how to measure serial receive byte speed. eg bytes per second. - c

Question

I'm receiving byte by byte via serial at baud rate of 115200. How to calculate bytes per sec im receiving in a c program?

Answer 1

There are only 3 ways to measure bytes actually received per second.
The first way is to keep track of how many bytes you receive in a fixed length of time. For example, each time you receive bytes you might do counter += number_of_bytes, and then every 5 seconds you might do rate = counter/5; counter = 0;.
The second way is to keep track of how much time passed to receive a fixed number of bytes. For example, every time you receive one byte you might do temp = now(); rate = 1/(temp - previous); previous = temp;.
The third way is to combine both of the above. For example, each time you receive bytes you might do temp = now(); rate = number_of_bytes/(temp - previous); previous = temp;.
For all of the above, you end up with individual samples and not an average. To convert the samples into an average you'd need to do something like average = sum_of_samples / number_of_samples. The best way to do this (e.g. if you want nice/smooth looking graphs) would be to store a lot of samples; where you'd replace the oldest sample with a new sample and recalculate the average.
For example:
double sampleData[1024];
int nextSlot = 0;
double average;
addSample(double value) {
double sum = 0;
sampleData[nextSlot] = value;
nextSlot++;
if(nextSlot >= 1024) nextSlot = 0;
for(int i = 0; i < 1024; i++) sum += sampleData[1024];
average = sum/1024;
}
Of course the final thing (collecting the samples using one of the 3 methods, then finding the average) would need some fiddling to get the resolution how you want it.

Answer 2

Assuming you have some fairly continuous input, just count the number of bytes you receive, and after some number of characters have been received, print out the time and number of characters over that time. You'll need a fairly good timestamp - clock() may be one reasonable source, but it depends on what system you are on what is the "best" option - as well as how portable you want it, but serial comms tend to not be very portable anyways, or your error will probably be large. Each time you print, reset the count.

Answer 3

To correct some odd comments in this thread about the theoretical maximum:
Around the time that 14400 Baud modems came to the pre-web world, the measure of Baud changed from Baud (wiki it) to match emerging digital technologies such as ISDN 64kbit. At that time, Baud became to mean Bits/second.
Being serial data in the format of 8N1, a common shorthand notation, there are eight bits, no parity bit, and one stop bit for every byte. There is no start bit.
So a theoretical maximum for 8N1 serial over 115200 Baud (bits/sec) = 115200/(8+1) = 12800 bytes/sec.
Similar (but not the same) to watching your download speeds, the rough ball-park way to work out bytes/sec from bits/sec, without a calculator, is to divide by 10.

Answer 4

Baud rate is measurement of how many times per second a signal is able to change. In one of that cycles, depending on the modulation you are using, you can send one or more bits (if you are using no modulation - bit rate is the same as baud rate).
Let's say you are using QPSK modulation, so you can transmit/receive 2 bits per baud. So, if you are receiving data at 115200 baud rate, 2 bits per symbol, you are receiving data with 115200 * 2 = 230400bps.