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CHAPTER 5 SAR FUNCTION
241
5.10.2 Use-Command-FIFO Mode
If the CFE bit of the GMR register is set to 1, commands are issued in use-command-FIFO mode. This
mode is valid only in PCI mode. In Generic mode, the CFE bit must be set to 0 so that commands are issued in
not-use-command-FIFO mode.
The
μ
PD98405 has a built-in command FIFO that can accommodate up to 20 host commands (10 CMR
commands and 10 CER commands). By using the command FIFO, the host can issue commands without
checking the busy bit, thus improving system performance.
When the host is about to issue a command (that is, about to access the CMR, CMR_L, CER, or CER_L in
slave mode), if the command FIFO is full, the
μ
PD98405 executes target termination using retry control at the
PCI interface. In addition, the host can detect whether the command FIFO is busy. Reading the command
FIFO register (CFR) enables detection of the number of commands in the command FIFO.
(1) Busy flag
The busy flag (B bit) indicates that the
μ
PD98405 has a command for which a response will be returned
(an indication in the command FIFO) or executed. The
μ
PD98405 sets the B bit to 1 only when the host
has issued a command for which an indication will be issued as a response. The B bit remains set to 1 not
only when the command is being executed but also when the command FIFO contains a command for
which an indication will be issued as a response.
For those commands for which an indication will not be issued as a response, the B bit is not set to 1. The
host can issue such a command at any time without checking the B bit. It need not check the end of the
command either.
After issuing a command for which an indication will be issued as a response, the host checks the B bit. If
the B bit is set to 0, the
μ
PD98405 terminates command execution and stores the indication. If the host
has issued a command for which an indication will be issued as a response, and if the B bit is set to 1, no
more commands for which an indication will be issued as a response can be issued. Before issuing such a
command, it is necessary to wait until the completion of the previous command and receive the
corresponding indication. A command for which an indication will not be issued as a response can be
issued at any time even if the B bit is set to 1. Table 5-12 lists those commands for which an indication is
issued as a response, as well as those that will not.
Table 5-12. Commands for Which an Indication Is Issued as a Response/Not Issued
Command for which indication is issued
Command for which indication is not issued
Open_Channel command
Deactivate_Channel command
Close_Channel command
Tx_Ready command
Indirect_Access read command
Add_Batches command
BRM_Tx command
NOP command
Indirect_Access write command
Set_Rx_Congestion command