Fix SP804 delay timer on FVP

This patch fixes several issues with the SP804 delay timer on FVP:

* By default, the SP804 dual timer on FVP runs at 32 KHz. In order
  to run the timer at 35 MHz (as specified in the FVP user manual)
  the Overwrite bit in the SP810 control register must be set.

* The CLKMULT and CLKDIV definitions are mixed up:

      delta(us) = delta(ticks) * T(us) = delta(ticks) / f(MHz)

  From the delay function:

      delta_us = (delta * ops->clk_mult) / ops->clk_div;

  Matching both expressions:

      1 / f(MHz) = ops->clk_mult / ops->clk_div

  And consequently:

      f(MHz) = ops->clk_div / ops->clk_mult

  Which, for a 35 MHz timer, translates to:

      ops->clk_div = 35
      ops->clk_mult = 1

* The comment in the delay timer header file has been corrected:
  The ratio of the multiplier and the divider is the clock period
  in microseconds, not the frequency.

Change-Id: Iffd5ce0a5a28fa47c0720c0336d81b678ff8fdf1

1 files changed, 1 insertions, 1 deletions

diff --git a/include/drivers/delay_timer.h b/include/drivers/delay_timer.h
index 4f3bdc88..0dec626c 100644
--- a/include/drivers/delay_timer.h
+++ b/include/drivers/delay_timer.h
@@ -38,7 +38,7 @@
  * The driver must be initialized with a structure that provides a
  * function pointer to return the timer value and a clock
  * multiplier/divider. The ratio of the multiplier and the divider is
- * the clock frequency in MHz.
+ * the clock period in microseconds.
  ********************************************************************/
 
 typedef struct timer_ops {