ENC424J600 datasheet(38/168 Pages) MICROCHIP

38 / 168 page

ENC424J600/624J600

DS39935C-page 36

 2010 Microchip Technology Inc.

3.5.5.2

Circular Wrapping with ERXDATA

As with the general purpose pointers, operations with

ERXDATA

normally

cause

the

ERXRDPT

ERXWRPT Pointer to automatically increment by one

byte address. However, if the end of the receive buffer

area (5FFFh) is reached, the pointer will increment to

the start of the receive FIFO buffer area instead, as

defined by ERXST (Figure 3-7).

The receive wrapping rules for the ERXDATA interface

are identical to the buffer wrapping rules used by the

receive hardware. Therefore, this register interface is

ideally suited to reading packet data from the receive

buffer. The host controller can set the ERXRDPT value

at the start of a packet in the receive buffer and sequen-

tially read out the entire packet contents without having

to write to the ERXRDPT Read Pointer again.

FIGURE 3-7:

CIRCULAR BUFFER

WRAPPING USING THE

ERXDATA WINDOW

EQUATION 3-2:

POINTER INCREMENT LOGIC FOR ERXRDPT AND ERXWRPT

3.5.5.3

Circular Wrapping with EUDADATA

The user-defined buffer area is primarily useful for

setting up a circular FIFO within the general purpose

area for use by TCP/IP stacks or other applications. The

wrap-around behavior of the user-defined buffer area is

somewhat more complicated than with the general

purpose or receive buffer cases. This is because the

user-definable boundaries set by EUDAST and

EUDAND take priority over normal wrapping behavior.

Like other pointers, EUDAST and EUDAND are fully

user-configurable from the host microcontroller. Unlike

ERXST, which must not be modified while the receive

hardware is enabled, EUDAST and EUDAND can be

modified at any time.

As in the previous instances, operations with

EUDADATA normally cause the EUDARDPT or

EUDAWRPT Pointer to automatically increment by one

byte address. If the value in EUDAND is reached, the

pointer will increment to the address specified by

EUDAST instead. However, if the end of memory

(5FFFh) is reached, and EUDAND is located at some

other address, the pointer will increment to the begin-

ning of memory (0000h). If EUDAND is set to 5FFFh,

the pointer address increments to the value of

EUDAST, instead of 0000h.

The increment behavior logic is explained in

Equation 3-3.

EQUATION 3-3:

POINTER INCREMENT LOGIC FOR EUDARDPT AND EUDAWRPT

General Purpose

Buffer

0000h

5FFFh

Circular RX FIFO

Buffer

ERXST

ERXST – 1

Unimplemented

if ERXRDPT/ERXWRPT = 5FFFh, then

ERXRDPT/ERXWRPT = ERXST

else

ERXRDPT/ERXWRPT = ERXRDPT/ERXWRPT + 1

if EUDARDPT/EUDAWRPT = EUDAND, then

EUDARDPT/EUDAWRPT = EUDAST

else if EUDARDPT/EUDAWRPT = 5FFFh, then

EUDARDPT/EUDAWRPT = 0000h

else

EUDARDPT/EUDAWRPT = EUDARDPT/EUDAWRPT + 1

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