[PATCH] TMS570 console driver, SCI frame error (baudrate calculation error)

[Gedare Bloom]

On Wed, Jul 12, 2023 at 8:06 AM Joel Sherrill <joel at rtems.org> wrote:
>
>
> Pavel this was filed as https://devel.rtems.org/ticket/4903. The ticket submitter
> didn't give a patch, just a code change snippet and explanation. Zack turned it into
> a patch. You will need to post that on the ticket to get the submitter's feedback.
>
> I hope you don't mind copying that to the ticket and seeing what you think is
> really up.
>
I posted it over there. Thanks Pavel and Joel.

> Thanks.
>
> --joel
>
> On Wed, Jul 12, 2023 at 8:49 AM Pavel Pisa <ppisa4lists at pikron.com> wrote:
>>
>> Hello Zack and Gedare,
>>
>> On Tuesday 11 of July 2023 19:52:27 Gedare Bloom wrote:
>> > Thanks for the patch. Someone should probably test it, or identify in
>> > the documentation why this calculation was off-by-1. Pavel, any clues?
>> > On Sun, Jul 9, 2023 at 10:09 PM zack <zakthertemsdev at gmail.com> wrote:
>> > > Fixes #4903
>> > > diff --git a/bsps/arm/tms570/console/tms570-sci.c
>> > > b/bsps/arm/tms570/console/tms570-sci.c index 768770a4c8..59a0b7e6f1
>> > > 100644
>> > > --- a/bsps/arm/tms570/console/tms570-sci.c
>> > > +++ b/bsps/arm/tms570/console/tms570-sci.c
>> > > @@ -311,7 +311,7 @@ bool tms570_sci_set_attributes(
>> > >    /* Apply baudrate to the hardware */
>> > >    baudrate *= 2 * 16;
>> > >    bauddiv = (BSP_PLL_OUT_CLOCK + baudrate / 2) / baudrate;
>> > > -  ctx->regs->BRS = bauddiv? bauddiv - 1: 0;
>> > > +  ctx->regs->BRS = bauddiv? bauddiv - 2: 0;
>>
>> I think that change is not correct. The actual used values
>> for BSP_PLL_OUT_CLOCK and baudrate should be provided to analyze
>> the case. The code can result in some rounding error and can
>> be enhanced if fractional divider is used or even super finegrained
>> fractional divider. But these options are available only for
>> for SCI/LIN peripheral case.
>>
>> According to
>>
>> TMS570LS31x/21x 16/32-Bit RISC Flash Microcontroller
>> Technical Reference Manual
>> Literature Number: SPNU499B
>>
>> 26.2.3 SCI Baud Rate
>>
>>   SCICLK Frequency = VCLK Frequency / (P + 1 + M / 16)
>>
>>   Asynchronous baud value = SCICLK Frequency / 16
>>
>> So the substraction of one corresponds to the manual.
>>
>> Actual code does not use M part. It would be problem if it is
>> leftover from some boot/monitor but it is part of BRS 32-bit
>> register which is overwritten in the whole, so such problem
>> should not appear either.
>>
>> So I vote against the proposed change for now and suggest
>> to do analysis what happens in the computation and what
>> are input values and output. Change would/could affect
>> negatively large number of combinations of the baudrate
>> and clocks.
>>
>> I would consider to discuss if the rounding formula
>> could/should be updated, but I think that it is the best
>> which cane be achieved for rations which do not result
>> in exact ratio.
>>
>>   (BSP_PLL_OUT_CLOCK + baudrate / 2) / baudrate;
>>
>> If there is interrest then code can be enhanced by fraction
>> dividers for SCI/LIN peripheral case. The field with variant
>> should be added into tms570_sci_context and in this case
>> the alternative formula can be used
>>
>>   long long bauddiv;
>>   bauddiv = (BSP_PLL_OUT_CLOCK * 16ll + baudrate / 2) / baudrate;
>>   ctx->regs->BRS = ((bauddiv >> 4) & 0xffffff) | ((bauddiv & 0xf) << 24);
>>
>> which should be rewritten after header for SCI/LIN update to
>>
>>   ctx->regs->BRS = TMS570_LIN_BRS_P(bauddiv >> 4) | TMS570_LIN_BRS_M(bauddiv & 0xf);
>
>
>
>>
>>
>> Best wishes,
>>
>>
>>                 Pavel
>> --
>>                 Pavel Pisa
>>     phone:      +420 603531357
>>     e-mail:     pisa at cmp.felk.cvut.cz
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>>
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