Datasheet
DS2480B
Maxim Integrated ........................................................................................................................................................................................... 10
The type of time slot b2 (write 1 or write 0) is determined by the DS2480B as follows:
b2 = r
n
if conflict (as chosen by the host)
= b
0
if no conflict (there is no alternative)
= 1 if error (there is no response)
The response the host will receive during a complete pass through a Search ROM function using the
Search Accelerator consists of 16 bytes as follows:
first byte
7
6
5
4
3
2
1
0
r’
3
d
3
r’
2
d
2
r’
1
d
1
r’
0
d
0
et cetera
16
th
byte
7
6
5
4
3
2
1
0
r’
63
d
63
r’
62
d
62
r’
61
d
61
r’
60
d
60
As before, the index (values from 0 to 63, “n”) designates the position of the bit in the ROM ID of a
1-Wire/iButton device. The character “d” marks the discrepancy flag in that particular bit position. The
discrepancy flag will be 1 if there is a conflict, or no response in that particular bit position, and 0
otherwise. The character “r” marks the actually chosen path at that particular bit position. The chosen
path is identical to b2 for the particular bit position of the ROM ID.
To perform a Search ROM sequence one starts with all bits r
n
being 0s. In case of a bus error, all
subsequent response bits r’
n
are 1s until the Search Accelerator is deactivated. Thus, if r’
63
and d
63
are both
1, an error has occurred during the search procedure and the last sequence has to be repeated. Otherwise
r’
n
(n = 0 ... 63) is the ROM code of the device that has been found and addressed.
For the next Search ROM sequence one reuses the previous set r
n
(n = 0 ... 63) but sets r
m
to 1 with “m”
being the index number of the highest discrepancy flag (that is, 1) and sets all r
i
to 0 with i > m. This
process is repeated until the highest discrepancy occurs in the same bit position for two consecutive
passes.
The table below shows an example for the communication between host and DS2480B to perform one
pass through the Search ROM function using the Search Accelerator. After a device has been identified
and addressed, a memory function (not specified here) is executed and finally a reset pulse is generated.
This example assumes that the DS2480B was in Command Mode and that standard 1-Wire speed is used.