5238-DS03-405-R datasheet(25/76 Pages) BOARDCOM

25 / 76 page

Advance Data Sheet

BCM5238

1/31/03

Broa d c om Co rp o r a t i o n

Document

5238-DS03-405-R

Jumbo Packet Mode

Page 17

extended FIFO modes to allow for extremely large packet sizes. These modes are enabled by setting appropriate bits in the

MII registers. The following table shows the number of effective FIFO bits supported.

When changing the receive FIFO size, it is necessary to identify the receive packet size requirement and set the descrambler

timer lock accordingly. The descrambler timer lock is controlled by Jumbo Packet Enable bit located in register 1Bh, bit 10.

The following table shows the descrambler lock timer allowed by this bit and the corresponding packet size.

Note that the packet size that can be reliably received is the lower number indicated by two settings:

•

Jumbo Packet Enable bit

•

Two extended FIFO Enable bits

Table 3: Receive FIFO Size Select

Jumbo Packet FIFO Enable

Reg 1Bh bit 9

Extended FIFO Enable

Reg 10h bit 2

Number of Effective FIFO

Bits Supported

Packet Size in Bytesb

b. Packet size calculation—based on 50 PPM clock tolerance for local and link partner.

12,500

25,000

40 (20)a

a. In this mode, the BCM5238 operation is guaranteed to only to 20 bits effective FIFO depth, although under some

circumstances, it could behave as if the FIFO depth is 40 bits.

50,000/25,000

Table 4: Jumbo packet enable and descrambler lock timer

Jumbo Packet Enable Reg 1Bh bit 10

Descrambler lock timer

Packet size

724 µs

9050

5792 µs

72400

5238-DS03-405-R Datasheet(PDF) 25 Page - Broadcom Corporation.