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CHAPTER 5 SAR FUNCTION
237
When data is read, a command is written to the command register, and the host takes over the data stored
to the command extension register (indication) once execution of the command has been completed. The
μ
PD98405 stores data to the command extension register once execution of the command has been
completed. The host reads the command extension register after it has confirmed that the B bit has been
cleared to "0."
The parameters to be set to the CER register depend on the command to be executed. For details, refer to
the description of each command.
5.10.1 Not-Use-Command-FIFO Mode
If the CFE bit of the GMR register is set to 0, the host is in not-use-command-FIFO mode. In Generic mode,
the CFE bit must be set to 0 so that commands are issued in not-use-command-FIFO mode.
(1) Busy flag
The host writes commands to the command register together with appropriate parameters.
The
μ
PD98405 internal operation is performed in units of states, each of which is equivalent to 36 system
clocks. The
μ
PD98405 receives a command from the host in one state, then executes that command in
the next state. Because the
μ
PD98405 can execute no more than one command in each state, whenever
it receives a command, it sets bit 31 (B bit) of the command register to "1," indicating that the next state is
busy. In other words, this B bit functions as a busy flag that indicates that the
μ
PD98405 is executing a
command.
While the B bit is set to "1," the
μ
PD98405 cannot receive a command. Even if the command register is
overwritten with a new command while the B bit is set to 1, the
μ
PD98405 regards that command as being
invalid and ignores it. The host can issue a new command only when the B bit is set to "0." The
μ
PD98405 clears the B bit to "0" upon the completion of command execution in the state next to that in
which it received the command.
Figure 5-50. Command Issue and B Bit
36 clocks
Internal
state
Command
issued
B bit
Command
<1>
1
0
<1>
Executes
<2>
Executes
<3>
Executes
Command
<2>
Command
<3>