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1

1

0

0

0

1

1

CRC (1 BYTE)

0

Reset

and

Presence

Signals

Match ROM (55h)

Family Code and Identification

Number (7 BYTES)

SEARCH ROM

BIT0

Reset

and

Presence

Signals

0

0

0

0

1

1

1

1

BITn

BIT63

Search ROM (F0h)

Data Read

SKIP ROM

1

0

0

0

1

1

0

Reset

and

Presence

Signals

Skip ROM (CCh)

1

bq2022

SLUS526F – OCTOBER 2002 – REVISED DECEMBER 2006

Figure 4. MATCH ROM Sequence

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 is

performed by comparing the bit and bit complement time-slots. If they are both zero, this indicates that a

collision has occurred, indicating multiple devices on the bus. The device decode is achieved in the third

transmission of the bit from the host back to the bq2022. If the bit transmitted by the host does not match the bit

transmitted by the bq2022, then the device with mismatch stops transmitting. Devices that did match, continue

transmitting. This process is continued until all bits of a single device are read. The SEARCH ROM command is

reissued and the process is repeated to read additional devices.

NOTE:

NOTE: If the number of devices on the bus is unknown, the SEARCH ROM

command should be used.

A.

B = bit(n): nth bit transmitted by bq2022

B.

C = bit(n): complement of nth bit transmitted by bq2022

C.

H = bit(n): nth bit transmitted by host

Figure 5. SEARCH ROM Sequence

This SKIP ROM command, CCh, allows the host to access the memory/status functions without issuing the

64-bit ROM code sequence. The SKIP ROM command is directly followed by a memory/status functions

command. Because this command can cause bus collisions when multiple SDQ devices are on the same bus,

this command should be issued in single device applications.

Figure 6. SKIP ROM Sequence

6

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