# Uart baud rate

Thread Starter

#### Dadu@

Joined Feb 4, 2022
127
I think baud rate refers to the speed at which data is being transferred or received.

In the serial port context, "9600 baud" means that the serial port is capable of transferring/receiving a maximum of 9600 bits per second.

I just want to know if I'm thinking right

#### ericgibbs

Joined Jan 29, 2010
15,526
hi Dadu,
If you have a data block as 1 Start, 8 Data, 1 Stop Bits' ie; 10 Bits total that approx 960 Data blocks/sec,
Usually there is a small time gap' in the sending device output after each block
E

#### BobTPH

Joined Jun 5, 2013
4,906
To put it another way, to send 8 bits of data requires 10 bits so, instead of 9600/8 or 1200 bytes per second, you can only transmit 9600/10 or 960 bytes of data per second.

Bob

#### nsaspook

Joined Aug 27, 2009
9,730
Thread Starter

#### Dadu@

Joined Feb 4, 2022
127
Thanks a lot almost got it.

if you want to tell the speed of your UART, How do you tell the speed for 8 Data bits?

Do you tell the speed of one bit or how many bits are being transferred in a second?

#### MrChips

Joined Oct 2, 2009
26,099
Bit rate and data rate are two different things.

Keeping it simple, 9600 baud means that each bit is transmitted in a time period of 1/9600 seconds or 104μs.

The number of bits transmitted in order to send 8 data bits is more than 8.

#### nsaspook

Joined Aug 27, 2009
9,730
Thanks a lot almost got it.

if you want to tell the speed of your UART, How do you tell the speed for 8 Data bits?

Do you tell the speed of one bit or how many bits are being transferred in a second?
You can look at the signal with a proper instrument for timing and try to decode it.

#### ronsimpson

Joined Oct 7, 2019
2,067

In your case, 1 start bit, 8 data bits, no parity, probably 1 stop bit, maybe a gap to the next start bit.

#### Ya’akov

Joined Jan 27, 2019
5,663
Just to muddy the waters in order to clear them…

Baud is the symbol rate not the bit rate. It is only the bit rate if the system can have only two states. It’s better to think of it as the number of times per second it is possible to transmit a state to the receiver.

In the cast of the UART, there are only two states so baud rate and bit rate are the same number. On the other hand, sophisticated schemes for using things like PAM (Pulse Amplitude Modulation) can increase the discrete states the system can take and make the bit rate greater than the baud rate.

For example, the 56Kb/s V.90 modem standard, on the downstream, used an 8K/s symbol rate with PAM modulation at 128 amplitudes per symbol which means they were able to get 7 usable bits per symbol out of the 8 bits they could encode.

So:

Baud Rate is symbol rate, the number of symbols per second transmitted
Modulation allows for more than one bit per symbol
Bit rate is baud Rate x bits per symbol

#### DickCappels

Joined Aug 21, 2008
8,710
My definition which is not in opposition to those already expressed by may be simpler: The baud rate is the frequency at which bits (including start, stop, and parity bits are clocked out of the sending UART.

When writing firmware UART routines I actually clock out the stop bit(s) like any other bit.

#### Ya’akov

Joined Jan 27, 2019
5,663
My definition which is not in opposition to those already expressed by may be simpler: The baud rate is the frequency at which bits (including start, stop, and parity bits are clocked out of the sending UART.

When writing firmware UART routines I actually clock out the stop bit(s) like any other bit.
Yes, that falls under bit rate which in the UART case happens to equals baud rate because of the number of symbols in use.

#### BobTPH

Joined Jun 5, 2013
4,906
Fir standard UART communication, the maximum byte rate is baud rate divided by 10.

Bob