DS2423 BLOCK DIAGRAM Figure 1 ADDRESS REGISTERS AND TRANSFER STATUS Because of the serial data transfer, the DS2423 employs three address registers called TA1, TA2, and E/S (Figure 6). Registers TA1 and TA2 must be loaded with the target address to which the data will be written or from which data will be sent to the master upon a Read command. Register E/S acts like a byte counter and Transfer Status register. It is used to verify data integrity with write commands. Therefore, the master only has read access to this register. The lower 5 bits of the E/S register indicate the address of the last byte that has been written to the scratchpad. This address is called Ending Offset. Bit 5 of the E/S register, called PF or “partial byte flag,” is set if the number of data bits sent by the master is not an integer multiple of 8. Bit 6 has no function; it always reads 0. Note that the lowest 5 bits of the target address also determine the address within the scratchpad, where intermediate storage of data will begin. This address is called byte offset. If the target address (TA1) for a Write command is 03CH for example, then the scratchpad will store incoming data beginning at the byte offset 1CH and will be full after only 4 bytes. The corresponding ending offset in this example is 1FH. For best economy of speed and efficiency, the target address for writing should point to the beginning of a new page, i.e., the byte offset will be 0. Thus the full 32-byte capacity of the scratchpad is available, resulting also in the ending offset of 1FH. However, it is possible to write one or several contiguous bytes somewhere within a page. The ending offset together with the Partial Flag support the master checking the data integrity after a Write command. The highest valued bit of the E/S register, called AA or Authorization Accepted, acts as a flag to indicate that the data stored in the scratchpad has already been copied to the target memory address. Writing data to the scratchpad clears this flag. Not Recommended for New Design 3 of 25
