4.2.1 Command Format

All Programming Executive commands have a general format, consisting of a 16-bit header and any required data for the command (see Figure 4-3). The 16-bit header consists of a 4-bit opcode field, which is used to identify the command, followed by a 12-bit command length field.

The command opcode must match one of those in the command set. Any command that is received, which does not match the list in Table 4-1, will return a “NACK” response (see Opcode Field).

The command length is represented in 16-bit words since the SPI operates in 16-bit mode. The Programming Executive uses the command length field to determine the number of words to read from the SPI port. If the value of this field is incorrect, the command will not be properly received by the Programming Executive.