Question about USdelay

Mauro Fantina said:

Hello,

I open 2806x library,

I see DELAY_US macro

#define DELAY_US(A)  DSP28x_usDelay(((((long double) A * 1000.0L) / (long double)CPU_RATE) - 9.0L) / 5.0L)

this macro multiply "us delay" x1000 to calculate "ns delay" (1us=1000ns),

then the macro divides "ns delay" for Tclock in ns to calculate the number of ticks.

I don't understand why the macro subtract 9 and it subsequently divide for 5.

Can someone help me ?

Mauro,

The source for this function is in the <device>_common\source folder in the <device>_usDelay.asm file.  The comments describe the function:

;There is a 9/10 cycle overhead and each loop
;takes five cycles. The LoopCount is given by
;the following formula:
;  DELAY_CPU_CYCLES = 9 + 5*LoopCount
; LoopCount = (DELAY_CPU_CYCLES - 9) / 5
; The macro DELAY_US(A) performs this calculation for you
;

Lori