Regarding UART modeling through FPGA.

[noloser]

I am modeling a UART interface using FPGA through VHDL, but there are some spec which i got confused. is the UART chipset found in most desktop PC decode a negative voltage as logic '1' or a positive voltage as logic '1'? I read some articles on RS232 communication, some saying one way while other saying the other, so i'm getting confused. can i safely assume that a start bit is logic '0' and end bit is logic '1' but not it's voltage level?

Thank for any useful information!!!

 

[Iouri]

driver MAX 232 is inverting your signal after UART controller

 

[noloser]

does that mean when a design a UART interface on FPGA to communicate with the serial port on a desktop computer, i can just treat logic '1' as high voltage and logic '0' as negative (or zero) voltage level? any link to spec sheets or articles to show these info?

thank alot!!

 

[Pont de Pedra]

After you have finshed your design in VHDL you'll need a driver RS-232 that is, it translate 3.3v or 5v or else logic level you have to a ±5V ÷ ±12V (the standard allow large voltage interval). Every converter, transmitter or receiver inverts the signal, then if you send a logic high (1) it is inverted by your transmitter (driver) and the receiver (driver in your computer) inverts the signal again.
Then for sure, a Start bit is allways '0' and Stop bit(s) always '1'.

This converters (drivers) are available from Linear Technology, Analog devices, Maxim, Texas Instruments .... and much more.

I hope this helps

 

[gliss]

MAX232 datasheet
**broken link removed**

 

[noloser]

which mean i will design the code such as logic '1' is high voltage and logic '0' is zero voltage and after completing, before using it, i need to design another driver circuit using the MAX232 chip, am i right?

Thank for your info!

 

[jelydonut]

Basically you can just invert the input as your shifting in/out (start bit is low).. then you use the max232 just for line drivers for the +12V/-12V.

jelydonut