uart3.txt

universal Serial is for send and receive from serial UART whit AVR

 DESCRIPTION

 This application describes how to make a half duplex software UART
 using the 8-bit Timer/Counter0 and the External Interrupt0 (ATmega128).

 As a lot of control applications communicate in one direction at a time
 only, a half duplex UART will limit the usage of MCU resources.

 The constants D and N determine the data rate. D selects the frequency
 divider as described in the T/C Prescaler in the AVR datasheet (the T/C
 prescaling factor). The following expression yields the data rate:

		 	 CLK
	 BAUD = ----- ;   (CLK = quartz frequency)
		 	 N*D

 The UART uses PD0 as receive pin because it must be an external interrupt.
 The transmit-pin is PD4 in this application, but it can be any other pin.

 Since the UART is half duplex, it can either send or receive data. It can't
 do both simultaneously. When idle it will automatically receive incoming
 data into the RX buffer, but if it is transmitting data while incoming data
 arrives, it will ignore it.

 Interrupt Usage :	External Interrupt0,
			 		Timer/Counter0 overflow interrupt.

 Remark: The constants 80 and 56 (inside the "defs.h" file), used to adjust
 		 the bit lengths, are achieved by using an oscilloscope.