DS1985

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EXTENDED READ MEMORY [A5H]

The Extended Read Memory command supports page redirection when reading data from the 16384-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 DS1985 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 DS1985.

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 DS1985 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 16384-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 2-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 DS1985 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 DS1985 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 16 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 16384-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.

The CRC received by the bus master directly following the Redirection Byte, is calculated in two

different ways. 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 passes through the Extended Read Memory flow chart will

generate a 16-bit CRC that is the result of clearing the CRC generator and then shifting in the Redirection

Byte only.