Electronic – the meaning of “half bit” in this context

adcrs232

In RS-232 specification, the stop bits can be 1, 1.5 or 2 stop bits…I wonder how it's possible to have half a bit?

Also some ADC math uses 0.5bit precision in the IC specifications/datasheet. Can someone elaborate in this subject?

Best Answer

I wonder how it's possible to have half a bit?

In UART based asynchronous serial communications (of which RS232 is an example), a serial byte of data is "enclosed" within a start bit and a stop bit(s). Following the stop bit there is a gap before the next data byte and this gap can be regarded as extending the stop bit(s): -

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A lot of serial data transmission uses one stop bit because the receiver is able to process the bytes in the time but sometimes extending the gap between data bytes by increasing the number of stop bits is useful. As both sending and receiving UARTs have a much higher internal clock rate, manufacturing a fractional bit length is easy.

For an ADC, I've attached a drawing: -

enter image description here

The blue line represents a linearly rising analogue input voltage and the staircase is the digital number from the ADC trying to represent that analogue voltage. In the middle I've shown in red what the theoretical maximum error will be - no surprise it being equivalent to 0.5bits.