bq2022

SLUS526B − OCTOBER 2002 − REVISED OCTOBER 2003

www.ti.com

5

Serial Communication

A host reads, programs, or checks the status of the bq2022 through the hierarchical command structure of the

SDQ interface. Figure 2 shows that the host must first issue a ROM command before the EPROM memory or

status can be read or modified. The ROM command either selects a specific device when multiple devices are

on the SDQ bus, or skips the selection process in single SDQ device applications.

Initialization

ROM Command Sequence

Memory/Status Command Sequence

Figure 2. General Command Sequence

Initialization

Initialization consists of two pulses, the RESET and the PRESENCE pulses. The host generates the RESET

pulse, while the bq2022 responds with the PRESENCE pulse. The host resets the bq2022 by driving the DATA

bus low for at least 480

µs. For more details, see the RESET section under SDQ Signaling.

ROM Commands

READ ROM

The READ ROM command sequence is the fastest sequence that allows the host to read the 8-bit family code

and 48-bit identification number. It is used if only one SDQ slave device is attached to the bus. The READ ROM

sequence starts with the host generating the RESET pulse of at least 480

µs. The bq2022 responds with a

PRESENCE pulse. Next, the host continues by issuing the READ ROM command, 33h, and then reads the

ROM and CRC byte using the READ signaling (see the READ and WRITE signals section) during the data

frame.

0

1

10

0

0

1

1

CRC (1 BYTE)

Reset

and

Presence

Signals

Read ROM (33h)

Family Code and Identification

Number (7 BYTES)

Figure 3. READ ROM Sequence

MATCH ROM

The MATCH ROM command, 55h, is used by the host to select a specific SDQ device when the family code

and identification number is known. The host issues the MATCH ROM command followed by the family code,

ROM number and the CRC byte. Only the device that matches the 64-bit ROM sequence is selected and

available to perform subsequent Memory/Status Function commands.

11

00

0

11

CRC (1 BYTE)

0

Reset

and

Presence

Signals

Match ROM (55h)

Family Code and Identification

Number (7 BYTES)

Figure 4. MATCH ROM Sequence

SEARCH ROM

The SEARCH ROM command, F0h, is used to obtain the 8-bit family code and the 48-bit identification number

and 8-bit CRC of any SDQ device when it is unknown. All devices on the bus are read under the SEARCH ROM

command with the use of a collision-detect and device-decode method. Figure 5 shows the SEARCH ROM

sequence started by the host, generating the RESET pulse of at least 480

µs. The bq2022 responds with a

PRESENCE pulse. The host then issues the command in the command frame by writing an F0h. During the

DATA READ of the SEARCH ROM sequence, each bit is transmitted three times. The bq2022 transmits the

bit followed by the complement of the bit. The host in turn retransmits the bit just read. Collision detection if