DS28EC20: 20Kb 1-Wire EEPROM

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MEMORY FUNCTION COMMANDS

The Memory Function Flow Chart (Figure 7) describes the protocols necessary for accessing the memory of the

DS28EC20. The target address registers TA1 and TA2 are used for both read and write. To prevent accidental

changes to the data memory or control registers the device employs a BS-flag indicating a “bad sequence”. The

communication between master and DS28EC20 takes place either at standard speed (default, OD = 0) or at

overdrive speed (OD = 1). If not explicitly set into the Overdrive mode, the DS28EC20 assumes standard speed.

WRITE SCRATCHPAD COMMAND [0Fh]

The Write Scratchpad command applies to the data memory and the writable addresses in the register page. After

issuing the Write Scratchpad command, the master must first provide the 2-byte target address, followed by the

data to be written to the scratchpad. The data is written to the scratchpad starting at the byte offset of T[4:0]. The

ES bits E[4:0] are loaded with the starting byte offset, and increment with each subsequent byte. Effectively, E[4:0]

is the byte offset of the last full byte written to the scratchpad. Only full bytes are accepted. If the last byte is

incomplete its content is ignored and the partial byte flag PF is set. The PF flag is also set if the master ends the

command before a complete target address is transmitted. The PF and BS flags are both cleared when a complete

target address is received.

When executing the Write Scratchpad command, the CRC generator inside the DS28EC20 (Figure 13) calculates a

16-bit CRC of the entire data stream, starting at the command code and ending at the last data byte as sent by the

master. This CRC is generated using the CRC16 polynomial (x

and then shifting in the command code (0Fh) of the Write Scratchpad command, the target addresses TA1 and

TA2 as supplied by the master, and all the data bytes. The master can end the Write Scratchpad command at any

time. However, if the end of the scratchpad is reached (E[4:0] = 11111b), the master can send 16 read-time slots to

receive the CRC generated by the DS28EC20.

If a Write Scratchpad is attempted to a write-protected location, the scratchpad is loaded with the data already in

memory, rather than the data transmitted. Similarly, if the target address page is in EPROM mode, the scratchpad

is loaded with the bitwise logical AND of the transmitted data and the data already in memory.

The DS28EC20’s memory address range is 0000h to 0A3Fh. If the bus master sends a target address higher than

this, the DS28EC20’s internal circuitry sets the four most significant address bits to zero as they are shifted into the

internal address register. The Read Scratchpad command reveals the modified target address. The master

identifies such address modifications by comparing the target address read back to the target address transmitted.

If the master does not read the scratchpad, a subsequent Copy Scratchpad command does not work since the

most significant bits of the target address the master sends do not match the value the DS28EC20 expects.

READ SCRATCHPAD COMMAND [AAh]

The Read Scratchpad command allows verifying the target address and the integrity of the scratchpad data. After

issuing the command code, the master begins reading. The first two bytes are the target address. The next byte is

the Ending Offset/Data Status byte (E/S) followed by the scratchpad data beginning at the byte offset (T[4:0]). The

scratchpad data can be different from what the master originally sent. This is of particular importance if the target

address is within the register page or a page in either Write Protection or EPROM modes. See the Write

Scratchpad Command section for details. The master should read through the end of the scratchpad, after which it

receives an inverted CRC16, based on data as it was sent by the DS28EC20. If the master continues reading after

the CRC, all data are logic 1s.