DS2703_07 Datasheet, PDF (10/20 Pages) Dallas Semiconductor – SHA-1 Battery Pack Authentication IC

DS2703 SHA-1 Battery Pack Authentication IC
The DS2703 will remain in thermistor measurement mode until the stored charge on the VB pin capacitor is
depleted causing the IC to power cycle back to standard mode of operation. While in thermistor measurement
mode, communication to the DS2703 is not possible. After measuring the thermistor, the host must wait until the
VB capacitor is depleted. Figure 2 shows the typical and worst case transition times over the full operating range
when using .22µF as the VB pin capacitor. Thermistor measurements should be made within the first 100ms after
issuing the command. The host system should then wait until at least 1000ms have passed before sending the next
communication sequence to the IC.
Table 6. Thermistor Function Command
COMMAND
HEX
FUNCTION
Activates the THM output for thermistor measurement. Activation
Activate Thermistor
A9 occurs within 50μs of command completion and continues until VB
capacitor depleted.
1-Wire BUS SYSTEM
The 1-Wire bus is a system that has a single bus master and one or more slaves. A multidrop bus is a 1-Wire bus
with multiple slaves, while a single-drop bus has only one slave device. In all instances, the DS2703 is a slave
device. The bus master is typically a microprocessor in the host system. The discussion of this bus system consists
of five topics: 64-bit net address, CRC generation, hardware configuration, transaction sequence, and 1-Wire
signaling.
64-BIT NET ADDRESS (ROM ID)
Each DS2703 has a unique, factory-programmed 1-Wire Net Address that is 64 bits in length. The term Net
Address is synonymous with the ROM ID or ROM Code terms used in earlier Dallas 1-Wire product documentation.
The first eight bits of the Net Address are the 1-Wire family code, (34h) for the DS2703. The next 48 bits are a
unique serial number. The last eight bits are a cyclic redundancy check (CRC) of the first 56 bits (see Figure 3.).
The 64-bit net address and the 1-Wire I/O circuitry built into the device enable the DS2703 to communicate through
the 1-Wire protocol detailed in this data sheet.
Figure 3. 1-Wire Net Address Format
8-BIT CRC
MSb
48-BIT SERIAL NUMBER
8-BIT FAMILY
CODE (34H)
LSb
CRC GENERATION
The DS2703 has an 8-bit CRC stored in the most significant byte of its 1-Wire net address. To ensure error-free
transmission of the address, the host system can compute a CRC value from the first 56 bits of the address and
compare it to the 8-bit CRC from the DS2703.
The host system is responsible for verifying the CRC value and taking action as a result. The DS2703 does not
compare CRC values and does not prevent a command sequence from proceeding as a result of a CRC mismatch.
Proper use of the CRC can result in a communication channel with a very high level of integrity.
The CRC can be generated by the host using a circuit consisting of a shift register and XOR gates as shown in
Figure 4, or it can be generated in software using the polynomial X⁸ + X⁵ + X⁴ + 1. Additional information about the
Dallas 1-Wire CRC is available in Application Note 27: Understanding and Using Cyclic Redundancy Checks with
Dallas Semiconductor Touch Memory Products (www.maxim-ic.com/appnoteindex).
In Figure 4, 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, then the serial number is
entered. After the 48th bit of the serial number has been entered, the shift register contains the CRC value.
10 of 20