Key Board & LED Interfacing

1. Key Board & LED Interfacing Amrita Vishwa Vidyapeetham

2. External World Interfacing • The peripheral now to be discussed – Key Board • Typical application of weak pull-ups • A key is the simplest mechanical device that can be interfaced to a μC. • Depending on the number of keys to be interfaced and the type of application, different approaches to interface are available. Amrita Vishwa Vidyapeetham

3. Key – On/Off Bounce Period ~ 20 ms Amrita Vishwa Vidyapeetham

4. Key - Bounce / Debounce • A key is a mechanical device with spring action. • When a Key is pressed, it may take a few ms to settle to the ON state. • In the interim period, the contact may be made and broken repeatedly due to the vibration of the moving mechanism. • When released, it reverts to the OFF state after a similar period of vibration. • The duration of bounce and the high frequency of vibration associated with it depend on the key structure. • The status of the key is not clearly defined during the bounce duration. • The key can be ‘debounced’ (the effect of bounce nullified) in two ways - hardware or software Amrita Vishwa Vidyapeetham

5. Hardware Debounce - 1 Amrita Vishwa Vidyapeetham

6. Hardware Debounce - 1 • The RC time constant can be of the order of 5 ms to filter out bounce generated voltage spikes. • When the key is pressed, the voltage at point X increases from 0V to 5V smoothly. • The buffer B has threshold voltages for the up and down transitions • Y changes to high state (from the earlier low state) when the voltage at X crosses the threshold. • The debounce circuit behavior is similar for the transition from high to low state. Amrita Vishwa Vidyapeetham

7. Hardware Debounce - 2 • Debouncing can be done by employing a simple RS latch. • The latch is normally in the reset state. • When the key is pressed, the latch changes to set state; output Q changes from low to high state. • The instant of transition is decided by the first spike at input which is wide enough to make the latch set. • Once the latch is set, it remains in set state even after the key is released. • The μC has to reset it by applying a high state pulse at R; it is to be done with a reasonable time gap of about 1s after key closure. Amrita Vishwa Vidyapeetham

8. Software Key Debounce • The debounce is handled by the software program • Usually a delay period is assumed over which the level of the key press signal should not change Amrita Vishwa Vidyapeetham

9. 4x3 Keypad Amrita Vishwa Vidyapeetham

10. 4x3 Key Pad • In this particular example there are 12 switches • Rather than use up all these scarce I/O pins it is hardware efficient to connect these switches in the form of a 4×3 matrix • Without matrix – 12 I/O pins are required • With matrix, total I/O pin count is 7. • Larger keypads show an even greater efficiency gain, with a 64-contact 8 × 8 keyboard needing only 16 I/O pins. Amrita Vishwa Vidyapeetham

11. Working of 4x3 Key Pad • The four rows are read in via RB7:4 with internal pull-up resistors enabled. • The three columns connected to RB1:3 can be individually selected in turn by driving the appropriate pin low, thus scanning through the matrix. • This is called key board scanning. • The switch contacts are normally open and, because of the pull-up resistors, read as logic 1. • When a switch connected to a low column line is closed then the appropriate row line is low. • Once the closed key row has been detected the column : row intersection is known (i.e. the key that is pressed is identified). Amrita Vishwa Vidyapeetham

12. Key Pad Subroutine • Let us write subroutine SCAN_IT for scanning a Key Pad. • SCAN_IT initializes the column count key to 1 and the column scan pattern to 11110111b. • This test vector is sent to Port B and each row is tested in turn adding 3, 6 or 9 to the value of key if a zero is found and loop is exited (break). • If after the four rows have been tested no outcome has been detected, the column scan pattern is shifted right and key is incremented. • The process is continued until either a 0 is found or count reaches 13. • In the latter case a key value of FFh (−1) is returned. Amrita Vishwa Vidyapeetham

13. Algorithm for SCAN_IT subroutine • 1. Two global variables KEY_COUNT, PATTERN • 2. Key 1 is the first key • 3. Set initial KEY_COUNT = 1 • 4. Set initial PATTERN = b’11110111’ • 5. Start a loop – SLOOP • 6. Output PATTERN to PORTB • 7. Move KEY_COUNT to W • 8. Check ROW1 - RB7 • 9. IF zero THEN found the key, exit the subroutine with Key identifier, ELSE increment KEY_COUNT by 3 • 10. Check ROW2 – RB6 • 11. IF zero THEN found the key, exit the subroutine with Key identifier, ELSE increment KEY_COUNT by 3 • 12. Check ROW3 – RB5 • 13. IF zero THEN found the key, exit the subroutine with Key identifier, ELSE increment KEY_COUNT by 3 • 14. Check ROW4 – RB4 • 15. IF zero THEN found the key, exit the subroutine with Key identifier, ELSE increment KEY_COUNT by 3 • 16. If no closed key, W = -1 • 17. Advance KEY_COUNT one column • 18. Shift (right) scan pattern once -> • 19. Check - has the 0 reached RB0? • 20. IF not DO another column. Go to SLOOP • 21. IF yes, return with KEY_COUNT in W Amrita Vishwa Vidyapeetham

14. a Flow chart Yes RB7 = 0 No W = W + 3 RB6 = 0 Yes SCAN_IT No KEY ID = W W = W + 3 KEY_COUNT = 1 PATTERN = b’11110111 RB5 = 0 Yes No W = W + 3 O/P PATTERN to PORTB RB4 = 0 Yes No W = KEY_COUNT W = h’FF KEY_COUNT + 1 PATTERN >> 1 a RB0 = 0 KEY ID = W Yes No Amrita Vishwa Vidyapeetham

15. Real World Issues • In the real world a subroutine like this would often read in rubbish due to switch bounce • Also possibly noise induced in the connections between keypad and the electronics. • One way of filtering out this unpredictability is debounce subroutine – S/W • Here the state of the keypad is interrogated using the SCAN_IT subroutine of Program • By keeping the state of the previous reading in Data memory, any change can be detected. • Only if no change over 256 readings occurs will subroutine GET_IT return with the keypad state. • Depending on the quality of the keypad, ambient noise and processor speed, the outcome can be improved at the expense of response time by including a short delay in the loop, or by using a 2-byte stability count. Amrita Vishwa Vidyapeetham

16. Keypad Subroutine with debounce • GET_IT keeps a number of tries count tally, last reading OLD_KEY and current reading NEW_KEY variables. • COUNT is incremented after calling SCAN_IT if the current reading is the same as the last reading. • If not, COUNT is reset and OLD_KEY is updated. • The loop exits if count reaches 255 (8-bit register), indicating that the last 255 readings are the same. Amrita Vishwa Vidyapeetham

17. Algorithm for GET_IT subroutine • 1. Three global variables COUNT, NEW_KEY, OLD_KEY • 2. Initial value of OLD_KEY& NEW_KEY is Zero • 3. Initial value of COUNT is zero. • 4. Call the subroutine SCAN_IT • 5. Raw value returned in W is moved to NEW_KEY • 6. Check if NEW_KEY = OLD_KEY • 7. IF same go to STEP 9 ; • 8. ELSE the readings are different, so: Make OLD_KEY = NEW_KEY and start all over again. Go to STEP 3 • 9. IF readings are the same THEN Increment COUNT • 10. IF not rolled around to 00 repeat ELSE we got the key identifier. Amrita Vishwa Vidyapeetham

18. Flow Chart GET_IT NEW_KEY = 0 OLD_KEY = 0 COUNT = 0 CALL SCAN_IT NEW_KEY = W NEW_KEY = OLD_KEY Yes No COUNT = COUNT + 1 OLD_KEY = NEW_KEY Yes COUNT = 0 No KEY_ID = OLD_KEY Amrita Vishwa Vidyapeetham

19. Seven Segment LEDs Amrita Vishwa Vidyapeetham

20. Multiplexing 7 Segment Displays - 1 Amrita Vishwa Vidyapeetham

21. Multiplexing 7 Segment Displays - 1 • Assuming each display requires eight lines (seven segments plus decimal point) then a budget of 8 × n parallel lines are required for an ndigit display. • The straightforward solution to this problem - a 3-digit display is driven from three parallel registers on a single bus. • The principle can be extended to six or more digits using the appropriate number of registers. Amrita Vishwa Vidyapeetham

22. Multiplexing 7 Segment Displays - 2 Amrita Vishwa Vidyapeetham

23. Multiplexing 7 Segment Displays - 2 • An alternative approach, is frequently used with LED-based displays. • Instead of using a register for each digit, all readouts are connected in parallel to the one PIC port. • Each readout is enabled in turn for a short time with the appropriate data from the output port. • Provided that the scan rate is greater than 50 per second (preferably greater than 100) the brain’s persistence of vision will trick the onlooker into visualizing the display as flicker free. • Of course this is how the brain interprets a series of 24 still frames per minute in a movie, each shown twice, as a moving image. Amrita Vishwa Vidyapeetham

24. WATCH DOG TIMER Amrita Vishwa Vidyapeetham

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28. During normal operation, a WDT time-out generates a device RESET. • If the device is in SLEEP mode, a WDT time-out causes the device to wake-up and continue with normal operation, this is known as a WDT wake-up. • The WDT can be permanently disabled by clearing the WDTE configuration bit. • The postscaler assignment is fully under software control, Amrita Vishwa Vidyapeetham

29. WDT Period • The WDT has a nominal time-out period of 18 ms, • If longer time-outs are desired, a postscaler with a division ratio of up to 1:128 can be assigned • Thus, time-out periods of up to 2.3 seconds can be realized. • The CLRWDT and SLEEP instructions clear the WDT and the postscaler (if assigned to the WDT). • and prevent it from timing out and generating a device RESET. • The TO bit in the STATUS register will be cleared upon a Watchdog Timer time-out (WDT Reset and WDT wake-up). • *********************************************************** Amrita Vishwa Vidyapeetham