TNETX3100 datasheet(32/80 Pages) TI1

32 / 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

transmit queue-length registers at 0x2*N + 0x00A4 through 0x2*N + 0x00A5

DIO ADDRESS

Transmit queue N length

0x2*N + 0x00A4

Transmit queues use a residual queue length to control their behavior. The value indicates how many more

buffers can be added to the queue, rather than how many buffers are on the queue. This is an advantage

because it is easy to detect that the queue is full (length goes negative) and can be adjusted

dynamically (2’s complement addition to the length).

After reset, all transmit queue-length registers are initialized to zero. These registers are part of the address

range that is read from the EEPROM. The first value written to the transmit queue-length registers is the initial

value. When you write again, the value entered is added (as a signed 16-bit integer) to the current value.

As frames are placed on the queue, the transmit queue length is decremented by the number of buffers queued.

If the transmit queue length becomes negative (most-significant bit is set and the queue is full), no new frames

are added until the length becomes positive by the transmission of buffers. Since a maximum-size frame (1518

bytes) is 24 buffers long, and whole frames are queued based on the current transmit queue-length value, the

queue consumes 23 buffers more than the initial residual length (i.e., if the transmit queue is set to length = 1,

a full-size Ethernet frame can still be queued). As buffers are transmitted, the transmit queue length is

incremented.

The transmit queue registers are used to initialize, alter, and provide status on transmit queue lengths. They

are used in three different ways:

To assign initial transmit queue-length value. The value in the register is used as its initial value when the

first frame is put on the queue.

To indicate current transmit queue-length value. The register is loaded with the transmit queue-length value

when it is updated.

To adjust transmit queue length.

After transmit queue initialization, a value written to this register is added to the current transmit queue-length

value the next time it is updated. The update bit in port status is used to detect initialization or that an update

operation is completed. The operation is a 16-bit addition, allowing the current queue length to increase or

decrease. The update operation is enabled only when the most-significant byte of the register (bits 15–8) is

written to prevent possible length corruption. Length bytes are always written with the least-significant byte first.

port VLAN registers at 0x2*N + 0x00B8 through 0x2*N + 0x00B9 (N = port number in hex)

The VLAN registers hold broadcast destination masks for each source port. Each bit in the VLAN register

(except bits 15–10) directly corresponds to a port (bit 9 = port 09 through bit 00 = port 00). Broadcast and

multicast frames are directed according to the VLAN register setting for the port on which the broadcast or

multicast frame was received. Each VLAN register is initialized at reset to send frames to all other ports except

itself. After reset, the registers contain the following initial values (see Table 6).

TNETX3100 Datasheet(PDF) 32 Page - Texas Instruments