Datasheet

ENC424J600/624J600
DS39935C-page 34  2010 Microchip Technology Inc.
3.5.5 INDIRECT SRAM BUFFER ACCESS
Indirect access to the SRAM buffer is available to all I/O
interfaces. For the SPI interface, it is the only method
available. For PSP interfaces, it may be used in
addition to the direct access method.
Three separate pointer pairs are available for the host
microcontroller to use when accessing the SRAM:
• General Purpose Buffer Read/Write Pointer
(EGPRDPT/EGPWRPT)
• Receive Buffer Read/Write Pointer
(ERXRDPT/ERXWRPT)
• User-Defined Area Read/Write Pointer
(EUDARDPT/EUDAWRPT)
Each of these pointer pairs provides an 8-bit virtual
window register (EGPDATA, ERXDATA and EUDATA)
through which the SRAM data is read or written. The
pointers and their associated data windows are shown
in Figure 3-5.
EGPDATA, ERXDATA and EUDADATA are all 8 bits
wide. When writing to them using a 16-bit PSP
interface, the low-order byte select or write enable must
be used; strobing the high byte Byte Select or Write
Enable has no effect. When reading from a 16-bit PSP
interface, one byte of useful data will be returned on the
lower 8 bits of the data bus; the upper 8 bits are to be
ignored.
When a data window register is read, the memory
contents at the address indicated by the corresponding
Read Pointer are obtained and presented to the host
microcontroller. Similarly, when a data window register
is written, the memory contents at the address
indicated by the corresponding Write Pointer are
updated by the data from the host microcontroller.
Following a read/write operation, the appropriate
pointer is automatically incremented in hardware.
FIGURE 3-5: POINTERS FOR INDIRECT BUFFER ACCESS
General Purpose
Buffer
0000h
5FFFh
Circular RX FIFO
Buffer
ERXST
ERXST – 1
EGPRDPT
EGPWRPT
ERXRDPT
ERXWRPT
EUDARDPT
EUDAWRPT
EUDAST
EUDAND
EGPDATA
ERXDATA
EUDADATA
EGPDATA
ERXDATA
EUDADATA
EUDADATA
EGPDATA
ERXDATA
Unimplemented
Read
Write
Read
Write
Read
Write
Data Windows
Buffer Pointers