5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
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 COM20020I guarantees a minimum of 200nS (at EF=1) 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 COM20020I guarantees a minimum of 200ns (at EF=1) 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 (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 COM20020I, 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 COM20020I 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 COM20020I contains digital filter circuitry
and a Schmitt Trigger on the nRESET signal to reject glitches in order to ensure fault-free operation.
The COM20020I 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
SMSC COM20020I 3.3V
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Revision 12-06-06
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