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DS1985 Datasheet(PDF) 5 Page - Maxim Integrated Products |
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DS1985 Datasheet(HTML) 5 Page - Maxim Integrated Products |
5 / 26 page DS1985 5 of 26 16384 BITS EPROM The memory map in Figure 4 shows the 16384 bits EPROM section of the DS1985 which is configured as 64 pages of 32 bytes each. The 8-bit scratchpad is an additional register that acts as a buffer when programming the memory. Data is first written to the scratchpad and then verified by reading a 16-bit CRC from the DS1985 that confirms proper receipt of the data and address. If the buffer contents are correct, a programming voltage should be applied and the byte of data will be written into the selected address in memory. This process ensures data integrity when programming the memory. The details for reading and programming the 16384 bits EPROM portion of the DS1985 are given in the Memory Function Commands section. EPROM STATUS BYTES In addition to the 16384 bits of data memory the DS1985 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 DS1985. The first 8 bytes of the EPROM Status Memory (addresses 000 to 007H) contain the Write Protect Page bits that 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 that 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 DS1985. 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 bits EPROM section have been invalidated by software and redirected to the page address contained in the appropriate redirection byte. The hardware of the DS1985 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 DS1985 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. |
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