website
(www.ibutton.com). This software also includes drivers
for the serial and USB port of a PC and routines to
access the general-purpose memory for storing appli-
cation-specific or equipment-specific data files.
All iButton data loggers are calibrated/validated against
NIST traceable reference devices. Maxim offers a web
application to generate validation certificates for the
DS1922L, DS1922T, DS1922E, and DS1923 (tempera-
ture portion only) data loggers. Input is the iButton’s
ROM code (or list of codes) and the output is a valida-
tion certificate in PDF format. For more information,
refer to Application Note 4629: iButton
Calibration and NIST Certificate FAQs .
The block diagram in Figure 1 shows the relationships
between the major control and memory sections of the
DS1922L/DS1922T. The devices have six main data
components: 64-bit lasered ROM; 256-bit scratchpad;
512-byte general-purpose SRAM; two 256-bit register
pages of timekeeping, control, status, and counter reg-
isters, and passwords; 64 bytes of calibration memory;
and 8192 bytes of data-logging memory. Except for the
ROM and the scratchpad, all other memory is arranged
in a single linear address space. The data-logging
memory, counter registers, and several other registers
are read only for the user. Both register pages are write
protected while the device is programmed for a mis-
sion. The password registers, one for a read password
and another one for a read/write password, can only be
written, never read.
______________________________________________________________________________________
Detailed Description
Temperature Logger iButton with 8KB
®
Data-Logger
Overview
Figure 2 shows the hierarchical structure of the 1-Wire
protocol. The bus master must first provide one of the
eight ROM function commands: Read ROM, Match
ROM, Search ROM, Conditional Search ROM, Skip
ROM, Overdrive-Skip ROM, Overdrive-Match ROM, or
Resume. Upon completion of an Overdrive ROM com-
mand executed at standard speed, the device enters
overdrive mode, where all subsequent communication
occurs at a higher speed. The protocol required for
these ROM function commands is described in Figure
11. After a ROM function command is successfully exe-
cuted, the memory and control functions become
accessible and the master can provide any one of the
eight available commands. The protocol for these mem-
ory and control function commands is described in
Figure 9. All data is read and written least significant
bit first.
The block diagram (Figure 1) shows the parasite-pow-
ered circuitry. This circuitry “steals” power whenever
the IO input is high. IO provides sufficient power as
long as the specified timing and voltage requirements
are met. The advantages of parasite power are two-
fold: 1) By parasiting off this input, battery power is not
consumed for 1-Wire ROM function commands, and
2) if the battery is exhausted for any reason, the ROM
may still be read normally. The remaining circuitry of
the DS1922 is solely operated by battery energy.
Each DS1922L/DS1922T contains a unique ROM code
that is 64 bits long. The first 8 bits are a 1-Wire family
code. The next 48 bits are a unique serial number. The
last 8 bits are a cyclic redundancy check (CRC) of the
first 56 bits (see Figure 3 for details). The 1-Wire CRC is
generated using a polynomial generator consisting of a
shift register and XOR gates as shown in Figure 4. The
polynomial is X
about the 1-Wire CRC is available in Application Note
27: Understanding and Using Cyclic Redundancy
Checks with Maxim iButton Products .
The shift register bits are initialized to 0. Then, starting
with the least significant bit of the family code, one bit
at a time is shifted in. After the 8th bit of the family code
has been entered, the serial number is entered. After
the last bit of the serial number has been entered, the
shift register contains the CRC value. Shifting in the 8
bits of CRC returns the shift register to all 0s.
Data-Log Memory
8
+ X
5
+ X
4
+ 1. Additional information
64-Bit Lasered ROM
Parasite Power
13