LPC3131的SPI驱动程序，哪位给解释一下？

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redgalaxy| 禁止发言

楼主 | 2010-8-27 15:01 | 只看该作者回帖奖励

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驱动程序, spi驱动, LPC, ge, ST

下面是LPC3131的SPI驱动程序，其中给DIV赋值的那句话，我没明白，哪位大侠给解释一下！
static STATUS spi_get_clock_div(UNS_32 target_clock,
                              UNS_32 *pDivSet)
{
  UNS_32 div, spi_clk, ps, div1;
  STATUS retvalue = _ERROR;
  /* The SPI clock is derived from the (main system oscillator / 2),
   so compute the best divider from that clock */
  spi_clk = cgu_get_clk_freq(CGU_SB_SPI_CLK_ID);
  /* Find closest divider to get at or under the target frequency.
   Use smallest prescaler possible and rely on the divider to get
   the closest target frequency */
  div = (spi_clk + target_clock / 2) / target_clock;//div为什么这样计算，没看明白？
  if ((div < SPI_MAX_DIVIDER) && (div > SPI_MIN_DIVIDER))
  {
ps = (((div - 1) / 512) + 1) * 2;
div1 = ((((div + ps / 2) / ps) - 1));
/* write the divider settings */
*pDivSet = SPI_SLV1_CLK_PS(ps) | SPI_SLV1_CLK_DIV1((div1));
retvalue = _NO_ERROR;
  }
  return retvalue;
}

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