High baud rate serial communication problem

Projects Interfacing w/ Software on the Computer
1

I've been trying to use my external DAC with an Arduino Nano to reproduce 8-bit audio coming from the serial port.

void setup() {
  Serial.begin(528000, SERIAL_8N1);
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
}
void loop() {
  byte inByte = Serial.read();
  digitalWrite(2, bitRead (inByte, 0));
  digitalWrite(3, bitRead (inByte, 1));
  digitalWrite(4, bitRead (inByte, 2));
  digitalWrite(5, bitRead (inByte, 3));
  digitalWrite(6, bitRead (inByte, 4));
  digitalWrite(7, bitRead (inByte, 5));
  digitalWrite(8, bitRead (inByte, 6));
  digitalWrite(9, bitRead (inByte, 7));
}

Despite the enormous baud rate, it actually kind of works. The biggest problem I have is that the 7th bit is stuck high. Reducing the baud rate kind of helps, but it makes a bunch of other things worse so I'm not considering it yet. I've tried outputting this bit on other pins, makes no difference. Any ideas on what's going on here?

2

Did some research on 328P baud rates and reducing it to exactly 500000 seems to have fixed the 7th bit issue. Maybe there is also a way to stabilize the execution speed? Despite the code being pretty straightforward digital outputs slightly stutter and degrade the audio.

3

If you look in the Atmega 328 datasheet you will see tables of typical baud rates and the timing errors associated with them. There is no timing error at 500,000 baud - or 1,000,000 baud. I use 500,000 baud regularly.

...R

4

Robin2:
There is no timing error at 500,000 baud - or 1,000,000 baud.

Well that's what i'm getting at 500 000. Should be a straight triangle.

5312×2988 3.05 MB

5312×2988 3.08 MB
5312×2988 3.07 MB

5

I wonder if all your digitalWrite()s are slowing things down for the high baud rate.

Try the digitalWriteFast.h library - it really is a lot faster but it must know the I/O pins at compile time - which you do.

Read about direct port adressing for even faster I/O. If you can choose your I/O pins to suit the I/O Ports on the nano you could probably write all 8 values with 2 or 3 instructions and without even needing bitRead(). Unfortunately none of the I/O ports on a nano has all 8 bits available so you would need to use at least 2 Ports.

...R

6

Or it could interrupts to service millis() and micros() and whatever goes on with loop().

Try putting all the writes inside while (1) { }.

7

Thanks everyone!
Nothing worked, and it really shouldn't have, because all along the real problem was Tera Term. As soon as I switched to the command line "copy" the glitches stopped.

Robin2:
Try the digitalWriteFast.h library

Thanks for the info! Even though regular digitalWrite would probably work fine, I ended up using digitalWriteFast anyways, just to be sure.