time to transmit and execute

[vpan]

hello everybody...

i try to estimate how long it takes to transmit
the following command

printf("A_%ld\n",value) where value 0-9999

and baud rate 9600

also,how long it takes for the system to execute the command?

i have a 16f628 controller with 4 MHz XT

[Ttelmah]

Will also depend on number of bits selected, and parity etc..
Each bit takes 1/9600th second at 9600bps.
Each character, has one start bit added, and one stop bit.
If you are running at 8bits, no parity, then you have a total of ten bit times per character. 1/960th second.
Then (assuming you are using the hardware UART, not 'software serial'), the hardware has two characters of buffering present. So, if you send :

"A_0\n" (four characters), the command will return after two character times.
2/960th second.

If instead you send:
"A_9999\n" (seven characters), the command will return after five character times. 5/960th second.

Now, on your chip, you won't quite get 9600bps, with a 4MHz clock. The closest 'match', will be 9615bps (0.15% error), so in fact the times become /961.5. Also, the code will take a tiny fraction of a second to load the first characer (typically about 20uSec). However the calculation of the latter characters, is done 'while' the hardware is transmitting, so doesn't affect the results.
The major 'time', will always be the actual time needed by the hardware to send the bytes. The alternative is to use interrupt driven buffered transmission (EX_STISR.C), in which case the code will return much faster, taking perhaps a couple of mSec in total for the 'worst case' divisions. Total tansmission time, will remain the same.

Best Wishes

[vpan]

thanks for answer
i want to send 7 values over 232.

can i send these values every 1ms?
i mean,do i have much time between printf() command
and UART (transmit) without PIC confuse?

if i have,what baud rate do you suggest?
else what is your suggestion about my case?
(different PIC, XT...)

UART settings is classic
1 start
1 stop
8 bits data
no parity
no flow control

[Ttelmah]

Calculate it from what I posted.....
7*10 bits, in 1mSec, implies a bit rate of at least 70000bps. Normally 115200 would be the 'next' standard rae over this.
However you won't achieve what you want now at 4MHz.
First, the chip cannot develop this rate from this clock, with an acceptable margin.
Second, in your example, _generating_ the data for the number, will take a (as I have already said), perhaps 2mSec. At 9600bps, this is less than the time needed to transmit the data, so the transmission time is the limiting factor, but at 115200, this is no longer the case.
You would need to at least double, and probably quadruple the processor clock rate to do this.

Best Wishes

[vpan]

if you were in my place
what PIC and clock (XT) will you choose?

i'm asking you because i'm confused
about how to calculate baud rate from datasheet
(tables,BRGH e.t.c)

[vpan]

let me say an example

with 16f877A and 20MHz XT will be ok?

[John P]

"I want to send 7 values over 232. Can I send these values every 1ms?"

It's an easy calculation. With 8 data bits, a start bit and a stop bit, at 9600Bd you need almost exactly 1msec to send 1 character.

7 values 1000 times a second is not even close. You need to rethink this setup from the start.

115.2KB is the highest rate that ordinary serial ports can handle, though if you're sending from one processor to another, you could go faster. At 115.2KB, you could get 11.5 characters over the line per msec, and I'm sure a PIC16F877 could send that many, but does the software run fast enough at the receiving end? And is 11500 chars/sec fast enough for what you want anyway?

If you need to, you could send the data in binary form and decode it at the receiving end. That would save a lot of transmission time.

[Freddie]

You could time it exactly in a test program.