COM20019I_06 Datasheet, PDF (36/65 Pages) SMSC Corporation – Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
software compatibility. This scheme is useful for applications which frequently use packet sizes of 256
bytes or 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 is being used,
512 bytes is still guaranteed to be free because Command Chaining only requires two pages for transmit
and two for receive (in this case, two 256 byte pages for transmit and two 512 byte pages for receive,
leaving 512 bytes free). Please note that it is the responsibility of software to reserve 512 bytes for each
receive page if the device is configured to handle long packets. The COM20019I does not check page
boundaries during reception. If the device is configured to handle only short packets, then both transmit
and receive pages may be allocated as 256 bytes long, freeing at least 1KByte at any given time.
Even if the Command Chaining operation is being used, 1KByte is still guaranteed to be free because
Command Chaining only requires two pages for transmit and two for receive (in this case, a total of four
256 byte pages, leaving 1K free).
The general rule which may be applied to determine where in RAM a page begins is as follows:
Address = (nn x 512) + (f x 256).
Rev. 03-07-06
Page 36
DATASHEET
SMSC COM20019I