Long delay for Serial1.begin() / missing updateBaudRate()

[gicking]

I have a library which provides LIN communication across a variety of architectures and serial interfaces. To generate LIN BREAK signal, I temporarily reduce the baudrate, send 0x00 and revert to the original baudrate.

However, when porting to Uno R4 Minima I found that changing the baudrate via Serial1.begin() and subsequent Serial1.write() adds a ~1.2ms pause between BREAK and SYNC, which causes LIN slaves to time-out (see also here). Specifically Serial1.begin() takes ~620µs and (only) the first Serial1.write() has a latency of ~520µs.

Can you please check what causes this long latency and improve it? Alternatively add a new method updateBaudRate(), similar to ESP32 (see here) with smaller latency? Attached please find a small sketch and screenshots to show what I mean.

Can you please look into that? Thanks a lot in advance!

Looking forward to a lot of fun with the new UNO :-)

Test_Baudrate_change.zip

[gicking]

Further analysis shows that the delay is caused by R_SCI_UART_BaudSet(). However, I cannot find its implementation

[gicking]

I went down to SCI register level and seemingly the observed is delay after changing the baudrate is caused by the SCI hardware initializing (for 1B duration) after re-enabling the UART transmitter (TE=1). But without RE=0 and TE=0, changing the baudrate registers has no effect. I don't see how one can generate a LIN frame using this UART.

Any idea how I can generate a 16bit low on TxD without this delay???

Here is the sketch I tested

#include "Arduino.h"
#include "hal_data.h"
#include <variant.h>

#define TESTPIN   4
#define R_SCI     R_SCI2    // Serial1 uses SCI2

void setup()
{
  pinMode(TESTPIN, OUTPUT);
  Serial1.begin(19200);
}

void loop()
{
  // set half baudrate
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));
  R_SCI->SCR_b.RE = 0; R_SCI->SCR_b.TE = 0;
  R_SCI->SEMR_b.BGDM = 0;
  //R_SCI->BRR = 255;
  R_SCI->SCR_b.RE = 1; R_SCI->SCR_b.TE = 1;

  // send BREAK=0x00
  R_SCI->TDR = 0x00;
  while (R_SCI->SSR_b.TEND == 0);
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));

  // restore baudrate
  R_SCI->SCR_b.RE = 0; R_SCI->SCR_b.TE = 0;
  R_SCI->SEMR_b.BGDM = 1;
  //R_SCI->BRR = 140;
  R_SCI->SCR_b.RE = 1; R_SCI->SCR_b.TE = 1;

  // send SYNC=0x55
  R_SCI->TDR = 0x55;
  while (R_SCI->SSR_b.TEND == 0);
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));

  delay(500);
}

[gicking]

In the meantime I also tried switching to GPIO mode temporarily, and also using control via Serial Port Register (SPTR). However, both require TE=0 to take effect. And when I re-enable TE=1 I again see the 1B delay for SCI initialization, which is the same as if I simply change the baudrate

Here the code I tried:

#include "Arduino.h"
#include "hal_data.h"
#include <variant.h>

#define TESTPIN   4

void lin_break() {
  R_SCI2->SPTR_b.SPB2DT = 0;
  R_SCI2->SPTR_b.SPB2IO = 1;
  R_SCI2->SCR_b.TE = 0;
  delayMicroseconds(700);
  R_SCI2->SCR_b.TE = 1;
}

void setup() {
  pinMode(TESTPIN, OUTPUT);
  Serial.begin(115200);
  Serial1.begin(19200);
}

void loop() {
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));
  lin_break();
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));
  Serial1.write(0x55);
  Serial1.flush();
  digitalWrite(TESTPIN, !digitalRead(TESTPIN));
  delay(500);
}

Here the resulting screenshot --> 540µs between BREAK and SYNC --> LIN slaves time-out

[gicking]

I also asked this question on Renesas support forum, see https://community.renesas.com/mcu/ra/f/forum/58381/lin-break-generation-on-ra4m1