Writing to the ID Register

Once the Protect Register has been locked, all registers less than the Protect Register may be written to by carrying out the WEN (Write Enable) instruction followed by the WRITE (Write) instruction. The device powers up in the Write Disable state. Hence, all programming operations must be preceded by a WEN command. Once this command has been executed, programming will remain enabled until a WDS (Write Disable) command is performed or Vcc is completely removed.

The WRITE function allows 16 bits of data to be written into a specified address. After the last bit of data is entered into the data-in (DI) pin, the CS (Chip Select) must be brought low before the next rising edge of the SK clock. The falling edge of the CS initiates the self-timed programming cycle. It is essential that the Program Enable (PE) pin be held at high while loading the WRITE instruction. After the loading process, the PE pin becomes neutral, or don't care. The DO pin indicates the ready or busy status of the chip if CS is brought high after the minimum CS low time (tCS) interval has been rendered. A DO reading of logical 0 indicates that the programming is still in process. When DO is logical 1 the register at the address specified in the instruction has been written to and is ready for another instruction to be performed.

More prosaically, data is written to the chip's memory at a specified address by following these steps:

1. set appropriate CS high to select a chip
2. enter write enable (WEN) mode by
   1. setting PRE low
   2. setting PE high
   3. clock in the bit pattern 10011, left-most bit first
3. keep PRE low
4. clock in the bit pattern 101xxAAAA into DI, left-most bits first, where x = (don't care) and A is the address of the particular register to write
5. clock in D15-D0, the data to enter into the selected register