ENC28J60
6.2
Transmission Buffer
6.0
INITIALIZATION
All memory which is not used by the receive buffer is
considered the transmission buffer. Data which is to be
transmitted should be written into any unused space.
After a packet is transmitted, however, the hardware
will write a seven-byte status vector into memory after
the last byte in the packet. Therefore, the host control-
ler should leave at least seven bytes between each
packet and the beginning of the receive buffer. No
explicit action is required to initialize the transmission
buffer.
Before the ENC28J60 can be used to transmit and
receive packets, certain device settings must be initial-
ized. Depending on the application, some configuration
options may need to be changed. Normally, these tasks
may be accomplished once after Reset and do not
need to be changed thereafter.
6.1
Receive Buffer
Before receiving any packets, the receive buffer must
be initialized by programming the ERXST and ERXND
Pointers. All memory between and including the
ERXST and ERXND addresses will be dedicated to the
receive hardware. It is recommended that the ERXST
Pointer be programmed with an even address.
6.3
Receive Filters
The appropriate receive filters should be enabled or
disabled by writing to the ERXFCON register. See
Section 8.0 “Receive Filters” for information on how
to configure it.
Applications expecting large amounts of data and
frequent packet delivery may wish to allocate most of
the memory as the receive buffer. Applications that
may need to save older packets or have several
packets ready for transmission should allocate less
memory.
6.4
Waiting For OST
If the initialization procedure is being executed immedi-
ately following a Power-on Reset, the ESTAT.CLKRDY
bit should be polled to make certain that enough time
has elapsed before proceeding to modify the MAC and
PHY registers. For more information on the OST, see
Section 2.2 “Oscillator Start-up Timer”.
When programming the ERXST or ERXND Pointer, the
internal hardware copy of the ERXWRPT registers will
automatically be updated with the value of ERXST. This
value will be used as the starting location when the
receive hardware begins writing received data. The
ERXWRPT registers are updated by the hardware only
when a new packet is successfully received.
Note:
After writing to ERXST or ERXND, the
ERXWRPT registers are not updated
immediately; only the internal hardware
copy of the ERXWRPT registers is
updated.
Therefore,
comparing
if
(ERXWRPT = = ERXST) is not practical in
a firmware initialization routine.
For tracking purposes, the ERXRDPT registers should
additionally be programmed with the same value. To
program ERXRDPT, the host controller must write to
ERXRDPTL first, followed by ERXRDPTH. See
Section 7.2.4 “Freeing Receive Buffer Space” for
more information.
© 2006 Microchip Technology Inc.
Preliminary
DS39662B-page 33
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