DS2506
After the 16-bit CRC of the last EPROM Status data page is read, the bus master will receive logical 1s
from the DS2506 until a Reset Pulse is issued. The Read Status command sequence can be ended at any
point by issuing a Reset Pulse.
EXTENDED READ MEMORY [A5H]
The Extended Read Memory command supports page redirection when reading data from the 65536-bit
EPROM data field. One major difference between the Extended Read Memory and the basic Read
Memory command is that the bus master receives the Redirection Byte first before investing time in
reading data from the addressed memory location. This allows the bus master to quickly decide whether
to continue and access the data at the selected starting page or to terminate and restart the reading process
at the redirected page address. A non-redirected page is identified by a Redirection Byte with a value of
FFH (see description of EPROM Status Bytes). If the Redirection Byte is different than this, the master
has to complement it to obtain the new page number. Multiplying the page number by 32 (20H) results in
the new address the master has to send to the DS2506 to read the updated data replacing the old data.
There is no logical limitation in the number of redirections of any page. The only limit is the number of
available memory pages within the DS2506.
In addition to page redirection, the Extended Read Memory command also supports “bit-oriented”
applications where the user cannot store a 16-bit CRC with the data itself. With bit-oriented applications
the EPROM information may change over time within a page boundary making it impossible to include
an accompanying CRC that will always be valid. Therefore, the Extended Read Memory command
concludes each page with the DS2506 generating and supplying a 16-bit CRC that is based on and
therefore always consistent with the current data stored in each page of the 65536-bit EPROM data field.
After having sent the command code of the Extended Read Memory command, the bus master follows
the command byte with a two byte address (TA1=(T7:T0), TA2=(T15:T8)) that indicates a starting byte
location within the data field. By sending eight read data time slots, the master receives the Redirection
Byte associated with the page given by the starting address. With the next sixteen read data time slots,
the bus master receives a 16-bit CRC of the command byte, address bytes and the Redirection Byte. This
CRC is computed by the DS2506 and read back by the bus master to check if the command word, starting
address and Redirection Byte were received correctly.
If the CRC read by the bus master is incorrect, a Reset Pulse must be issued and the entire sequence must
be repeated. If the CRC received by the bus master is correct, the bus master issues read time slots and
receives data from the DS2506 starting at the initial address and continuing until the end of a 32 byte
page is reached. At that point the bus master will send sixteen additional read time slots and receive a
16-bit CRC that is the result of shifting into the CRC generator all of the data bytes from the initial
starting byte to the last byte of the current page.
With the next 24 read data time slots the master will receive the Redirection Byte of the next page
followed by a 16-bit CRC of the Redirection Byte. After this, data is again read from the 65536-bit
EPROM data field starting at the beginning of the new page. This sequence will continue until the final
page and its accompanying CRC are read by the bus master. The Extended Read Memory command
provides a 16-bit CRC at two locations within the transaction flow chart: 1) after the Redirection Byte
and 2) at the end of each memory page. The CRC at the end of the memory page is always the result of
clearing the CRC generator and shifting in the data bytes beginning at the first addressed memory
location of the EPROM data page until the last byte of this page. With the initial pass through the
Extended Read Memory flow chart the 16-bit CRC value is the result of shifting the command byte into
the cleared CRC generator, followed by the two address bytes and the Redirection Byte. Subsequent
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DALLAS [ DALLAS SEMICONDUCTOR ]
