Lpc2000

Art Sobel wrote:

> In the process of attaching some SPI peripherals to the chip I ran
> into some strange  behaviour.  There are 4 pins to use, SPICLK, MISO,
> MOSI and SSCS, First the SSCS pin must be configured as an SPI PIN or
> the other parts of the SPI don't work at all, Next the SSCS signal is
> not generated, This means that I must use another GPIO as the CS
> signal. So now it uses 5 pins - with one more GPIO pin for every SPI
> peripheral. 
> 
> Using the fastest selection of clocks the output rate is about 12,500
> D/A samples a second (each D/A sample requires 2 bytes of data
> output). This just about scratches the idea that the chip could
> produce medium quality sound output. One channel just barely then if
> you want to have analog in on the SPI too then the throughput just
> dives. How the heck did they ever think this chip could do modems?

A couple of thoughts:

  - If the 21XX is always only going to serve as SPI master
    (i.e. it will never be selected as a SPI slave), then
    it makes sense to use a pullup on SSCS.
  - Not sure I follow your throughput math.  The max PCLK
    rate is 60 MHz (divide by 1 with a 60 MHz CCLK).  The
    max SPI clock is PCLK / 8, or 7.5 MHz.  At 7.5 MHz
    2 bytes should take 2.13 usec (468,750 samples/sec), no?

Ryan