STM32F4 DMA question

On 09/13/13 21:10, Tim Wescott wrote:

>
> I'm not even sure that the serial port is double-buffered -- I know that
> I figured out how to get just one byte at a time out via DMA, and my
> problems disappeared.
>

I didn't mean to be critical, just didn't understand what the problem was.

A lot of uarts aren't multi buffered, but they all at least have a
holding register for the previously received byte, while the next byte 
is being
loaded by the input shift register. This means you have at least a byte's
time to receive the interrupt and process it. Even at 100K bits/second,
assuming async, 8,N,1 you have 100uS to process the interrupt and do the
houskeeping, ie: modify bits in the uart control register, whatever. On the
Cortex M3, for example, interrupt latency, 72MHZ clock, is < 200nS worst
case, so it's unlikely to affect anything for serial comms. Instruction 
cycle
times are down on the 10's of nS range as well, so interrupt driven serial
comms should be a breeze without having to resort to dma transfer, which 
just
complicates everything,

The problems might have disappeared, but it wouldn't be enough for me not to
understand why. Hence the question :-)...

Chris

On Fri, 13 Sep 2013 21:46:49 +0000, chris wrote:

> On 09/13/13 21:10, Tim Wescott wrote:
> 
> 
>> I'm not even sure that the serial port is double-buffered -- I know
>> that I figured out how to get just one byte at a time out via DMA, and
>> my problems disappeared.
>>
>>
> I didn't mean to be critical, just didn't understand what the problem
> was.

You have my permission to be critical -- I shouldn't have that much 
interrupt latency.

> A lot of uarts aren't multi buffered, but they all at least have a
> holding register for the previously received byte, while the next byte
> is being loaded by the input shift register.

That's what I meant by double-buffered, and this one is.  So I really 
have no excuse for the interrupt response time.

Dang.  Now I wonder what was going on...

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com