TNETX3100 datasheet(19/80 Pages) TI1

19 / 80 page

TNETX3100

ThunderSWITCH™ 10-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS031D – JUNE 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

DRAM and EAM interfaces

All valid frames are passed across the DRAM interface (see Figure 8). The EAM hardware can detect the start of

a new frame from the flag-byte information. The first flag nibble on the DRAM bus (bits 35–32) corresponds to

bits 7–4 of the frame flag (see Figure 6). In conjunction with the DRAM column address strobe, external logic

accesses the frame addresses and performs external address lookups as detailed in the following:

D Use row-address strobe (DRAS) and column-address strobe (DCAS) to identify the position of the forward

pointer, the top nibble of the flag byte, and whether the nibble contains the start of frame code 01XX. Bit

35 of the forward pointer is zero to denote a start of frame. If it is high, the frame is an in-order broadcast

(IOB) link buffer and not the start-of-data frame (bits 34, 33, and 32 contain parity information for the three

forward pointers or data bytes).

D Bits 31–28 of the forward pointer contain the pre-tag nibble applied to the uplink (when used in uplink mode).

See description of port 00 (uplink port).

D Bits 27–24 denote the active port number (port 00 = 0000, port 01 = 0001, etc.).

D Use the DRAM column-address strobe (DCAS) to identify the presence of destination and source address

data on the DRAM interface.

D Perform address processing.

D Present the destination-channel bit map not more than 12 memory cycles after the high nibble of the start

flag is transmitted on the DRAM interface.

D External address timing is shown in Figure 21.

NOTE: Diagram shows end-of-frame flag format. Multiple buffer frames use this flag format only on the last buffer. (End-of buffer flags are used

between a frame’s buffers.)

C = Channel code/source port code

P = Parity bits for forward pointer

B = Number of valid bytes in data word

S = Frame status (error conditions)

Tg = Pretag (uplink mode)

U = Reserved

1 U U

Byte 3

Byte 7

Byte 2

Byte 6

Byte 1

Byte 5

Byte 0

Byte 4

00 U U

CCC

CCCC

00 U U

CCCC

00 U U

CCCC

00 U U

CCCC

00 U U

CCCC

0 0U U

CCCC

1 S S S

Byte 63

Byte 62

Byte 61

Byte 60

Flags

Buffer Data

PTg

Tg Tg Tg CC C C

Forward Pointer

NOTE: IOB Index buffer bit = 0; this is

a data buffer.

DA03

DA02

DA01

DA00

SAD1

SAD0

DA05

DA04

Figure 6. Buffer Diagram Showing Flag Format for a 64-byte Frame

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