DS2407 datasheet(7/31 Pages) DALLAS

7 / 31 page

DS2407

012099 7/31

STATUS MEMORY

The Status Memory can be read or written to indicate

various conditions to the software interrogating the

DS2407. These conditions include special features for

the data memory, definition of the power–on default and

actual settings for the Conditional Search as well as the

channel flip–flops and the external power supply indica-

tion. How these functions are assigned to the bits of the

Status Memory is detailed in Figure 5. The channel flip–

flops and power supply indication are also included in

the Channel Info Byte of the Channel Access command

protocol (see Figure 6).

The first four 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 iButton operating software TMEX. Their purpose is

to indicate which memory pages are already in use.

Originally, all of these bits are unprogrammed, indicat-

ing 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 DS2407.

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 invali-

dated by software and redirected to the page address

contained in the appropriate redirection byte. The hard-

ware of the DS2407 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 pos-

sible to simply rewrite a page if the data requires chang-

ing or updating. But with space permitting, an entire

page of data can be redirected to another page within

the DS2407. Under TMEX, a page is redirected by writ-

ing 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 corre-

sponds to that page is valid. If a Page Address Redirec-

tion 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 is programmed to 00h at the

factory. Status Memory location 6 contains the pow-

er–on default settings for the Conditional Search Select

(CSS0 to CSS4, bits 0 to 4) and the PIO channels. The

power–on settings become valid as they are internally

transferred by the device into Status Memory location 7

after the device has powered up and the bus master

sends a ROM Function Command byte for the first time.

The codes for the Conditional Search Settings are

detailed with the description of the Conditional Search

command later in this data sheet. If both CSS1 and

CSS2 in Status Memory Location 7 are set to zero, the

DS2407 will enter a “Hidden Mode” where it will keep its

status but only responds to Match ROM and Conditional

Search. To respond to Conditional Search the polarity

(CSS0) needs to be 1. The “Hidden Mode” can be ended

either by a power–on reset or by matching the device’s

registration number and setting CSS1 or CSS2 to 1.

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