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'UART,I2C'
1998\01\05@113125 by andre

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Hi to all engineers.

I need to learn more about uart, i2c serial communications is there any
web site available?
few questions I have.

1. since asynchronies method first sends start bit, data , parity then
stop bit what if my data is four bit
long how does it work since stop bit is 10th bit how does receiver
figure out that data is less then 8 bit.

2. since start bit always is logic 1 stop bit is logic 0  is there any
rule for this  to be always 1 for
start bit 0 for stop bit . what if some one modifies it just to be
opposite.

3. is there any UART sample code that I can play with it?



thank you



Andre

1998\01\05@145359 by Mike Keitz

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On Mon, 5 Jan 1998 07:17:01 -0800 Andre Abelian <spam_OUTandreTakeThisOuTspamcompufire.com>
writes:

>1. since asynchronies method first sends start bit, data , parity then
>stop bit what if my data is four bit
>long how does it work since stop bit is 10th bit how does receiver
>figure out that data is less then 8 bit.

The receiver always expects 8 data bits, so to send 4 bits just  don't
use the other 4.  Sending a start bit, 4 data bits, and a stop bit will
not work because the receiver would still be collecting data bits when
the start bit for the next 4-bit group arrives.  So there is a lot of
wasted time sending 4 bits, unless you start packing two 4-bit data
groups into each 8-bit transmission.

Some UARTs can be set for fewer than 8 bits.  The 16X50 chip in the PC
can be set for 5, 6, 7, or 8 bits.  Modes other than 7 or 8 bits are very
rare.  I don't know of anything that uses 6 bits.  Five bits are used for
the Baudot code of old Teletypes, but there is little to no PC software
that supports it in this manner.

>
>2. since start bit always is logic 1 stop bit is logic 0  is there any
>rule for this  to be always 1 for
>start bit 0 for stop bit . what if some one modifies it just to be
>opposite.

Then it won't work.  Actually the start bit is a logic 0 and a stop bit
is a logic 1.  This is how the data comes out at TTL level from most UART
chips.  The signal is then inverted so than on the RS-232 line, a
positive voltage is a start bit or logic 0, and a negative voltage is a
stop bit or logic 1.  There is a rule for this, it's what makes the
RS-232 interface useful by making every device use the same format.

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