1-Wire 4Kb EEPROM

8

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Memory Access

Address Registers and Transfer Status

The DS24B33 employs three address registers: TA1,

TA2, and E/S (Figure 6). Registers TA1 and TA2 must

be loaded with the target address to which the data is

written or from which data is read. Register E/S is a

read-only transfer status register used to verify data

integrity with write commands. ES bits E[4:0] are

loaded with the incoming T[4:0] on a Write Scratchpad

command and increment on each subsequent data

byte. This is, in effect, a byte-ending offset counter

within the 32-byte scratchpad. Bit 5 of the E/S register,

called the partial byte flag (PF), is set if the number of

data bits sent by the master is not an integer multiple of

8 or if the data in the scratchpad is not valid due to a

loss of power. A valid write to the scratchpad clears the

PF bit. Bit 6 has no function; it always reads 0. The

highest valued bit of the E/S register, called authoriza-

tion accepted (AA), is valid only if the PF flag reads 0. If

PF is 0 and AA is 1, the data stored in the scratchpad

has already been copied to the target memory address.

Writing data to the scratchpad clears this flag.

Writing with Verification

To write data to the DS24B33, the scratchpad must be

used as intermediate storage. First, the master issues

the Write Scratchpad command to specify the desired

target address, followed by the data to be written to the

scratchpad. Under certain conditions (see the

Write

Scratchpad [0Fh] section) the master receives an

inverted CRC-16 of the command, address, and data at

the end of the Write Scratchpad command sequence.

Knowing this CRC value, the master can compare it to

the value it has calculated itself to decide if the commu-

nication was successful and proceed to the Copy

Scratchpad command. If the master could not receive

the CRC-16, it should send the Read Scratchpad com-

mand to verify data integrity. As a preamble to the

scratchpad data, the DS24B33 repeats the target

address TA1 and TA2 and sends the contents of the

E/S register. If the PF flag is set, data did not arrive cor-

rectly in the scratchpad or there was a loss of power

since data was last written to the scratchpad. The mas-

ter does not need to continue reading; it can start a

new trial to write data to the scratchpad. Similarly, a set

AA flag together with a cleared PF flag indicates that

the device did not recognize the write command. If

everything went correctly, both flags are cleared and

the ending offset indicates the address of the last byte

written to the scratchpad. Now the master can continue

reading and verifying every data byte. After the master

has verified the data, it can send the Copy Scratchpad

command, for example. This command must be fol-

lowed exactly by the data of the three address registers

TA1, TA2, and E/S. The master should obtain the con-

tents of these registers by reading the scratchpad. As

soon as the DS24B33 has received these bytes correctly,

it starts copying the scratchpad data to the requested

location.

BIT NUMBER

7

6

5

4

3

2

1

0

TARGET ADDRESS (TA1)

T7

T6

T5

T4

T3

T2

T1

T0

TARGET ADDRESS (TA2)

T15

T14

T13

T12

T11

T10

T9

T8

ENDING ADDRESS WITH

DATA STATUS (E/S)

(READ ONLY)

AA

0

PF

E4

E3

E2

E1

E0

Figure 6. Address Registers