DS1922E(2008) Просмотр технического описания (PDF) - Maxim Integrated

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DS1922E
(Rev.:2008)

High-Temperature Logger iButton with 8KB Data-Log Memory

Maxim Integrated 

High-Temperature Logger iButton with 8KB

Data-Log Memory

Detailed Description

With its extended temperature range, the DS1922E is

well suited to monitor processes that require tempera-

tures well above the boiling point of water, such as pas-

teurization of food items. Note that the initial sealing

level of the DS1922E achieves the equivalent 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 environ-

ments, it is recommended to place the DS1922E in the

DS9107 iButton capsule. The DS9107 provides a water-

tight 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 application- or equipment-specific data files.

Overview

The block diagram in Figure 1 shows the relationships

between the major control and memory sections of the

DS1922E. The device has five main data components:

64-bit lasered ROM; 256-bit scratchpad; 576-byte gen-

eral-purpose SRAM; two 256-bit register pages of time-

keeping, control, status, and counter registers, and

passwords; and 8192 bytes of data-logging memory.

Except for the ROM and the scratchpad, all other mem-

ory 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 pro-

grammed for a mission. The password registers, one for

a read password and another one for a read/write pass-

word, can only be written, never read.

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 Command. Upon completion of an Overdrive

ROM command byte 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 executed, the memory and control func-

tions become accessible and the master can provide

any one of the eight available commands. The protocol

for these memory and control function commands is

described in Figure 9. All data is read and written

least significant bit first.

Parasite Power

The block diagram (Figure 1) shows the parasite-pow-

ered circuitry. This circuitry “steals” power whenever the

I/O input is high. I/O 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 can still be read

normally. The remaining circuitry of the DS1922E is sole-

ly operated by battery energy.

64-Bit Lasered ROM

Each DS1922E 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 regis-

ter and XOR gates as shown in Figure 4. The polynomi-

al is X8 + X5 + X4 + 1. Additional information about the

1-Wire CRC is available in Application Note 27:

Understanding and Using Cyclic Redundancy Checks

with Maxim iButton Products and in Application Note

937: Book of iButton Standards.

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.

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