LPTCOM. Bruce Misner Lakehead University h d3 RD2 pin 21. RD3 pin h d4. RD4 pin 27 RD5 pin h d5. RD6 pin 29 RD7 pin H d6

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1 LPTCOM By Bruce Misner Lakehead University LPTCOM is a demonstration of a robust bi-directional communcation between the PIC microcontroller and the printer port of your PC. Incorporating more handshaking signals we can develop a compact and complete read and write cycle for either for either communicating device. 37Ah /d1 37AH /d3 LPT Request LPT Ready 378h d0 378h d1 378h d7 379h d6 379h d5 PIC Request P1 J1 PIC Interface LPT Request PIC Request RB6 pin h d2 RD0 pin RD1 pin h d3 RD2 pin RD3 pin h d4 RD4 pin 27 RD5 pin h d5 RD6 pin 29 RD7 pin H d6 PIC Ready LPT DB25 Connector LPT Ready PIC Ready RB7 pin 40 RE1 pin 9 RE0 pin 8 The schematic indicates how all the signals are connected to each other. It does not show that the two connectors are joined by a straight through ribbon cable. The other thing to take note of is that the control signals from the LPT port are inverted at the output. Hence, when we write to the PIC on either the LPT Request line or the LPT Ready line must write out a logic low to actually raise the signal to its active high state. The last thing of note is that the ground line pin 25 on the LPT connector side and ground on your microcontroller board must be joined as well.

2 Recv Request Recv Data Send Ready Send Data Send Request Above is the timing diagram for the different states for a read/write cycle. The cycles are the same no matter which is the sender and which is the receiver. Let s go through the timing states of the communication. At 1 the sender has raised its request line to signal that it wants to send a byte of data. The sender then places the data on its data port in state t2. Once the data is present on the port the sender now raises its ready line. The sender has proceeded to this point without requiring feedback from the receiver. Once the receiver detects that the senders ready line is high and the data is ready to be read, it can do just that. Now, the receiver has read the data off of the data port and signals the sender that it has done this by raising its request line. The sender responds to the receivers action by lowering its ready signaland to respond back the receiver lowers its request line and in turn the sender lowers its request line. This completes the timing cycle.

3 PIC Receive routine: intvect btfss INTCON,RBIF retfie readdata movlw b' ' ;turn off interupts bsf STATUS,RP0 movlw b' ' movwf TRISD ;change port direction bcf STATUS,RP0 poll4rdy btfss PORTB,6 goto poll4rdy ;wait for ready from lpt movf PORTD,W ;read data movwf readvalue ;save in readvalue bsf PORTE,0 poll4notrdy btfsc PORTB,6 goto poll4notrdy bcf PORTE,0 ;raise request ;wait for ready from lpt to drop ;drop ready poll4notreq btfsc PORTB,7 goto poll4notreq ; movlw b' ' ;reenable interupts ; retfie

4 The previous code selection is the receive routine for the PIC. The first thing we notice is that it is tied to Port B Change interupt. Hence, when the PC wants to communicate with the PIC it raises the LPT Request line and this generates an interupt. Now, to try and step you through the code. First, we test to see if the interupt was generated by the Port B Change interupt by testing bit RBIF to see if the flag bit is set. If it was not the Port B Change then we end the interupt routine and go back to our main program. We then disable the interupts while we service our request. If we did not disable interupts the other control signals changing state would generate interupts and we would end up in a predicament. We now change the Port D direction to input by writing all ones to the TRISD register. We then poll the LPT Ready line for it to change state. Once the Ready line is high we can read the data byte and save it in a previously defined variable readvalue. Now that the data is read, we must inform the LPT Port that we have read by setting the PIC Request Line high. We now wait for the LPT Port to drop its ready line, then we drop our PIC Ready Line and then wait for the LPT Port to drop its Request line. This completes the cycle so now we can re-enable our interupts and return from our interupt routine. VB Send Code Private Sub sendbutton_click() Timer1.Enabled = False i = OutPortB(&H37A, 8) 'raise request and output i = OutPortB(&H378, Val(Text2.Text)) i = OutPortB(&H37A, 0) 'raise ready End Sub Do While (InPortB(&H379) And 32) = 0 i = OutPortB(&H37A, 8) Do While (InPortB(&H379) And 32) <> 0 i = OutPortB(&H37A, 10) Timer1.Enabled = True

5 On the PC side we write the following code tied to a button click event. First we will disable the Timer so the Timer time event does not interupt our transfer. Next, we change our LPT1 data port 378h to an output port by clearing bit 5 on port 37Ah, also at this time we will raise the LPT Request Line. Now, we can place our data on the 378h Data port and then raise the LPT Ready Line. We now loop until the PIC sends back our PIC Request Line signal, at which time, we can drop our LPT Ready Line. Again we loop until the PIC Request Line goes low this time and then we drop our LPT Request line and then enable the Timer again. Remember all bits used on port 37Ah are inverted, for example when we OutportB(&H37A,0) we are raising both LPT Request and LPT Ready lines high. Let s stay on the PC side and go through the PC receive code. For this we use Timer1 to poll the PIC s Request Line. An interupt could be used as well but, that would take some fancy coding I will leave for later. PC Receive Code Private Sub Timer1_Timer() Timer1.Enabled = False i = InPortB(&H379) If (i And 32) <> 0 Then 'test if request high i = OutPortB(&H37A, &HFF) 'change direction Do While (InPortB(&H379) And 64) = 0 'wait for ready i = InPortB(&H378) 'read data Text1.Text = Str$(i) i = OutPortB(&H37A, 8) 'raise request Do While (InPortB(&H379) And 64) <> 0 ' wait for not ready End If i = OutPortB(&H37A, &HF) 'change to output and drop signals do while (inportb(&h379) and 32)<>0 loop Timer1.Enabled = True End Sub

6 The PC timer on timing out reads in from port 379h to test to see if the PIC is requesting to send a byte of data. If the PIC is not requesting transfer the timer which we disabled at the start of the subroutine is re-enabled and the subroutine is exitted. If the PIC is requesting we turn our 378h data port to an input and wait till the PIC signals it is ready. When the PIC signals ready we can read our byte then we will raise the LPT Request Line to signal the data has been read then we wait for PIC to drop it s ready line. We now turn the 378H port back to an output being careful to leave the other bits on the 37Ah port low (high as far as the VB code is concerned). We should probably wait in a loop till the PIC Request Line goes low before we exit the if statement. Finally, we reenable the Timer1 and exit our subroutine. PIC Write Code writedata movlw b' ' ;turn off interupts bsf PORTE,0 ;raise request line bsf STATUS,RP0 movlw b' ' movwf TRISD bcf STATUS,RP0 ;change direction of PORTD movf writevalue,w ;load writevalue into W movwf PORTD ;send out value bsf PORTE,1 ;raise ready poll4done btfss PORTB,7 goto poll4done bcf PORTE,1 ;wait for lpt signal ;clear ready poll4reqdrop btfsc PORTB,7 goto poll4reqdrop bcf PORTE,0 ;wait for lpt signal ;clear request movlw b' ' ;reenable interupts return

7 When we want the PIC to send a byte of data we call the writedata subroutine. Writedata sends the value that is saved in the writevalue variable. First, we have to turn off the interupts because when the LPT Request Line goes high to signal it has read the data it would cause an interupt and start into its receive routine. We raise the PIC Request Line to signal the PC we want to send a byte. We then change PORTD to an output port, load the value of writevalue into the W register, and then to PORTD. We then raise the PIC Ready Line to signal that the data is ready to be read and then we sit in a loop till the LPT Request Line goes high to signal that the PC has read the data. We then lower the PIC Ready Line and wait for the PC to lower the LPT Request Line. Once that line has dropped we can lower the PIC Request Line re-enable our interupts and return from our subroutine. Other tricks that have been tried include: Testing both request and ready lines to see if they are both high, indicating that there is no connection between the two devices. In conclusion, there can be more elaborate ways to achieve higher data throughput, coupling the PIC Request Line with the LPT interupt as not to use the timer. However, this works well and for small data transfers it would more that sufficient.