[En-Nut-Discussion] UART looses Bytes

[Harald Kipp]

Hello Kai,

Gosh, good question. That very much depends on what's going
on else in the system. For example, timer processing disables
interrupts and the duration depends on the number of times
to process.

20us is a lot of time and the UART hardware can buffer 2 bytes.
But still...roughly calculated, context switching takes about
7us on a 16 MHz ATmega with interrupts disabled almost all
the time.

You are receiving limited length telegrams, right? Wouldn't
it be better then to receive the full telegram in a single
interrupt call? This will block all other interrupts,
which is usually acceptable for Ethernet and Timer processing.

But I agree that this may not solve the problem for the
first two bytes of the telegram.

We had similar problems MP3 streaming and solved it by
explicitly disabling the MP3 interrupt and re-enabling
the global interrupt within the MP3 interrupt routine,
which takes quite long to process. This allows nesting
interrupts, which were required to continuously receive
PPP characters from the serial port. In this case you
should make sure, that the separate interrupt stack feature
is disabled.

One last idea: If you're using Ethernut 1 (with RTL8019),
then the interrupt routine in the driver can become
quite slow during overflow processing:

#if !defined(__AVR_ATmega128__) && !defined(__AVR_ATmega103__)
         cbi(EIMSK, RTL_SIGNAL_IRQ);
         sei();
#endif
         NicOverflow();
#if !defined(__AVR_ATmega128__) && !defined(__AVR_ATmega103__)
         cli();
         sbi(EIMSK, RTL_SIGNAL_IRQ);
#endif

The part for allowing nested interrupts had been disabled
for AVR, because when this was done, Oliver's code didn't
allow interrupt nesting to that time. Try to remove the
#ifs.

Any other Nut/OS drivers you're using in your app?

Harald

At 17:51 24.11.2004 +0100, you wrote:
>Hi!
>
>I try to receive data at 1MBaud on the UART using the receive interrupt.
>I changed the oszillator, it is now 16MHz.
>I do not use the Nut-OS routines for UART communication.
>It all works fine, but sometimes(too often) I loose one or more bytes.
>Something from the Nut-OS delays the uart receive interrupt.
>This I could see on the oscilloscope.
>The thing is, now I send byte after byte with no delay.
>Later I will have 20µs between the startbits of two following bytes.
>Will that be enough?
>My receive interrupt routine takes no more than 5µs.
>
>I would like to know, what is interfering me and how long it will take.
>And if it might be disabled, for a period of time(complete telegramm).
>
>Thanks in advance,
>Kai Tönnies