COM20019IP datasheet(39/81 Pages) SMSC

39 / 81 page

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The software flow for controlling the Configuration,

Node ID, Tentative ID, and Next ID registers is

generally limited to the initialization sequence and

the maintenance of the network map.

Additionally, it is necessary to understand the

details of how the other Internal Registers are

used in the transmit and receive sequences and to

know how the internal RAM buffer is properly set

up. The sequence of events that tie these actions

together is discussed as follows.

Selecting RAM Page Size

During normal operation, the 2K x 8 of RAM is

divided into four pages of 512 bytes each. The

page to be used is specified in the "Enable

Transmit (Receive) from (to) Page fnn" command,

where "nn" specifies page 0, 1, 2, or 3.

This

allows the user to have constant control over the

allocation of RAM.

When the Offset bit "f" (bit 5 of the "Enable

Transmit (Receive) from (to) Page fnn" command

word) is set to logic "1", an offset of 256 bytes is

added to the page specified. For example: to

transmit from the second half of page 0, the

command "Enable Transmit from Page fnn"

(fnn=100 in this case) is issued by writing 0010

0011 to the Command Register. This allows a

finer resolution of the buffer pages without

affecting software compatibility. This scheme is

useful for applications which frequently use packet

sizes

256 bytes

less,

especially for

microcontroller

systems

with

limited memory

capacity.

The remaining portions of the buffer

pages which are not allocated for current transmit

or receive packets may be used as temporary

storage for previous network data, packets to be

sent later, or as extra memory for the system,

which may be indirectly accessed.

If the device is configured to handle both long and

short

packets

(see

"Define

Configuration"

command), then receive pages should always be

512 bytes long because the user never knows

what the length of the receive packet will be. In

this case, the transmit pages may be made 256

bytes long, leaving at least 512 bytes free at any

given time.

Even if the Command Chaining

operation

being

used,

COM20019IP датащи(PDF) 39 Page - SMSC Corporation

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