Re: [avr-gcc-list] interrupts

[Oliver Kasten]

here is my two pennies worth. (i'm pretty pretty sure the following is 
right, but wouldn't mind if someone could confirm or disprove.)

if interrupts are disabled in ISRs interrupts can get lost when two or 
more interrupts occur while servicing an ISR. then you might also miss a 
byte, for example, from the USART. if your ISR-code makes assumptions 
about the form of the incoming data (e.g., you expect the bytes from the 
USART stream to alternate sign), your assumption might brake and your 
code might enter into an endless loop or so.  the problem then would be 
your code, not the interrupt handling of the controller.

now, how do interrupts (and data) get lost exactly?
let's assume interrupts are disabled in ISRs (as in your case). 
internally the interrupt condition (timer compare match etc.) sets an 
interrupt-specific boolean flag (there is one flag per interrupt type). 
if interrupts are enabled, the ISR is entered immediately and the flag 
is cleared. if the previous ISR is still executing and a second 
interrupt of the same type occurs, the flag is again set, but the IRS is 
not executed until the previous has finished. the flag is used to keep 
track of pending interrupt requests. however, only one pending interrupt 
can be remembered. if two or more interrupts occur during the execution 
of an ISR, only one can be remembered.

some interrupts are associated with data, for example, the reception of 
a data byte on the USART. typically, there is a one or two-byte FIFO 
queue for storing the data until it is read.  if the data comes in 
faster than it is read, old values are obviously overwritten and are 
lost for good. so loosing an interrupt might also mean loosing data.

we had a case like that, when a timer ISR did some lengthy bookkeeping 
every now and then. when at the same time there was high-speed serial 
data transfer, we lost some bytes. took us some time to figure that out...

hmm, maybe that was obvious. anyway, it hope it helps someone.

best,
olli*


Jamie Morken wrote:
> Hi,
>
> When in an interrupt service routine, and another interrupt is
>
> [...]
>
> cheers, Jamie Morken

--
oliver kasten
eidgenössische technische hochschule (eth) zürich
distributed systems group, eth zentrum, ch-8092 zuerich, switzerland
tel: +41-1-632 06 63  fax: +41-1-632 16 59  www.inf.ethz.ch/~kasten/