UART more popular than USART?

[dknguyen]

I was just curious why (on PICs at least) the UART is present on so many more chips than the USART? Even the newer 24F/H and dsPIC30F/33F only have a UART. Very few PIC seems to have a USART.

 

[Pommie]

Isn't this simply that the synchronous bit is now it's own peripheral with it's own section (section 16 in DSPic30F data sheet) in the data sheet?

Mike.

 

[dknguyen]

Oh, you mean the synchronous part of the USART has been split up into the UART and SPI? I didn't think about that...I always kind of thought of SPI and USRT as two different things.

 

[hjames]

I don't think I've ever used any sort of USART functionality. I've noticed on the newer Atmel parts, they've documented up the USART so that you can use the unit as a second SPI port.

I think the only real reason for USARTs is if you need the extra flexibility in the clock generation - if you were building a modem or other circuit where the timing is nontrivial.

 

[Nigel Goodwin]

It's far more common to use a UART than a USART, like 'hjames' I've never had occasion to use a USART.

 

[Papabravo]

The other reason could be that the microchip implementation of the USART function has a few holes that make it not syncronous. There was a fellow trying to implement a syncronous protocol and found that the 16F628A inserted extra bits between bytes making it useless for his purposes. Do a search on USART and see the scope pictures he took. I think the post was on this forum. The user was kenmac and his original post was April 11th of this year.

https://www.electro-tech-online.com/threads/16f628a-usart-in-sync-mode.20847/?highlight=USART

 

[Blueteeth]

Hi,

I was also after the same thing 'kenmac' was, using the USART in sync mode. Although my app relied on there being no gaps inbetween bytes, regardless of clock (wanted to stream bytes without framing bits). The USART is still useful in sync mode, using a shiftregister at the reciever, but there is a 'gap' between bytes that does not appear in the datasheet. So effectively it sends a byte, then waits one bit period, with the clock line idling high, then sends another...

With an inverter or two, I've used the USART as an SPI port (output only) and it does work well, if only I could find a way to stream bytes back to back at high speed.....I'll try some ATMEGA chips next.

But yes, I agree, the UART is more useful thatn the USART, althuogh they can both be used for async, the sync part seems to be redundant for most people.

Blueteeth

 

[Pommie]

Surely, nobody has every used a USART. They have used a UART or a USRT. The Synchronous/Asynchronous bits are mutually exclusive. That is why on more powerful Pics, the 2 have been separated and the synchronous part extended.

Nigel, your tutorial 7.7a uses the USART in asynchronous mode.

Mike.

 

[Nigel Goodwin]

Nigel, your tutorial 7.7a uses the USART in asynchronous mode.

Actually it uses it solely as a UART - I've no interest in it being configurable as a USRT at all, and neither have the vast majority of PIC users. I can see why it's there, it's an extra feature which costs little to add, and maintains compatibility with old syncronous gear.