Synchronizing between ISR and main code

Altera_Forum · ‎09-16-2005

Hello all,

What is the best way to ensure safe access to shared data between an ISR and the main code ?

In my application, the ISR reads a packet from the FIFO of a custom UART and saves it in a buffer provided to it upon registration. It then asserts a "ready" flag on the data. The main code polls for this flag and processes the data when it's ready. How can I ensure that during the main code's processing the data the ISR won't jump again and override it ? It may cause corrupted data. Say the main code has read 5 out of 10 bytes and then the ISR has overwritten them. When the main code resumes it will read the remaining 5 bytes from the new message.

Is disabling interrupts during the main's code reading of the data the best option ?

What are the common guideline when dealing with data synchronization in ISRs for Nios ?

Thanks in advance

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Altera_Forum · ‎09-16-2005

Hi eliben,

> What is the best way to ensure safe access to shared data between an

> ISR and the main code ?

That depends on what you are trying to do -- there's usually not a single "best way".

> Is disabling interrupts during the main's code reading of the data the best option ?

In most cases, no. This usually leads to those "mysterious" bugs that show up when

baud rates, cpu clocks, etc. change.

> What are the common guideline when dealing with data synchronization

> in ISRs for Nios ?

Here's a good reference: http://www.xml.com/ldd/chapter/book/ch09.html#t8 (http://www.xml.com/ldd/chapter/book/ch09.html#t8)

A very simple and common technique is to use a circular buffer. The size of the buffer can

be adjusted to meet your requirements ... provided you have adequate memory resources.

Here's a basic overview:

1. The isr reads a "put" pointer, and writes its data. Then updates the put pointer.

2. The app compares the put pointer to a "get" pointer, when unequal, there is data.

3. The app reads (copies, consumes, whatever) the data, then updates the get pointer.

Regards,

--Scott

Altera_Forum · ‎09-16-2005

The circular buffer is indeed useful, implementing a FIFO in software. I wanted, however, to avoid it and use something simpler, as I don't need a FIFO. I can, however, use a "trivial" FIFO which has depth 1 and "loses" all subsequent packets until the old ones has been read.

Which brings me to the question - are variable assignment and test atomic in Nios ? What I mean is:

int ready;
// is this atomic ?
ready = 1;
// and is this atomic 
if (ready == 1)
{
 ...
}

I ask because I can make my main code "deassert" the 'ready' flag when it finished reading the packet, and the ISR to ignore a new packet if 'ready' is still asserted. This way no stomping occurs (overflow messages are lost, which is acceptable). So I wonder whether it's possible for the ISR to run in the middle of asserting 'ready' ? If "ready = 1" is atomic, it certainly can't.

Altera_Forum · ‎09-16-2005

Don't forget to specify variables as "volatile" to prevent them being optimised out.

Sometimes I do tricks such as implementing S/R Flip Flops or counters in hardware, with different memory addresses to set/reset increment/decrement/read them.

For simple FIFO buffers I'll do e.g. "ISR owns write pointer", "main thread owns read pointer" as Scott says.

I will also use a "main thread is using this" flag as you've suggested. So

//shared
volatile int MT_Access;
//main thread
MT_Access = 1;
<do something>
MT_Access = 0;
//ISR
if (0 == MT_Access)
{
<update something>
}