44
44
command is issued, thus resetting the TTA bit
and clearing the interrupt.
Note that only
the
"Clear Transmit Interrupt" command will
clear the TTA bit and the interrupt.
It is not
necessary, however, to clear the bit or the
interrupt right away because the status of the
transmit operation is double buffered in order to
retain the results of the first transmission for
analysis by the processor. This information will
remain in the Status Register until the "Clear
Transmit Interrupt" command is issued.
Note
that the interrupt will remain active until the
command is issued, and the second interrupt will
not occur until the first interrupt is acknowledged.
The ARCNET core guarantees a minimum of
200nS interrupt inactive time interval between
interrupts. The TMA bit is also double buffered
to reflect whether the appropriate transmission
was a success.
The TMA bit should only be
considered valid after the corresponding TTA bit
has been set to a logic "1". The TMA bit never
causes an interrupt.
When the token is received again, the second
transmission will be automatically initiated after
the first is completed by using the stored "Enable
Transmit
from
Page
fnn"
command.
The
operation is as if a new "Enable Transmit from
Page fnn" command has just been issued. After
the first Transmit status bits are cleared, the
Status Register will again be updated with the
results of the second transmission and a second
interrupt resulting from the second transmission
will occur.
The ARCNET core guarantees a
minimum of 200ns interrupt inactive time interval
before the following edge.
The Transmitter Available (TA) bit of the Interrupt
Mask Register now masks only the TTA bit of the
Status Register, not the TA bit as in the non-
chaining mode.
Since the TTA bit is only set
upon transmission of a packet (not by RESET),
and since the TTA bit may easily be reset by
issuing a "Clear Transmit Interrupt" command,
there is no need to use the TA bit of the Interrupt
Mask Register to mask interrupts generated by
the TTA bit of the Status Register. In Command
Chaining
mode,
the
"Disable
Transmitter"
command will cancel the oldest transmission.
This permits canceling a packet destined for a
node not ready to receive.
If both packets
should be canceled, two "Disable Transmitter"
commands should be issued.
Receive Command Chaining
Like the Transmit Command Chaining operation,
the processor can issue two consecutive "Enable
Receive from Page fnn" commands.
After the first packet is received into the first
specified page, the TRI bit of the Status Register
will be set to logic "1", causing an interrupt.
Again, the interrupt need not be serviced
immediately.
Typically, the interrupt service
routine will read the Status Register.
At this
point, the RI bit will be found to be a logic "1".
After reading the Status Register, the "Clear
Receive Interrupt" command should be issued,
thus resetting the TRI bit and clearing the
interrupt.
Note that only the "Clear Receive
Interrupt" command will clear the TRI bit and the
interrupt. It is not necessary, however, to clear
the bit or the interrupt right away because the
status of the receive operation is double buffered
in order to retain the results of the first reception
for analysis by the processor, therefore the
information will remain in the Status Register
until the "Clear Receive Interrupt" command is
issued. Note that the interrupt will remain active
until the "Clear Receive Interrupt" command is
issued, and the second interrupt will be stored
until the first interrupt is acknowledged.
A
minimum of 200nS interrupt inactive time interval
between interrupts is guaranteed.
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