DS2502

5 of 22

1024-BITS EPROM

The memory map in Figure 5 shows the 1024-bit EPROM section of the DS2502 which is configured as

four 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 an 8-bit

CRC from the DS2502 that confirms proper receipt of the data. 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 1024-bit EPROM portion of the DS2502 are given in the Memory Function Commands

section.

EPROM STATUS BYTES

In addition to the 1024 bits of data memory the DS2502 provides 64 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 DS2502. The first byte of the EPROM Status Memory contain the Write Protect Page

bits which inhibit programming of the corresponding page in the 1024-bit main memory area if the

appropriate write protection bit is programmed. Once a bit has been programmed in the Write Protect

Page byte, the entire 32-byte page that corresponds to that bit can no longer be altered but may still be

read.

The next 4 bytes of the EPROM Status Memory contain the Page Address Redirection Bytes, which

indicate if one or more of the pages of data in the 1026-bit EPROM section have been invalidated and

redirected to the page address contained in the appropriate redirection byte. The hardware of the DS2502

makes no decisions based on the contents of the Page Address Redirection Bytes. These additional bytes

of Status 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 DS2502 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.

If a Page Address Redirection Byte has an 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 DS2502 are given in the Memory Function Commands section.

MEMORY FUNCTION COMMANDS

The “Memory Function Flow Chart” (Figure 6) describes the protocols necessary for accessing the

various data fields within the DS2502. The Memory Function Control section, 8-bit scratchpad, and the

Program Voltage Detect circuit combine to interpret the commands issued by the bus master and create

the correct control signals within the device. A 3-byte protocol is issued by the bus master. It is

comprised of a command byte to determine the type of operation and two address bytes to determine the

specific starting byte location within a data field. The command byte indicates if the device is to be read