;
;	Example program #2 for use with the OATLEY Electronics PICAXE Development System
;
;	This program is an example of using a UHF transmitter to send 3 bytes of data to a receiver
;
;	Author		Jeff Monegal
;
;	Microcontroller	PICAXE 18X
;
;	Compile date	28/3/2010
;
;--------------------------------------------------------------------------------------------------
;
;	NOTES
;
;--------------------------------------------------------------------------------------------------
;
;	The 3 bytes of data to be transmitted are contained in the 3 byte registers. The checksum
; value is first calculated by exclusivly ORing byte_1 with byte_2. The result is stored in the
; checksum register which is then placed on the end of the data frame to be sent. A checksum is
; used t oensure that the received data is as error free as possible. At the receiver the checksum
; is compared with the result of exclusively ORing the same 2 registers.
;
;--------------------------------------------------------------------------------------------------

symbol	byte_1 = b2
symbol	byte_2 = b3
symbol	byte_3 = b4
symbol	checksum = b5	rem checksum used to ensure the data is received correctly
	
;--------------------------------------------------------------------------------------------------
;
;	First the 3 bytes to be transmitted are loaded into the 3 byte registers. The checksum is then
; calculated and it is used as the 4th byte to be sent. This means the data part of the frame 
; consists of 4 bytes. Also part of the complete data frame is the value 85 and the word 'code'. 
;
;	Because many receivers use what is termed a 'data or bit slicer' in their design the receiver 
; should first be calibrated. By calibrating we mean the RX should be synchronized to the incoming data 
; stream. This may seem a bit strange but often the RX will sit with no incoming data for long periods
; of time. Its internal data slicer will drift high or low. What happens now is that the first few bits
; of received data can be missed until the RX again becomes synchronized. By first sending the value
; 85 (10101010) we effectively wake up the receiver so it is calibrated and ready to correctly receive
; digital data.
;
;	The next value 'code' is used to ensure the receiver only receives or decodes the data stream that
; is meant for it. PICAXE calls this word the QUALIFIER. CODE can be any combination of alphanumerics 
; and is not limited in length. Be careful however as the longer the Qualifier the longer the time
; taken to send the frame. By using this qualifier we can produce a data TX/RX system that is
; secure from interference caused by other PICAXE data TX/RX systems operating in the same enviroment.
;
;	Serout 7,t2400 means use output pin 7 (can be what ever the user wants) and send the data at a 
; baud rate of 2400 bits per second. Refering to the relavent PICAXE chip data will better explain
; this command
;---------------------------------


;	byte_1 is used to store any 8 bit data that the user wants to send to the receiver
;	byte_2 is used to store 2nd 8 bit word that is to be sent to the RX
;	byte_3 is the same as byte_2

start:

	if pin2 = 0 then send_frame

tx_done:

;	continue here with users own program. Note the push button should use debounce detect code

	goto start
	
;---------------------------------
	
send_frame:

	checksum = byte_1 xor byte_3				 ;calculate the checksum byte
		
	serout 7,t2400,(85,"code",byte_1,byte_2,byte_3,checksum) ;send the frame.
	
	goto tx_done						 ;go back to the users program
	
;--------------------------------------------------------------------------------------------------