ENC424J600 datasheet(39/168 Pages) MICROCHIP

39 / 168 page

 2010 Microchip Technology Inc.

DS39935C-page 37

ENC424J600/624J600

The user-defined area start address, EUDAST, is a

read/write register. For wrapping to work correctly, the

hardware enforces 16-bit even word alignment of this

tied off to ‘0’. Similarly, the user-defined area end

address, EUDAND, is a read/write register that is

forced to an odd memory address. The Least Signifi-

cant bit of EUDAND is internally tied to ‘1’. Applications

wishing to set up general purpose circular FIFOs in

memory using these hardware features must observe

these same alignment requirements.

If the user-defined area end address, EUDAND, is at a

higher memory address relative to the start address,

EUDAST, the buffer wraps to either EUDAST or the

beginning of memory, depending on where the

EUDARDPT or EUDAWRPT Pointers are located. This

is shown in Case 1 of Figure 3-8.

In some cases (for example, when accessing

fragmented data), it may be useful to place the

EUDAST Pointer at a higher memory address relative

to the end address. When organized in such a manner,

an “exclusion zone” in the middle of the memory range

is created; sequential read/write operations with the

user-defined area pointers will jump over the range of

addresses between EUDAND and EUDAST. This is

shown in Case 2.

If the user-defined buffer is not needed, it can

effectively be disabled by setting EUDAST and

EUDAND to addresses outside of the implemented

memory area. For example, if EUDAST is set to 6000h

and EUDAND is set to 6001h, EUDARDPT and

EUDAWRPT will never reach these addresses.

Instead, they wrap from the end of implemented RAM

to its beginning, as shown in Case 3.

When the user-defined buffer is disabled, the host con-

troller can use the EUDADATA interface as a second

general purpose window into RAM. Unlike the original

general purpose pointers, however, EUDARDPT and

EUDAWRPT do not wrap at the ERXST boundary,

thereby allowing access to the full SRAM buffer area.

This may be beneficial for debugging and testing

purposes where it may be desirable to read or write the

entire SRAM buffer in a single operation.

FIGURE 3-8:

CIRCULAR BUFFER WRAPPING USING THE EUDATA WINDOW

General Purpose

Buffer

0000h

5FFFh

Circular RX FIFO

Buffer

Unimplemented

EUDAST

EUDAND

General Purpose

Buffer

0000h

5FFFh

Circular RX FIFO

Buffer

Unimplemented

EUDAST

EUDAND

General Purpose

Buffer

0000h

5FFFh

Circular RX FIFO

Buffer

Unimplemented

EUDAST

EUDAND

Case 1:

EUDAND > EUDAST

Case 2:

EUDAST > EUDAND

Case 3:

EUDAST and EUDAND > 5FFFh

User-Defined Buffer Disabled

User-Defined Buffer with

“Exclusion Zone”

Normal User-Defined Buffer

Excluded

User-Defined

Buffer

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