Serial interface - bit transmission - on byte wise manner

This page is targeted TTL serial. See index.html for further explanation.

this page is for getting better understanding and a help if you must debug by oscilloscope

In short:

A serial interface can transmit and receive characters (normally 8 bit char- can be 5,6,7,8)
Often it is on a 0-5V or 0-3.3V line (aka TTL levels also if its 1.8V)
two wires a common TX/RX and a ground
1. tx-device1 –> rx-device2
2. tx-device2 –> rx-device1
3. ground device1 <–> ground device2

Data as bytes is transmitted as a sequence of bits surrounded with a start and a stop bit

Bit and bytes on the wire - transmission

Data is normally 8 bits (can be 5,6,7,8)
Surrounded by a startbit and stopbit and maybe a parity bit(odd,even,non) before stopbit
Idle level is high (eg 5V, 3.3V dependig on arch)
bit value 1 is high (eg 5V, 3.3V dependig on arch)
bit value 0 is low (0V)
startbit starts when line goes from high to low eg startbit is 0V or logical 0
stopbit starts when line goes from low to high eg startbit is 5V or logical 1

Transmit examples

Transmission goes from left to right in the figures
Each bit has a fixed length in time
9600 baud -> duration 1 bit = 1/9600 sec ~= 104 micro sec
- 1 byte (incl start and stop bit - 8+2) takes 1040 micro sec ~= 1 milli sec
- max capacity <= 961 bytes pr second
115200 baud -> duration 1 bit = 1/115200 sec ~= 8.68 micro sec
- 1 byte (incl start and stop bit - 8+2) takes 86,8 micro sec
- max capacity <= 11520 bytes pr second

So if you do have pulled receive code this means you have to call code like

int ch = getByte();  // -1 if nothing for you otherwise 0x00-0xff

and do this at least every 104.2 microsec for 9600 baud

You can see at 0 msec line goes low for 1 msec(startbit - which is low) and after that the following 8 bit are HHLLLHL and last H(stopbit)
You can see below in figures that TX order is least significant bit first

Low level (0V) = logic 0 -- High level (5V) = logic 1

-We want to TX 0X7C (binary 0111.1100)

- TX'ed as <startbit>"0X7C" with lsb first<stopbit>

-  which is     00011110111
                         ^^- 2 bit idle - can be less
                        ^--- stopbit
                       ^---- msb bit is 0x7C
                 ^---------- lsb bit in 0X7C
                ^------------ startbit

You can see at 0 msec line goes low for 1 msec(startbit - which is low) and after that the following 8 bit are HHLLLHL and last H(stopbit)

Images is recorded by a *Analog Discovery 2’

Startbit is marked 'S’, Stopbit 'P’, and paritybit(if present) 'X’ in all the following images.

below 0x7C at 9600 baud

below 0x8F at 9600 baud

below 0x55 at 9600 baud - with bit duration measured(104.1 microsec)

Note

You may find serial interfaces where

bit value 1 is low and bit value i high
startbit is high
stop bit is low
and therefore idle level is low

But these implementation is rare.

Parity

Normally we just transmit 8 databits but we can add a prirty bit after the 8 databit:

Parity

None - no parity
Even - extra bit which is high if a odd number of 1's in the databit
Odd - extra bit which is high if a even number of 1's in the databit

From https://en.wikipedia.org/wiki/Parity_bit

NB we do normally use 8 bit for data (a byte) but parity rule is the same

Howto parity bit

In Arduino world:s
All at 9600 baud
- standard - no parity bit - Serial.begin(9600);
- odd parity - 8 bit data  - Serial.begin(9600,SERIAL_8O1);
- even parity- 5 bit data  - Serial.begin(9600,SERIAL_5E1);

8: 8 databit - can be 5,6,7,8
O: odd parity
E: even parity


My test code

void setup() {
  Serial.begin(9600,SERIAL_8E1);
}

void loop() {
  Serial.write(0xa1);
  delay(100);
}

In the following examples we use Even parity.
The parity bit is marked 'X’

0xa5, even parity - 9600 baud

We do TX 0xa5 or 1010.0101 which is 4 high bits so even parity is here 0
Parity bit is by analog discovery marked 'X’,startbi 'S’ and stopbit 'P’

0xa1, even parity - 9600 baud

We do TX 0xa1 or 1010.0001 which is 3 high bits so even parity is here 1