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[avr-gcc-list] Re: USART interrupts

From: Bob Halford
Subject: [avr-gcc-list] Re: USART interrupts
Date: Tue, 15 Apr 2003 10:05:42 +1200
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This is most likely an FAQ but it would seem (with the mega8 at least) that you
have to use SIGNAL rather than INTERRUPT for SIG_UART_RECV as
the code produced by the C-compiler does not read the UDR until after whole bunch of registers have been pushed on the stack..... Difficult to say with the limited in-circuit tools to hand but my guess is that when using INTERRUPT, as soon as the interrupt is re-enabled, the un-cleared RXC flags immediately causes another interrupt
and so on until the stack is full.

Is this really what happens? If so, surely this is a bug.... the interrupt arbritation logic should not allow this (ie one way of avoiding this is to make the interrupt transition rather than level sensitive.. but preferable is to disallow recursive interrupts).

You can kind of work round the problem by using SIGNAL and doing an sei() just after reading the UDR but does this mean that if the SIG_UART_RECV s/r does not get done
before the next RXC, we are in the shit?

Cheers, Bob

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