[RTEMS Project] #4903: TMS570 console driver, SCI frame error (baudrate calculation error)

[RTEMS trac]

#4903: TMS570 console driver, SCI frame error (baudrate calculation error)
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 Reporter:  Usha      |       Owner:  (none)
     Type:  defect    |      Status:  new
 Priority:  normal    |   Milestone:  6.1
Component:  arch/arm  |     Version:  6
 Severity:  normal    |  Resolution:
 Keywords:            |  Blocked By:
 Blocking:            |
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Comment (by Gedare Bloom):

 From Pavel:

 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);

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Ticket URL: <http://devel.rtems.org/ticket/4903#comment:2>
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