COM20020_00 Datasheet, PDF (55/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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
(at EF=1) interrupt inactive time interval between
interrupts is guaranteed.
The second reception will occur as soon as a
second packet is sent to the node, as long as the
second "Enable Receive to Page fnn" command
was issued. The operation is as if a new "Enable
Receive to Page fnn" command has just been
issued. After the first Receive status bits are
cleared, the Status Register will again be updated
with the results of the second reception and a
second interrupt resulting from the second
reception will occur.
In the COM20020, the Receive Inhibit (RI) bit of
the Interrupt Mask Register now masks only the
TRI bit of the Status Register, not the RI bit as in
the non-chaining mode. Since the TRI bit is only
set upon reception of a packet (not by RESET),
and since the TRI bit may easily be reset by
issuing a "Clear Receive Interrupt" command,
there is no need to use the RI bit of the Interrupt
Mask Register to mask interrupts generated by
the TRI bit of the Status Register. In Command
Chaining mode, the "Disable Receiver" command
will cancel the oldest reception, unless the
reception has already begun. If both receptions
should be canceled, two "Disable Receiver"
commands should be issued.
RESET DETAILS
Internal Reset Logic
The COM20020 includes special reset circuitry to
guarantee smooth operation during reset. Special
care is taken to assure proper operation in a
variety of systems and modes of operation. The
COM20020 contains digital filter circuitry and a
Schmitt Trigger on the nRESET signal to reject
glitches in order to ensure fault-free operation.
The COM20020 supports two reset options;
software and hardware reset. A software reset is
generated when a logic "1" is written to bit 7 of the
Configuration Register. The device remains in
reset as long as this bit is set. The software reset
does not affect the microcontroller interface
modes determined after hardware reset, nor does
it affect the contents of the Address Pointer
Registers, the Configuration Register, or the
Setup1 Register. A hardware reset occurs when a
low signal is asserted on the nRESET input. The
minimum reset pulse width is 5TXTL. This pulse
width is used by the internal digital filter, which
filters short glitches to allow only valid resets to
occur.
Upon reset, the transmitter portion of the device is
disabled and the internal registers assume those
states outlined in the Internal Registers section.
After the nRESET signal is removed the user may
write to the internal registers. Since writing a non-
zero value to the Node ID Register wakes up the
COM20020 core, the Setup1 Register should be
written before the Node ID Register. Once the
Node ID Register is written to, the COM20020
reads the value and executes two write cycles to
the RAM buffer. Address 0 is written with the data
D1H and address 1 is written with the Node ID.
The data pattern D1H was chosen arbitrarily, and
is meant to provide assurance of proper
microsequencer operation.
INITIALIZATION SEQUENCE
Bus Determination
Writing to and reading from an odd address
location from the COM20020's address space
causes the COM20020 to determine the
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