DS1977-F5 Просмотр технического описания (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1977-F5

32KB EEPROM iButton

Dallas Semiconductor -> Maxim Integrated 

Writing With Verification

To write data to the DS1977 iButton, the scratchpad has to be used as intermediate storage. First

the 1-Wire 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 Write

Scratchpad command) the 1-Wire master will receive an inverted CRC16 of the command,

address and data at the end of the write scratchpad command sequence. Knowing this CRC

value, the 1-Wire master can compare it to the value it has calculated itself to decide if the

communication was successful and proceed to the Copy Scratchpad command. If the 1-Wire

master could not receive the CRC16, it has to send the Read Scratchpad command to read back

the scratchpad to verify data integrity. As preamble to the scratchpad data, the DS1977 iButton

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 correctly in the scratchpad or there was a loss of power since data was

last written to the scratchpad. The 1-Wire master 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 Write command was not recognized by the device. 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 1-Wire master can continue reading and verifying every data byte. After the

1-Wire master has verified the data, it has to send the Copy Scratchpad command. This

command must be followed exactly by the data of the three address registers TA1, TA2 and E/S.

The 1-Wire master may obtain the contents of these registers by reading the scratchpad or derive

it from the target address and the amount of data to be written. As soon as the DS1977 iButton

has received these bytes correctly and the 1-Wire master has provided an acceptable password,

the DS1977 will copy the scratchpad data to the requested location beginning at the target

address.

Memory and Control Functions

The DS1977 supports the following functions:

• Write Scratchpad (iButton version only)

• Read Scratchpad (iButton version only)

• Copy Scratchpad with Password (iButton version only)

• Read Memory with Password (iButton version only)

• Verify Password (iButton version only)

The following pages provide a short discussion of these functions. For the detailed flow charts

refer to Figure ??. The TA1, TA2, E/S logic is described in section Address Registers and

Transfer Status.

Write Scratchpad Command [0Fh]

After issuing the write scratchpad command, the 1-Wire master must first provide the 2-byte

target address, followed by the data to be written to the scratchpad. The data will be written to the

scratchpad starting at the byte offset (T5:T0). The ending offset (E5: E0) will be the byte offset at

which the 1-Wire master stops writing data. Only full data bytes are accepted. If the last data byte

is incomplete its content will be ignored and the partial byte flag PF will be set. When writing to a

password address, internal circuitry of the chip will force the 3 least significant address bits to 0.

Only full 8-byte passwords are accepted. The ending offset will be 07 or 0F, depending on the

password(s) to be changed.

When executing the Write Scratchpad command the CRC generator inside the DS1977 iButton

(see Figure ??) calculates a CRC over the entire data stream, starting at the command code and

ending at the last data byte sent by the 1-Wire master. This CRC is generated using the CRC16

polynomial by first clearing the CRC generator and then shifting in the command code (0FH) of

the Write Scratchpad command, the Target Addresses TA1 and TA2 as supplied by the 1-Wire

master and all the data bytes. The 1-Wire master may end the Write Scratchpad command at any

time. However, if the ending offset is 3Fh, the 1-Wire master may send 16 read time slots and will

receive the CRC generated by the DS1977 iButton.

Page 5

1/31/03

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