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Figure 17.10 A 16-button keypad would be wired as shown here, requiring only eight PIC I/O pins.
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connected to the columns are left in tristate (input) mode. When a column is being scanned, the column pin is output-enabled, driving a 0, and the four input bits are scanned to see if any are pulled low. In this case, the keyboard can be scanned for any closed switches (buttons pressed) using the code
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int KeyScan() when a { int int i = 0; key = -1; // // Scan the Keyboard and Return key is pressed
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while (key == -1) { for (i = 0; (i < 4) & ((PORTB & 0x00F) == 0x0F0); i++); switch (PORTB & 0x00F) { case 0x00E: key = i; break; case 0x00D: case 0x00C: key = 0x04 + i; break; case 0x00B: case 0x00A: case 0x009: case 0x008: key = 0x08 + i; break; else key = 0x0C + i; break; } // hctiws } // elihw return key; } // End KeyScan // // Find Key that is Pressed Row 0
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The KeyScan function will only return when a key has been pressed. This routine will not allow keys to be debounced or for other code to execute while it is executing. These issues can be resolved by putting the key scan into an interrupt handler that executes every 5 ms:
Interrupt KeyScan( ) { // 5 msec Interval Keyboard Scan
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int int
i = 0; key = -1
for (i = 0; (i <4) & ((PORTB & 0x00F) == 0x00F)); i++); if (PORTB & 0x00F) != 0x00F) { // Key Pressed switch (PORTB & 0x00F) { // Find Key that is Pressed case 0x00E: // Row 0 key = i; break; case 0x00D: // Row1 case 0x00C: key = 0x04 + i; break; case 0x00B: // Row2 case 0x00A: case 0x009: case 0x008: key = 0x08 + i; break; else // Row3 key = 0x0C+i; break; } // hctiws if (key == KeySave) { keycount = keycount + 1;// Increment Count // <-- Put in Auto Repeat Code Here if (keycount == 4) keyvalid = key; // Debounced Key } else keycount = 0; // No match Start Again KeySave = key; // Save Current key for next // 5 ms } // // Interval } // End KeySave
This interrupt handler will set keyvalid variable to the row/column combination of the key button (which is known as a scan code) when the same value comes up four times in a row. This time scan is the debounce routine for the keypad. If the value doesn t change for four intervals (20 ms in total), the key is determined to be debounced. There are two things to notice about this code. The rst is that in both routines I handle the row with the highest priority. If multiple buttons are pressed, then the one with the highest bit number will be the one that is returned to the user. The second point is that this code can have an autorepeat function added to it very easily. To do this, a secondary counter has to be rst cleared and then incremented each time the keycount
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variable is 4 or greater. To add an autorepeat key every second (200 intervals), the following code is added in the interrupt handler
if (keycount == 4) { keyrepeat = keyrepeat - 1; // Decrement the Key Auto Repeat Value if (keyrepeat == 0) { keyrepeat = 200; // Restart the 1 second Auto // Repeat count keycount = 3; // Reset the counter keyvalid = key; // Return the key } } else // Reset the Auto Repeat Counter keyrepeat = 1; // End Outputting the Value with Auto // Repeat
LCDs
LCDs can add a lot to your application in terms of providing a useful interface for the user, debugging an application, or just giving it a professional look. The most common type of LCD controller is the Hitachi 44780, which provides a relatively simple interface between a processor and an LCD. Using this interface is often not attempted by new designers and programmers because it is dif cult to nd good documentation on the interface, initializing the interface can be a problem, and the displays themselves are expensive. I have worked with Hitachi 44780 based LCDs for a while now, and I have to say that I don t believe any of these perceptions. LCDs can be added quite easily to an application and use as few as three digital output pins for control. As for cost, LCDs often can be pulled out of old devices or found in surplus stores for less than a dollar. The purpose of this section is to give you a brief tutorial on how to interface with Hitachi 44780 based LCDs. I have tried to provide the all the data necessary for adding LCDs successfully to your application. In the book I use Hitachi 44780 based LCDs for a number of different projects. The most common connector used for 44780-based LCDs is 14 pins in a row, with pin centers 0.100 in apart, with the pinout listed in Table 17.1. As you probably would guess from this description, the interface is a parallel bus that allows simple and fast reading/writing of data to and from the LCD. This waveform to write an ASCII byte out to the LCD s screen is shown in Fig. 17.11. The ASCII code to be displayed is 8 bits long and is sent to the LCD either 4 or 8 bits at a time. If 4-bit mode is used, two nybbles of data (sent high 4 bits and then low 4 bits with an E clock pulse with each nybble) are sent to make up a full 8-bit transfer. The E clock is used to initiate the data transfer within the LCD. Sending parallel data as either 4 or 8 bits is the primary mode of operation. While there are secondary considerations and modes, deciding how to send the data to the LCD is the
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