Divide by zero warning results in wrong value in constant calculation

Question

I have the following code in a header file for an AVR32 program I'm writing:

#define PULSE_FREQ 150
#define TC_FREQ (60000000 / 8)                  
#define PULSE_PER (1 / PULSE_FREQ)
#define TC_PER (1 / TC_FREQ)
#define TTC ((PULSE_PER / TC_PER) - 1)

The goal is to be able to give a frequency and calculate the constant timer count value given by this formula:

Target Count = ((1 / Target Frequency) / (1 / Timer Frequency)) - 1

For example, for a timer frequency of 150Hz, a timer count value of 49999 is required. When manually entering that value for the timer, it works and gives a frequency of 150Hz. However, when using the code above, I get a frequency of 57Hz.

I'm also getting warnings about a division by zero, could the numbers in the constants be so small that they're getting chopped off to zero?

Would it be better to go about this at runtime instead of with #define macros?

you're doing integer division with numbers that end being between 0 and 1 (like `(1 / PULSE_FREQ)`), so integer division threats them as 0. — Federico klez Culloca, Jun 21 '17 at 15:25
The math equivalent `Target_Count = (Timer_Frequency / Target_Frequency) - 1` would work better. (some rounding issues remain) — chux - Reinstate Monica, Jun 21 '17 at 15:31

Carcigenicate · Accepted Answer · 2017-06-21T15:45:51.507

1

The integer division is probably returning a number less than 1 at some point, and it's being truncated to 0.

Try forcing floating-point division by making one of the constants a double:

// The .0 should "infect" the other numbers
#define PULSE_FREQ 150.0
#define TC_FREQ (60000000 / 8)                  
#define PULSE_PER (1 / PULSE_FREQ)
#define TC_PER (1 / TC_FREQ)
#define TTC ((PULSE_PER / TC_PER) - 1)

edited Jun 21 '17 at 15:45

answered Jun 21 '17 at 15:25

Carcigenicate

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"less than 0" did you mean less than 1? – Federico klez Culloca Jun 21 '17 at 15:26
@FedericoklezCulloca Whoops, corrected. Thanks. – Carcigenicate Jun 21 '17 at 15:27

Divide by zero warning results in wrong value in constant calculation

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