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Topic: Mods to HardwareSerial to handle 9-bit data (Read 7 times) previous topic - next topic

Nick Gammon

Feb 11, 2012, 06:21 am Last Edit: Oct 30, 2012, 01:55 am by Nick Gammon Reason: 1
Following on from a discussion in this thread:


The posters there wanted to use the "9th" bit in async comms to communicate with some gadget that uses that extra bit as a "command" bit.

This turned out to be a bit non-trivial, so I have made an amended HardwareSerial library, which can be downloaded from here:


The diffs for any developers interested are:


Because of the way that hardware serial is integrated into the IDE it is, unfortunately, necessary to find your existing files:

Code: [Select]

... and replace them with the ones in the download.

Note that this is for version 1.0 1.0.1 of the IDE. No guarantees are given that this will work with other versions. Be warned.


  • The internal buffers have been changed from 8-bit characters to 16-bit characters (to hold the 9th bit). Thus the buffers double in size.

  • There is a new argument to the Serial.begin() function, which is a boolean, whether or not you want 9-bit mode. It defaults to false.

  • There is a new function [font=Courier]Serial.write9bit ()[/font]. This takes an unsigned int argument, letting you supply a character with the 9th bit set. I didn't want to change the existing write function because it is used in the Print class.

  • The read function, which already returns an int, now will return the 9th bit where required.

Test sketch, run on a Mega2560:

Code: [Select]

void setup ()
 Serial.begin (115200);  // debugging prints
 Serial1.begin (115200, true);  // 9 bit mode
 Serial2.begin (115200, true);  // 9 bit mode
 Serial.println ("--- starting ---");
}  // end of setup

int i;

void loop ()

 Serial1.write9bit (i++);  // send another byte
 // display incoming on Serial2
 if (Serial2.available ())
   Serial.println ((int) Serial2.read (), HEX);
 // check if we have sent all possible characters
 if (i >= 0x200)
   delay (100);
   while (Serial2.available ())
     Serial.println ((int) Serial2.read (), HEX);
   delay (5000);
   i = 0;
   }  // end of sent 512 bytes
}  // end of loop

The sketch uses Serial (pins D0 and D1) for debugging. To test jumper D18 (Tx1) and D19 (Rx2).

The sketch will send all possible 512 bytes out from Serial1, and read them into Serial2, displaying them on Serial for manual evaluation.


Wow Nick! You have been on a role lately with all of the things you have been doing! I find myself referring to your posts and your blog quite a lot lately.

I am curious would it be difficult to change to 7-bit data? In some automotive applications I am lead to believe they use 7-bit but, don't quote me on that.

Nick Gammon

When I was doing that change I noticed there didn't seem to be any provision for different bit lengths, let alone 9 which was the trickiest.

Changing to other bit lengths should be trivial.

However for 7-bits you can "fudge" that by setting the high-order bit of the data you are sending. Since the high-order bit is sent last, and a stop bit is a 1, if you make sure that the high-order bit is a 1, then 8 bits of data with the high-order bit set, would look like 7 bits followed by a stop bit (well, 2 stop bits).

For receiving, just mask out the high-order bit (eg. "and" it with 0x7F).


Ok, masking bits would be a lot simpler than changing library definitions. Thanks Nick!


could you check the download url?  I get looped back to this page.

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