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Output the Data to PORTB
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With the circuit and interface code working, I was then able to focus on doing the serial input read and then output to the LCD. As I indicated earlier, I wanted to create an interface that would simulate the operation of a TMR0 over ow interrupt that polls the serial line three times for each bit. As I envisioned the serial LCD interface presented here, I wanted to have it work for 9,600 and 1,200 bps. A 9,600 bps data rate has a bit period of 104.167 s (which I usually round off to 104 s), and the 1,200 bps data rate has a bit period of 833.333 s (which I round off to 833 s). Using these timings, I had to come up with code that would poll either once every 35 s (or 35 instruction cycles) for 9,600 bps or 278 s for 1,200 bps. Polling every 35 s would give me an effective data rate of 105 s for 9,600 bps for an error of 0.8 percent. The 278- s polling rate gave me an effective data rate of 1,199.041 bps with an error of 0.08 percent to an ideal of 1,200 bps. In either case, this error rate is low enough as to not result in incorrect data being read in. To actually poll the data, I created a macro that would check for data already coming in, and if it were, or if a start bit was detected, it would call a serial processing subroutine. This macro only required three cycles (allowing up to 32 cycles to execute outside of it) to check the incoming data:
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SerCheck Macro btfss SerActive btfss SerIn call RSSerialPos endm ; ; ; Check Serial Data Reading to be Done Yes - Check the RS-232 Line Input
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In the code, the SerActive ag is set to indicate that data is coming in and to jump to the serial read routine. If this ag is not set, then if the incoming data line is a 0 (start bit), the serial read routine is called to start reading the incoming data. The serial read routine can be written in high-level pseudocode as follows:
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SerialRead() { // Read Incoming Serial Data
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Temp = inp(PORTB); // Get the Contents of the Serial Data // for Checking if (SerActive == 0) // Is this a Start Bit if ((Temp & (1 << SerInBit)) == 0) { Dlay( 1/3 Bit ); // Delay 1/3 Bit
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if ((inp(PORTB) & (1 << SerInBit)) == 0) { SerActive = 1; BitCount = 9; Dlay( 2/3 Bit ); } } else ; else { BitCount = BitCount 1; // // // Not a Glitch, Read in Byte Going to Read through 9 Bits Delay Full Bit
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Not Low Glitch Else, Reading in Data Decrement the Bit Count
if (BitCount != 0) // Read Bit RXData = (RXData >> 1) + ((Temp & (1 <<SerInBit) != 0); else { // Last Bit Is there a Stop Bit SerActive = 0; // In any Case, not Reading Bits
if ((inp(PORTB) & (1 << SerInBit)) != 0) SaveData(RXData); // Valid Stop Bit End } Dlay( 1/3 Bit ); } } // End SerialRead // Delay Remaining Bit
In this code, notice that that under all circumstances there is a delay for at least a third of a bit. In the application code, this meant that anytime SerialRead is called,
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at least two-thirds of a bit period is used in the subroutine. This leaves only one-third of a bit period for actually executing mainline code. This is not a terrible hardship because in the nine bits required for each bit, up to ve characters could be sent to the LCD with a polling routine on the busy ag to wait for the LCD to complete writing to its display. In the start bit, notice that I end up waiting a full bit period before returning to the mainline code. This code performs two glitch checks and makes sure that the data poll takes place in the one-third bit period that is closest to the middle of the bit. As I indicated earlier, the possible bit time errors are anywhere from 0.08 to 0.8 percent, which means that any kind of movement from this position should keep the poll well within the bit. The reason why I capture the serial data at the start of the serial input routine is to make sure that it is polled at the same location during the application s execution. The assembly code itself is quite a bit more complex, with the most important aspect of it being the delay code. In the preceding pseudocode, I label delays simply like Dlay( 1/3 bit ), but in the actual application code, it is quite a bit more complex. In the application code, I rst debugged the application to run properly at 9,600 bps. To do this, I had to make sure that the line is polled at exactly one-third bit period. To do this, I counted through the number of cycles for each action and then put in a delay to make sure that the number of cycles between the SerCheck polling macros was exactly 35 s using a delay macro similar to the one described elsewhere in this book. Once I had RS-232 input (which is essentially the complement of TTL/CMOS NRZ serial input) running at 9,600 bps, I then went ahead and made the changes to the code that would allow the switch selection between 9,600 and 1,200 bps. I was looking ahead at doing this, and to accomplish it, I simply selected between two different time delays. The code I normally used looked something like this:
btfsc SpeedSet goto Speed3P Delay 35 - 17 - 4 goto Speed4P Speed3P: Delay 278 - 17 - 4 nop Speed4P: ; Delay 1/3 Cycle
which executes a delay speci c to the speed at which the application is running. If the SpeedSet ag is set, then the application is running at 1,200 bps; otherwise, 9,600 bps. In this code, the 4 operator is used to include the four cycles that were added to select the actual data speed. With the application running for RS-232, I put the SerialRead code into a macro with the ability to select positive (TTL/CMOS) or negative (RS-232) inputs. This further adds to the complexity of trying to see what is happening inside the source code. If you look over the different copies of the application code, you will see how this macro was developed.
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