Description

CENG 336
Introduction to Embedded Systems Development

1 Objectives
Clarifications and revisions on the content, scope and requirements of the assignment will be posted to the course page discussion forum in ODTUClass.

Figure 1: Old phones with keypads used this text input scheme.
2 Specifications
2.1 Scenario

Figure 2: Keypad located on the board used for text input.
The individual letters are input using multiple key presses, for instance ’A’ is input using only one key press on button ’2’ but ’F’ requires 3 key presses in quick succession on button ’3’. The input letters will be shown in the 7segment display using an appropriate 7-segment font. A real 7-Segment font is very constrained and somewhat unreadable, so the “font” will be shared with you in the Encoding section.
The system will be in one of the three states at any given time; Message Write, Message Review or Message Read.
Using the RB4 button interrupt will immediately switch the system between Message Write State and Message Review State.
After all 6 characters are entered in Message Write State or when countdown reaches to 0 at Message Write State or Message Review State, the system will enter the Message Read State.
2.2 Initial Configuration
Initially, the system should be waiting for the RB3 button action. At this stage, 7 segment display screen should be blank. After pressing & releasing the RB3 button, the system should be brought to Message Write state and the countdown should start.
2.3 Encoding
With the provided 7-Segment font, your alphabet will look like the following.
1 2 ABC 3 DEF
4
GHI 5
JKL 6
MNO
7
PRS 8 TUV 9
YZ
* 0 #
Table 1: Input encoding overview

(a) Alphabet used in the THE with the 7-Segment Font

(b) Reference letters
Figure 3: How your message will look in the 7-Segment display
2.4 Message Write State
This state accepts inputs through the keypad and displays the countdown, last letter input and current input.

Figure 4: 7-Segment Display after RB3 button action
Going from left to right; leftmost two displays (DIS1 and DIS2, see Hints section) will be used for displaying the countdown. The countdown should start from 20 seconds and should be updated by TIMER0 Interrupt. The next 7-segment display will display the last letter input and the rightmost one will display the current input. The key presses should immediately affect the current input.

Figure 5: 7-Segment Display after pressing ’2’ for 3 times and ’3’ for once on keypad

Figure 6: 7-Segment Display after waiting for 1 second to commit the character and switch to the next one during the Message Write State
Table 1 shows which letter is accessible through which button. Every button can input 3 characters, each push and release on the same button cycles through the available characters under that button. 4th press will cycle back to the first letter. See Table 2 for an example.
If the RB4 button interrupt occurs during Message Write State then the system switches to Message Review State. This state will also immediately
Times 1st 2nd 3rd 4th 5th …
Character A B C A B …
Table 2: Required behaviour when button 2 is pressed multiple times
switch to Message Read State, if all 6 characters have been input or the global timeout duration is reached.
• Any empty positions are shown as “ ”. At the start, all of the letters in the message text are shown on the 7 segment display as “ ”.
• There is a space character which is accessible through the button ’9’. Contrary to the empty positions, if a space character is entered, none of the segments on that 7-segment display will be on (see Figure 8).
• Pressing ’1’, ’0’, ’*’ or ’#’ on the keypad will have no effect at this state.
• The current letter input is saved after 1 second of inactivity (see Figure 6) or when a new letter is input with a different button.
• To input two consecutive letters encoded with the same button, you have to wait for the 1 second timeout duration.
• The RB4 interrupt will discard the current character as well if the 1 second timeout period has not been hit yet. So any incorrect letter input can be avoided with switching to Message Review State by pressing and releasing RB4 button before the 1 second period.
2.5 Message Review State
In this state, you can review the message by going back and forth using over the letters with “*” and “#” buttons on the keypad.
Initially, thefirst4letterswillbedisplayedonthe7-segmentdisplayscreen. “*” and “#” buttons will be used to scroll left and right respectively. At each button press and release, the text on the display will scroll left or right by one letter only.
If an RB4 button interrupt occurs during this state, the system switches to Message Write State.
This state will immediately switch to Message Read State, if the global timeout duration is reached.
• The other keypad buttons have no effect at this state.
• If all 6 letters have not been entered yet, the blank positions will be shown with “ ” on the 7-Segment display (see Figure 8).
• If you have reached the end of the message, “#” button will have no effect and accordingly, if you have reached the start of the message the “*” button will have no effect on the 7-segment displays.

Figure 7: 7-Segment Display in Message Review State after inputting 3 letters

Figure 8: 7-Segment Display in Message Review State after pressing ’#’ twice
2.6 Message Read State
In this state, the final message text will start scrolling horizontally on the 7-segment display. The system should be unresponsive to any button action. It will start by displaying the first 4 letters. Every half a second, it will introduce the next letter by scrolling left. Once it reaches the 6th letter, it will start scrolling right. This behaviour continues until powering off the PICGenios board.
3 Regulations and Submission
• You will code your program using PIC assembly language.
• Your program should be written for PIC18F4620 working at 40 MHz.
• You MUST properly comment your code, including a descriptive and informative comment at the beginning of your code explaining your design. %5 of your grade will depend on the quality of your comments.
• You will use 3 ports in this assignment; PORTA, PORTB, PORTD.
• PORTB and PORTD will be used to input letters through multiple keypresses. There is a detailed explanation on how to read input from the keypad on Hints section.
• PORTA and PORTD will be used as output ports to display the resulting message on 7-Segment displays. You can find some useful information on Hints section (4).
• You should use the Timer0 interrupt as stated above and configure the Timer0 to work in 16-bit mode.
• You should use PORTB button interrupts for implementing the RB4 button action. The specified behaviour should occur after releasing the RB4 button.
• When you are writing your code, you can use the lecture notes on Input/Output and Interrupts, Recitation documents. It is also highly recommended that you make extensive use of the PIC data sheet. Please consult these resources first before you ask any questions to the assistants or on the forums.
• There is a discussion forum already setup for THE2 for your questions, you can also ask specific questions through mail to either Merve or Yigit.
• The final .asm file will be submitted through ODTUClass.
4 Hints
4.1 7-Segment Displays
There are four common cathode 7-segment displays mounted on the PICGenios board. PORTD and PORTA are used as data bus and selection pins, respectively, as illustrated in Figure 9. Notice that PORTD pins are connected

Figure 9: 7-Segment Display Reference
to all displays. As an example, if you want to show number 4 on leftmost display (DIS1), you should first select it by setting RA2 and clearing RA3, RA4 and RA5, then send binary 01100110 to PORTD. Hence, a, d, e segments on DIS1 will be turned off, and b, c, f, g segments will be turned on. Note that RD7 pin is used for the decimal point (dp) of displays.

Figure 10: Graphical illustration to show characters 7-Segment displays simultaneously.
If you want to show, for instance some value (1234) on the displays, you should select only DIS1 by using PORTA, write the byte necessary to turn on segments to PORTD, and wait for a while. Then you should do the same for DIS2, DIS3 and DIS4. This is illustrated in Figure 10. If you adjust on times properly and repeat on-off cycles continuously, you show your values on the displays in a smooth manner without flicker.
4.2 Keypad
We will use 4×3 Keypad located on the PICGenios board. There are 7 pins connected to the keypad but 12 buttons to interact with. See Figure 11.

Figure 11: Keypad Reference
In order to use the keypad the pins connected to the columns of the keypad matrix should be set as output and the pins connected to the rows of the keypad matrix should be set as input. When a key press occurs on the keypad, the relevant pin on the row will set LOW if the relevant pin on the column is LOW. So by setting the column pins LOW in turns and checking the input from row pins, we can match the key being pressed.
5 Cheating
Adapted from http://www.seas.upenn.edu/cis330/main.html