# In asynchronous communication how is the frequency same but clocks of both devices different?

Am currently studying 8051 microcontroller and in the course the lecturer told that in communication the frequencies of sender and receiver should be same else they will get different bits of data at different speeds and that would result in either data loss or extra useless data.

In synchronous communication both clock and frequency is same and in asynchronous communication clocks are different.

But as per my understanding the frequency generates the clock , then how are they both different?

• Async uses 16x clock to sync on 1st edge Jan 4, 2020 at 5:37
• Although there are purely asynchronous systems in theory, almost all systems are fundamental synchronous, it is just that two communicating systems that are each based on a local clock do not have their clocks synchronized even if they might be at the same frequency. They communicate asynchronously by each using synchronous digital logic. Jan 4, 2020 at 6:08
• Google UART. How it works. The 16x sampling like Tony said. The two devices can communicate as long as their internal idea of frequency is not too far off from each other and time is allocated to resynchronize at the end of each byte so timing errors don't accumulate. Jan 4, 2020 at 7:21
• Just FYI. I've used devices that employed 64X (and also 4X) over-sampling to perform the same job that is preformed by devices using 16X. So, don't buy the idea of only 16X. (These were standard IC devices and not me using bit-bang.)
– jonk
Jan 4, 2020 at 10:03
• Also to the OP. Even if the frequencies of the two MCUs are nominally the same, where they are driven by separate clock sources with the same nameplate frequency, they won't be the same and they will drift around relative to each other. So they are only the same if both driven by the same clock source.
– jonk
Jan 4, 2020 at 10:08