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No clock is required and the timing of the bits is determined from the leading edge of each character.
The very name UART, being an Asynchronous communication method, is that it is not SYNCHRONIZED with a clock. Rather, unintelligent UART methods only rely on being set with the same parameters such as baud, parity, and stop bits. Intelligent UARTs(not an official name, just one I use) can look at a frame of data and determine it's parity, baud rate and stop bits by itself, but it's only used in more complex devices.
EDIT: It's also important to mention that the I2C interface supports clock stretching, where devices that need more time to receive and process data can pull the clock line low (due to being designed to be open-drain) to tell the master device that it's not ready. Though I2C can theoretically support 127 devices, your average I2C slave device can't have it's address fully configured,
Uh, no, if the data isn't synchronized with a clock pulse, what's the point of it? A UART only needs to work with 3 lines: Tx, Rx, and ground. The only clock signals involved here are at each device, where they both have to generate a timing signal for internal UART handling. There is NO clock signal connection between the two devices. Also, if you're referring to the clock element of the UART itself, of course it needs a clock, all electronics need a time element to actually progress in execution.
A PLL is commonly used to sync the clock with the incoming received data.
Sorry, but that isn't so - as it's an asyncronous process there's no clock signal to be recovered, so PLL's aren't used - and couldn't be. You use a completely separate clock at the receiving end, which needs to be close enough to remain in sync only for a single word length, as it resyncronises every start bit.
Wouldn't a real UART use a Delay-Locked loop to change the phase of the baud rate's internal clock to be aboots 90 (whoops, not 45) degrees out of phase (to facilitate center-sampling), and a Fractional Baud Rate Generator to create the actual baud rate clock?
Sorry, but that isn't so - as it's an asyncronous process there's no clock signal to be recovered, so PLL's aren't used - and couldn't be. You use a completely separate clock at the receiving end, which needs to be close enough to remain in sync only for a single word length, as it resyncronises every start bit.
To say that you could not use a PLL is not true. I have experience working on such systems that used a PLL for
I didn't say 'could' not, I said 'do' not - it would be a silly way to do it, far easier to use a higher frequency and divide down.
Do you know of any hardware UART's that use a PLL?, presumably if you have experience using such a device, it was built from scratch to overcome a specific problem?. Many of the older UART's don't even have their own clock, you have to provide it, at a multiple of that required.
Actually you said PLLs "couldn't be" used. Read your post. And yes, many UARTs don't provide their own clock but I don't know what you mean by "...at a multiple of that required." Multiple of the baud rate?
Actually you said PLLs "couldn't be" used. Read your post.
My apologies.
But not entirely incorrectif it is used, it's external to the UART, and not part of it
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