DS2505

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EPROM STATUS BYTES

In addition to the 16384 bits of data memory the DS2505 provides 704 bits of Status Memory accessible

with separate commands.

The EPROM Status Bytes can be read or programmed to indicate various conditions to the software

interrogating the DS2505. The first 8 bytes of the EPROM Status Memory (addresses 000 to 007H)

contain the Write Protect Page bits which inhibit programming of the corresponding page in the 16384-bit

main memory area if the appropriate write protection bit is programmed.

Once a bit has been

programmed in the Write Protect Page section of the Status Memory, the entire 32-byte page that

corresponds to that bit can no longer be altered but may still be read.

The next 8 bytes of the EPROM Status Memory (addresses 020 to 027H) contain the Write Protect bits

which inhibit altering the Page Address Redirection Byte corresponding to each page in the 16384-bit

main memory area.

The following 8 bytes within the EPROM Status Memory (addresses 040 to 047H) are reserved for use

by the iButton operating software TMEX. Their purpose is to indicate which memory pages are already

in use. Originally, all of these bits are unprogrammed, indicating that the device does not store any data.

As soon as data is written to any page of the device under control of TMEX, the bit inside this bitmap

corresponding to that page will be programmed to 0, marking this page as used. These bits are

application flags only and have no impact on the internal logic of the DS2505.

The next 64 bytes of the EPROM Status Memory (addresses 100H to 13FH) contain the Page Address

Redirection Bytes which indicate if one or more of the pages of data in the 16384-bit EPROM section

have been invalidated by software and redirected to the page address contained in the appropriate

redirection byte. The hardware of the DS2505 makes no decisions based on the contents of the Page

Address Redirection Bytes. These additional bytes of Status EPROM allow for the redirection of an

entire page to another page address, indicating that the data in the original page is no longer considered

relevant or valid. With EPROM technology, bits within a page can be changed from a logical 1 to a

logical 0 by programming, but cannot be changed back. Therefore, it is not possible to simply rewrite a

page if the data requires changing or updating, but with space permitting, an entire page of data can be

redirected to another page within the DS2505 by writing the one’s complement of the new page address

into the Page Address Redirection Byte that corresponds to the original (replaced) page.

This architecture allows the user’s software to make a “data patch” to the EPROM by indicating that a

particular page or pages should be replaced with those indicated in the Page Address Redirection Bytes.

To leave an authentic audit trail of data patches, it is recommended to also program the write protect bit

of the Page Address Redirection Byte, after the page redirection is programmed. Without this protection,

it is still possible to modify the Page Address Redirection Byte, making it point to a different memory

page than the true one.

If a Page Address Redirection Byte has a FFH value, the data in the main memory that corresponds to

that page is valid. If a Page Address Redirection Byte has some other hex value, the data in the page

corresponding to that redirection byte is invalid, and the valid data can now be found at the one’s

complement of the page address indicated by the hex value stored in the associated Page Address

Redirection Byte. A value of FDH in the redirection byte for page 1, for example, would indicate that the

updated data is now in page 2. The details for reading and programming the EPROM status memory

portion of the DS2505 are given in the Memory Function Commands section.