COM20020D datasheet(30/72 Pages) SMSC

30 / 72 page

Table 6 - Command Register

DATA

COMMAND

DESCRIPTION

0000 0000

Clear

Transmit

Interrupt

This command is used only in the Command Chaining operation.

Please refer to the Command Chaining section for definition of

this command.

0000 0001

Disable

Transmitter

This command will cancel any pending transmit command

(transmission that has not yet started) and will set the TA

(Transmitter

Available)

status

bit

logic

"1"

when

the

COM20020D next receives the token.

0000 0010

Disable

Receiver

This command will cancel any pending receive command. If the

COM20020D is not yet receiving a packet, the RI (Receiver

Inhibited) bit will be set to logic "1" the next time the token is

received. If packet reception is already underway, reception will

run to its normal conclusion.

b0fn n100

Enable

Receive to

Page fnn

This command allows the COM20020D to receive data packets

into RAM buffer page fnn and resets the RI status bit to logic "0".

The values placed in the "nn" bits indicate the page that the data

will be received into (page 0, 1, 2, or 3). If the value of "f" is a

logic "1", an offset of 256 bytes will be added to that page

specified in "nn", allowing a finer resolution of the buffer. Refer to

the Selecting RAM Page Size section for further detail.

If the

value of "b" is logic "1", the device will also receive broadcasts

(transmissions to ID zero). The RI status bit is set to logic "1"

upon successful reception of a message.

00fn n011

Enable

Transmit from

Page fnn

This command prepares the COM20020D to begin a transmit

sequence from RAM buffer page fnn the next time it receives the

token. The values of the "nn" bits indicate which page to transmit

from (0, 1, 2, or 3). If "f" is logic "1", an offset of 256 bytes is the

start of the page specified in "nn", allowing a finer resolution of the

buffer. Refer to the Selecting RAM Page Size section for further

detail. When this command is loaded, the TA and TMA bits are

reset to logic "0". The TA bit is set to logic "1" upon completion of

the transmit sequence. The TMA bit will have been set by this

time if the device has received an ACK from the destination node.

The ACK is strictly hardware level, sent by the receiving node

before its microcontroller is even aware of message reception.

Refer to Figure 1 for details of the transmit sequence and its

relation to the TA and TMA status bits.

0000 c101

Define

Configuration

This command defines the maximum length of packets that may

be handled by the device. If "c" is a logic "1", the device handles

both long and short packets.

If "c" is a logic "0", the device

handles only short packets.

000r p110

Clear Flags

This command resets certain status bits of the COM20020D. A

logic "1" on "p" resets the POR status bit and the EXCNAK

Diagnostic status bit. A logic "1" on "r" resets the RECON status

bit.

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