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MATCH ROM
1 10 0 01 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
bq2022A
SLUS724C – SEPTEMBER 2006 – REVISED AUGUST 2007
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.
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 μ s. The bq2022A 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 bq2022A 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 bq2022A. If the bit transmitted by the host does not match the bit transmitted by
the bq2022A, 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 bq2022A
B. C = bit(n): complement of nth bit transmitted by bq2022A
C. H = bit(n): nth bit transmitted by host
Figure 5. SEARCH ROM Sequence
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