DS2705: SHA-1 Authentication Master

6 of 18

Figure 1. Block Diagram

EEPROM

1-Wire

Master/

Slave

160-bit MAC

Control FSM

64-bit

Challenge

CHAL

Pullup

Control

VDD

VSS

16-bit

Configuration

CHP = 0

SDQ

MDQ

DETAILED DESCRIPTION

The DS2705 orchestrates a challenge/response SHA-1 authentication procedure by accessing a Dallas Battery

Management SHA-1 Token product, such as the DS2703 or DS2704. The remote SHA-1 token is accessed with

the MDQ pin acting as the 1-Wire bus master. The DS2705 issues the appropriate 1-Wire command sequence on

MDQ to write the 64-bit challenge, initiates a SHA-1 computation in the token, and then reads back the 160-bit

MAC result. The DS2705 compares the 160-bit MAC received from the battery token with the preprogrammed

MAC. An exact bit for bit match is required for the authentication to be successful. The result of the operation,

PASS or FAIL, is indicated on active low status output pins which can be used to drive status LEDs and/or enable

cell charging.

The DS2705 can be configured to automatically authenticate by detection of a presence pulse on MDQ or

authentication can be controlled by the state of the CHAL input pin. The DS2705’s SDQ pin is a 1-Wire slave

interface for programming the behavior of the I.C.. All EEPROM values can be permanently locked to prevent

corruption.

Figure 2 shows a example application circuit for a standalone battery charger. The DS2705 is preprogrammed for

automatic authentication on MDQ and also contains a known good challenge/response pair. Programming occurs

during assembly through PCB test points shown on the right side of the circuit. When a battery pack is inserted into

the charger, a presence pulse on MDQ will cause the DS2705 to automatically authenticate the pack. The result of

the authentication will be displayed through the LEDs and the DS2705 will either enable or disable the charging

circuit.