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PIC16F630 and RS232

 
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Fabri



Joined: 22 Aug 2005
Posts: 275

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PIC16F630 and RS232
PostPosted: Tue Dec 13, 2005 2:30 am     Reply with quote
Hi Everybody,

This is a first time I use RS232 in software with CCS compiler.

I'm using RS232 in PIC16F630 without USART with internal RC oscillator 4 Mhz.
Quote:

#use delay(clock=4000000, RESTART_WDT)
#use rs232(baud=9600, xmit=PIN_C4,rcv=PIN_C3)


the PIC send out a character but it isn't 9600 baud and the PC doesn't see the comunication.

One character at 9600 baud take 1,87 ms time, my pic send the same character in 1,37 ms.

I'm using PCWH V3.235.

Have you something to suggest me ?

Thanks for support,
Fabri



Joined: 22 Aug 2005
Posts: 275

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PostPosted: Tue Dec 13, 2005 2:55 am     Reply with quote
Hi,

Because I use an RTCC interrupt every 200uS, I disabled it and the comunication start to work correctly.

I stay in interrupt 26 uS, just to take a count of time.

Why does it?

Thanks,
asmallri



Joined: 12 Aug 2004
Posts: 1636
Location: Perth, Australia

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PostPosted: Tue Dec 13, 2005 3:04 am     Reply with quote
Fabri wrote:
Hi,

Because I use an RTCC interrupt every 200uS, I disabled it and the comunication start to work correctly.

I stay in interrupt 26 uS, just to take a count of time.

Why does it?

Thanks,


You have answered your own question. I am not sure about the specific numbers you have mentioned but every 200us you spend 26us in the interrupt handler. The software UART counts bit time in software. It is not able to determine that the the interrupt handler has repeasted interrupted the process.

At 9600 baud a standard 8 bit no parity character transmission takes 1.04ms including the start and stop bit. Five or 6 interrupts will occur during this period adding an additional 120us to 156us plus the call return overhead if you have not already included this in your number.
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Ttelmah
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PostPosted: Tue Dec 13, 2005 4:58 am     Reply with quote
Also, the actual interrupt tie will be longer than 26uSec. I'd suspect this is the duration in the 'handler', rather than the total interrupt time. On a PIC, when an interrupt occurs, the global handler is called, which saves the registers, then calls the actual 'event handler'. When this finishes, the code returns to the global handler, which clears the interrupt flag, restores the registers, and finally returns to the main code.
Unfortunately, even if the event handler itself, only takes 26 instructions, the global handler will typically add perhaps another 50uSec to this. With an RTC at 200uSec, and an event handler that lasts 26uSec, the processor will be in the global handler and the event handler for this, about 76uSec out of every 200uSec. About 1/4 the processor time.....

Best Wishes
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