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最新的LPC214X SPI驱动程序

--
--  $Id: README 331 2008-11-09 16:59:47Z jcw $
--  $Revision: 331 $
--  $Author: jcw $
--  $Date: 2008-11-09 11:59:47 -0500 (Sun, 09 Nov 2008) $
--  $HeadURL: http://tinymicros.com/svn_public/arm/lpc2148_demo/trunk/spi/README $
--

The SPI EEPROM support and uIP are mutually exclusive, primarily because the
chip select line (SSEL0) is shared.  The SPI EEPROM code does a check at the
start of every operation to see if uIP is running.  If it is, an error is
returned.  When uIP is stopped, the SPI EEPROM interface is re-initialized.

The ENC28J60 only operates in mode 0,0.  The 25LC512 can operate in mode 1,1 or
0,0.  Typically, most SPI EEPROMs that I've dealt sith have been run in 1,1
mode.  Since the idea was to re-use as much code as possible, mode 0,0 is used
for the 25LC512.

It's possible to run both at the same time, provided two conditions are met: 1)
The 25LC512 and the ENC28J60 have separate chip select lines (this is not much
trouble), and 2) Access to the SPI controller is protected with
portENTER_CRITICAL()/portEXIT_CRITICAL() sections.  

The latter is necessary to make sure that a context switch doesn't take place
while the uIP task or monitor task is waiting for a SPI transaction to
complete.