ADC, rs232

[Guest]

Hi all, need help about ADC and rs232 transmit.

Briefing of my project,
I have a signal that looks a like a full-wave rectifier with a period of 1000Hz. For continuous monitoring I have used 8 bit ADC conversion and program as below:

[Ttelmah]

You are not going to get anywhere near 25 samples/cycle.

The ADC conversion itself, takes time. You don't show your clockrate, but the use of an /32 divider, suggests it is up at perhaps 40MHz. You don't say what chip, but the conversion takes typically something like 11 cycles of the ADC clock. So, 352 cycles ofthe master clock. If 40MHz, about 9uSec. Then you are performing floating point arithmetic, taking perhaps another 35uSec (this will depend on how the compiler optimises what you show - I'm assuming the 'best case', that it optimises this into val*0.0196 - if instead it choses to use val/51.0, then the time will increase to perhaps 145uSec...). Then, the output will vary in size, but will typically be about 5 characters. The chip's buffer can hold only two, so the output will have to wait for three characters to be sent. At 115200bps, another 260uSec. It also takes time to perform the floating point divisions here, slowing it yet further. So you are running at a loop time, of perhaps something over 400uSec, and only a cople of samples per cycle...

You need to do a lot of optimising.
Buffer the serial transmit.
Don't delay in the loop. Use a timer, and perform the conversion if it has completed.
Reduce the complexity of the output calculations. Consider sending an unconverted hex value, or at the least, use fixed precision arithmetic, rather than FP.
Remember it takes typically ten bits of data to send one byte. If you want to send 25000 samples per second, you need to be pushing the comms up to about 500kbps, even if you only send two for each sample.....

Best Wishes

[takumi]

Not sure why it set to a guess when I sending this problem...
Btw forget to tell that i'm using
PIC16f877A, 20MHz,
Btw about you said using a timer but how? I really didn't know how to?
Is it ok if I put change