 ### Network # Is using term "BaudRate" correct when talking about SPI?

#### Priyank Agarwal

Dec 14, 2017
1
Hello,
I am having a couple of questions:

1. When we talk about asynchronous protocol such as UART, we do have the term Baud Rate.
But whenever I read about SPI or I2C, I have always heard the term "speed".
Does that mean, term "Baud Rate", shouldn't be used with SPI? Or, is it legit to use baud rate in SPI?

2. Afaik, Baud Rate: no of signals per second so while bit rate: bits per second
I am hoping someone could provide me a deeper explanation like what will happen if I send say 50 (0 bits i.e. always low signal), in one second. How do they differ?

#### davenn

Moderator
Sep 5, 2009
14,126
2. Afaik, Baud Rate: no of signals per second so while bit rate: bits per second
I am hoping someone could provide me a deeper explanation like what will happen if I send say 50 (0 bits i.e. always low signal), in one second. How do they differ?

Baud Rate
a unit of transmission speed equal to the number of times a signal changes state per second. For signals with only two possible states one baud is equivalent to one bit per second.

so baud and bit rates are tied

https://en.wikipedia.org/wiki/Baud

https://en.wikipedia.org/wiki/Bit_rate

#### dorke

Jun 20, 2015
2,342
Welcome to EP.

The terms are used interchangeably.
The term "Baud Rate" should be understood to be
the number of Symbols per second ).
In a "binary system communication" the number of symbols per a single time interval is 2.
Thus the bit-rate and the baud-rate are the same.
The more appropriate term here should be Bit Rate since it is less ambiguous(not that the other isn't correct).

Since in a multilevel communication scheme there can be more than 2 possible symbols per a single time interval.
we get :
[Baud Rate] <= [Bit Rate]
[Bit Rate]
= [Baud Rate] x log2[#of Symbols]

e.g. mostly we think of "0" and "1" as the "symbols" ,that means 2 voltage levels only.
But think about the possibility of using 4 voltage levels ,that means we now have 4 symbols per time interval possible.
That means for each single time interval we can deliver 2 bits instead of only one.
A=00,B=01,C=10,D=11 ,the letters represent different voltage levels.

Entering the numbers of the above in the 2nd equation we get:

Bitrate1=[ Baud Rate] xlog2 = [Baud rate]
Bitrate2=[ Baud Rate] xlog2 = [Baud rate]x2

There is another term worth understanding :
The [effective bit rate] this is the "net" bit rate taking only the "data bits into account".
removing all the protocols overheads(at all levels).

for a UART (physical level) that would mean removing start,stop,parity bits etc.

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