TNETX4080 datasheet(23/64 Pages) TI1

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TNETX4080

ThunderSWITCH II™ 8-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS050A – AUGUST 1998 – REVISED NOVEMBER 1998

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

state of DIO signal pins during hardware reset

The CPU can perform a hardware reset by writing to an address in the range of 0x40–0x5F (writes to a DMA

address in this range have no effect on reset); this is equivalent to asserting the hardware RESET pin with the

following exceptions. During hardware reset, the output and bidirectional DIO pins behave as shown in Table 7.

D DIO interface continues to operate. The reset condition remains active until SCS is driven high. SRDY does

not become high impedance or resistively pulled high (unlike a true hardware reset), so it still can be used

as a normal acknowledge in this case.

D Following the reset, no EEPROM autoload is performed.

Table 7. DIO Interface During Hardware Reset

DIO INTERFACE

STATE DURING HARDWARE RESET

SRDY

High impedance – resistively pulled up

SDATA7–SDATA0

High impedance – resistively pulled up

STXRDY

Driven low

SRXRDY

Driven high

IEEE Std 802.1Q VLAN tags on the NM port

Frames received from the host via the NM port are required to contain a valid IEEE Std 802.1Q header (frames

that do not contain a valid IEEE Std 802.1Q header are incorrectly routed). They also can be corrupted at the

transmission port(s) as the tag-stripping process does not check that the four bytes after the source address

actually are a valid tag. The four bytes are a valid tag under all other circumstances.

When a frame is transmitted by the NM port (received by the host), no tag-stripping occurs, so the frame may

contain one or two tags, depending on how the frame originally was received.

frame format on the NM port

The frame format on the NM port differs slightly from a standard Ethernet frame format. The key differences are:

the frame always contains an IEEE Std 802.1Q header in the four bytes following the source address (see

Figure 2). The tag protocol identifier (TPID) or ethertype field, however, is used in the switch for other purposes,

so a frame transmitted out of the switch on the NM port does not have the IEEE Std 802.1Q TPID of 81–00

(ethertype constant) value in these two bytes.

The first TPID byte output contains:

D The frame source port number in the least significant bits. This allows the frame source port number to be

carried within the frame, which is useful for processing BPDUs, for example.

D A cyclic redundancy check (CRC) type indicator (crctype) in the most significant bit (bit 7).

–

If crctype = 1, then the CRC word in the frame excludes the IEEE Std 802.1Q header.

–

If crctype = 0, then the CRC word in the frame includes the IEEE Std 802.1Q header. This CRC word is

for a regular IEEE Std 802.1Q frame format with the value in the IEEE Std 802.1Q TPID of 81–00

(ethertype constant) in the TPID field. Because the internal frame format uses the TPID field for other

purposes in the manner being described, it is necessary to insert the IEEE Std 802.1Q TPID of 81–00

(ethertype constant) value into the TPID field if the frame needs to be restored to a normal

IEEE Std 802.1Q frame format, which passes a CRC check.

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