Efficient Average of Three Values in C
We are reading some signals from pins and setting some more events based on this reading.
To be safe, I want to sample the pins 3 times, compare the three values and use the most common value (ie sample A is 1, B is 3 and C is 1, I want to use 1, if AB and C are all 2 then use 2 but if A is 1 , B is 2 and C is 3, I want to capture the three samples again).
Currently I'm using:
int getCAPValues (void)
{
// Get three samples to check CAP signals are stable:
uint32_t x = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // First set of CAP values
for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t y = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // second set
for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t z = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // third set
if (x == y) || (x == z)
{
//use the x value
}
else if (y == z)
{
// use the y value
x = y;
}
else
{
x = -1;
}
return x;
}
But this doesn't seem very efficient to me, is there a better way to do this?
This is on a SAMD21 Xplained Pro development board in C.
EDIT:
I have changed the code as per answers, only reading the "z" value if it will be used, and using delay_us() instead of the dummy loops:
int getCAPValues (void)
{
// Get three samples to check CAP signals are stable:
uint32_t x = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // First set of CAP values
delay_us(1);
//for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t y = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // second set
// Using most common value, or error code of -1 if all different
if (!(x == y))
{
delay_us(1);
//for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t z = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // third set
if (x == z)
{
// use the x/z value
return x;
}
else if (y == z)
{
// use the y/z value
return y;
}
else
{
return -1;
}
}
return x;
}
If x==y you're going to use the value of x . So in that case you can dodge the third reading.
I don't know how volatile your values are but it could effectively almost double peformance to avoid that second delay if disputed values are in fact rare.
Indeed if they're not rare the whole rationale might be invalid.
int getCAPValues (void)
{
// Get three samples to check CAP signals are stable:
uint32_t x = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // First set of CAP values
for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t y = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // second set
if(x!=y){
//x & y are different. Get a tie-breaker...
for (uint32_t i = 0; i < 7; i++) dummy = i; // Pause
uint32_t z = (PORT->Group[IN_PORT_CAP].IN.reg & IN_PORT_CAP_MASK) >> IN_PORT_CAP_PIN; // third set
if (y == z) {
// use the y value
x = y;
} else if(x!=z){
//tie-breaking failed...
x=-1;
}
}
return x;
}
PS: I also think you should use `usleep()' rather than dummy loops. It depends what is available on your platform.
If you can assume that x , y and z are either 0 or 1 . Then you can just use x+y+z>1 (if most of them are 1 then the sum is larger than 1 and the comparison evaluates to 1 and otherwise it evaluates to 0 ).
Otherwise your (second) solution is probably the most efficient. At least if you don't know the probabilities of the outcomes. Besides by sleeping for one microsecond that would be the most time consuming operation.
What you should consider though is that as the reads are probably non-volatile it might differ if you actual do the third read or not. Also you should maybe consider if you need to do complete re-read. If you for example figure that if the first three reads from the ports are all different and the fourth read are equal to the second or third you could go for that you could use that to as an optimization. For example:
x = read_port();
y = read_port();
if( x == y )
return x;
for(;;) {
z = read_port();
if( z == x || z == y )
return z;
x = y;
y = z;
}
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