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CHAPTER 5 SAR FUNCTION
182
To activate the channel, enter "VC NUMBER" and set the enable bit in the lookup table. For an
explanation of lookup table specification, see
Section 5.5.4
. As a result, the channel enters the active
status and, when the
μ
PD98405 receives a cell that has the corresponding VPI/VCI from a PHY layer
device, it starts receive processing. The channel remains in this status as long as the enable bit of the
lookup table entry is set to 1. The channel subsequently returns to the idle status when the enable bit of
the lookup table entry is disabled and the Deactivate_Channel command is issued.
To disable this channel, the host disables the lookup table and issues the Deactivate_Channel command,
returns the channel to the idle status, and issues the Close_Channel command. This returns the VC table
to the free block pool, and results in the channel no longer existing, such that the non-existent status is set.
The Deactivate_Channel command must be issued at least 72 clocks after the lookup table has been
disabled. Create this timing by issuing the NOP command twice.
Figure 5-36. Receive Channel Status
Non-existent status
Idle status
Active status
Close_Channel command
Open_Channel command
Enables lookup table entry.
Disables lookup table entry.
Deactivate_Channel command
(4) Closing a receive channel
To terminate the use of a channel, the host issues a Close_Channel command while the channel is in the
idle state. The channel is placed in the idle state by first disabling the lookup table entry and issuing an
NOP command twice, then issuing a Deactivate_Channel command.
When the host issues a Close_Channel command, a Close_Channel indication is returned as a response.
If the indication includes the VC NUMBER of the channel to be closed, it means that execution of the
Close_Channel command was successful. The
μ
PD98405 returns the VC table to the free block pool.
Consequently, the channel cease to exist and therefore enters the non-existent state.
A Deactivate_Channel command must be issued for any VC for which a Close_Channel command has not
been issued. Even if two or more VCs are to be closed, it is impossible to issue Deactivate_Channel
commands continuously.
If the host issues a Close_Channel command to a channel for which a Deactivate_Channel command has
been issued to set the channel in the idle state, a Close_Channel indication may be returned in response,
indicating a failure in closing the channel. This is because the channel is in the idle state, but processing of
those cells in the internal receive FIFO has not yet been completed. In this case, the
μ
PD98405 is
performing internal processing related to the channel and does not allow the VC table to be released. If the
host receives a close failure indication, it must continuously issue Close_Channel commands until one is
accepted. Once a receive indication corresponding to the Deactivate_Channel command is issued, the
μ
PD98405 will accept a Close_Channel command.