[Tinyos-devel] Nested interrupts

[Michael Newman]

John did excellent work on this. We both agreed that using reentrant
interrupts is not a very good practice and can lead to many issues. 

-----Original Message-----
From: tinyos-devel-bounces at Millennium.Berkeley.EDU
[mailto:tinyos-devel-bounces at Millennium.Berkeley.EDU] On Behalf Of John
Regehr
Sent: Wednesday, August 15, 2007 12:24 PM
To: 'TinyOS-Devel list'
Subject: Re: [Tinyos-devel] Nested interrupts

> 1/3 to 1/10 seems reasonable.

Ok, Michael and I argued about this a bit more off-list, and I wrote a bit 
of code, and the conclulsion seems to be as follows.

If the goal is to read an AVR I/O port with as little latency as possible, 
both a reentrant and non-reentrant interrupt can read the port in the 10th 
cycle following the interrupt trigger, including the 7 cycles that the 
processor requires to take the interrupt and jump to the handler.  Of 
course this requires bypassing the compiler.

Then, the fastest non-reentrant interrupt enables subsequent interrupts 
after 45 cycles (when it returns) and the reentrant one does this after 11 
cycles.  So the worst-case sampling latency is roughly 55 cycles vs. 22, 
for a 2-3x improvement.

This improvement is only useful if the rest of the program keeps its 
critical sections in the same ballpark as 11 cycles.

Code attached is my fastest non-reentrant handler which assumes a 
32-element ring buffer for samples.  I expect someone clever could shave a 
few cycles off this but it seems pretty close to optimal in terms of both 
sampling latency and overall cycles.

Hopefully this is useful for people doing tight sampling...

John