DS2778 Maxim Integrated Products, DS2778 Datasheet - Page 35

• Manufacturer Part Number: DS2778
• Description: 2-Cell Stand-Alone Li+ Fuel-Gauge IC
• Manufacturer: Maxim Integrated Products

Table 11. Secret Management Function Commands
the new secret value to EEPROM. See Figure 26 for
command timing.
This command write protects the 64-bit secret to pre-
vent accidental or malicious overwrite of the secret
value. The secret value stored in EEPROM becomes
"final." The host must wait t
to write the lock secret bit to EEPROM. See Figure 28
for command timing.
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 DS2775/
DS2776 are slave devices. 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.
The DS2775/DS2776 have an 8-bit CRC stored in the
MSB of its 64-bit net address and generates a CRC
during some command protocols. To ensure error-free
transmission of the address, the host system can com-
pute a CRC value from the first 56 bits of the address
and compare it to the CRC from the DS2775/DS2776.
Figure 23. 1-Wire CRC Generation Block Diagram
iButton is a registered trademark of Maxim Integrated Products, Inc.
Clear Secret
Compute Next Secret without ROM ID
Compute Next Secret with ROM ID
Lock Secret
Protector and Optional SHA-1 Authentication
2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
COMMAND
MSb
______________________________________________________________________________________
(DS2775/DS2776 Only)
1-Wire Bus System
EEC
CRC Generation
for the DS2776/DS2778
Lock Secret [60h]
HEX
5Ah
30h
33h
60h
Clears the 64-bit secret to 0000 0000 0000 0000h.
Generates new global secret.
Generates new unique secret.
Sets lock bit to prevent changes to the secret.
XOR
The host system is responsible for verifying the CRC
value and taking action as a result. The DS2775/
DS2776 do not compare CRC values and do not pre-
vent 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 23, or it can be generated in software using
the polynomial X
about the Maxim 1-Wire CRC is available in Application
Note 27: Understanding and Using Cyclic Redundancy
Checks with Maxim iButton
In the circuit in Figure 23, the Shift register bits are ini-
tialized to 0. Then, starting with the LSb 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 num-
ber is entered. After the 48th bit of the serial number has
been entered, the Shift register contains the CRC value.
During some command sequences, the DS2775/
DS2776 also generate an 8-bit CRC and provide this
value to the bus master to facilitate validation for the
transfer of command, address, and data from the bus
master to the DS2775/DS2776. The DS2775/DS2776
compute an 8-bit CRC for the command and address
bytes received from the bus master for the Read
Memory, Read Status, and Read/Generate CRC com-
mands to confirm that these bytes have been received
correctly. The CRC generator on the DS2775/DS2776 is
XOR
8
FUNCTION
+ X
5
+ X
®
4
Products .
+ 1. Additional information
LSb
www.DataSheet4U.com
XOR
INPUT
35