5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
MSB
TRI
LSB
RI
TA
POR
TEST
RECON
TMA
TMA
TTA
TTA
TRI
FIGURE 10 - COMMAND CHAINING STATUS REGISTER QUEUE
7.4 Command Chaining
The Command Chaining operation allows consecutive transmissions and receptions to occur without host
microcontroller intervention.
Through the use of a dual two-level FIFO, commands to be transmitted and received, as well as the status bits, are
pipelined.
In order for the COM20020I to be compatible with previous SMSC ARCNET device drivers, the device defaults to the
non-chaining mode. In order to take advantage of the Command Chaining operation, the Command Chaining Mode
must be enabled via a logic "1" on bit 6 of the Configuration Register.
In Command Chaining, the Status Register appears as in Figure 10.
The following is a list of Command Chaining guidelines for the software programmer. Further detail can be found in the
Transmit Command Chaining and Receive Command Chaining sections.
The device is designed such that the interrupt service routine latency does not affect performance.
Up to two outstanding transmissions and two outstanding receptions can be pending at any given time. The
commands may be given in any order.
Up to two outstanding transmit interrupts and two outstanding receive interrupts are stored by the device, along with
their respective status bits.
The Interrupt Mask bits act on TTA (Rising Transition on Transmitter Available) for transmit operations and TRI
(Rising Transition of Receiver Inhibited) for receive operations. TTA is set upon completion of a packet
transmission only. TRI is set upon completion of a packet reception only. Typically there is no need to mask the
TTA and TRI bits after clearing the interrupt.
The traditional TA and RI bits are still available to reflect the present status of the device.
Transmit Command Chaining
When the processor issues the first "Enable Transmit to Page fnn" command, the COM20020I responds in the usual
manner by resetting the TA and TMA bits to prepare for the transmission from the specified page. The TA bit can be
used to see if there is currently a transmission pending, but the TA bit is really meant to be used in the non-chaining
mode only. The TTA bits provide the relevant information for the device in the Command Chaining mode.
In the Command Chaining Mode, at any time after the first command is issued, the processor can issue a second
"Enable Transmit from Page fnn" command. The COM20020I stores the fact that the second transmit command was
issued, along with the page number.
After the first transmission is completed, the COM20020I updates the Status Register by setting the TTA bit, which
generates an interrupt. The interrupt service routine should read the Status Register. At this point, the TTA bit will be
found to be a logic "1" and the TMA (Transmit Message Acknowledge) bit will tell the processor whether the
transmission was successful. After reading the Status Register, the "Clear Transmit Interrupt" 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.
Revision 12-06-06
40
SMSC COM20020I 3.3V
DATASHEET
SMSC [ SMSC CORPORATION ]
