DS2406

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STATUS MEMORY

The Status Memory can be read or written to indicate various conditions to the software interrogating the

DS2406. These conditions include special features for the data memory, definition of the settings for the

Conditional Search as well as the channel flip-flops and the external power supply indication. How these

functions are assigned to the bits of the Status Memory is detailed in Figure 6.

The first 4 bits of the Status Memory (address 0, bits 0 to 3) contain the Write Protect Page bits which

inhibit programming of the corresponding page in the 1024-bit data 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 remaining 4 bits of Status Memory location 0 are reserved for use by the 1-Wire File

Structure, indicating which memory pages are already in use.

Originally, all of these bits are

unprogrammed, indicating that the device does not contain 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 DS2406.

The next four bytes of the Status Memory (addresses 1 to 4) contain the Page Address Redirection Bytes

which indicate if one or more of the pages of data in the 1024-bits EPROM memory section have been

invalidated by software and redirected to the page address contained in the appropriate redirection byte.

The hardware of the DS2406 makes no decisions based on the contents of the Page Address Redirection

Bytes. Since with EPROM technology bits can only be changed from a logical 1 to a logical 0 by

programming, 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 DS2406. Under

TMEX, a page is redirected 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.

Under TMEX, 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 than FFh,

the data in the page corresponding to that redirection byte is invalid. According to the TMEX definitions,

the valid data will 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. Since the data memory consists of

four pages only, the 6 most significant bits of the redirection bytes cannot be programmed to zeros.

Status Memory location 5 serves as a test byte and is programmed to 00h at the factory. Status Memory

location 6 has no function with the DS2406. It is factory-programmed to 00h to distinguish the DS2406

from the DS2407, which both share the same family code. A DS2407 with Status Memory location 6

programmed to 00h will power-up into hidden mode and will only respond if the bus master addresses it

by a Match ROM command followed by the correct device ROM code. Conversely, a device that does

respond to a Read ROM command with family code 12h can only be a DS2406 if its Status Memory

location 6 reads 00h.