In the present, different types of systems for data communication, such as computers, are widely used in almost every field of work. So now we have super computers, personal computers(desktops / laptops) even in Tabs the very basic idea is to communicate with certain hosts,to do that we simply input raw data, we process it and finally we get the data as a useful information, as a output.
To develop such a system each task should be considered separately and the first task is data logging. So to develop or to observe the task simply we need an input device and a method of communicating with it along with a device to observe the input data. Since these devices are different for different systems interfacing the input device to host of a particular system, plays the major role.
The ‘PS/2 keyboard based character logger’ uses ‘PS/2’ serial communication for communicate with the input device, in this case a keyboard is interfaced with a 16×2 LCD Module using PIC16f877A microcontroller.
Materials and Methods
Materials
PIC16F877A Microcontroller
The microcontroller is the most important part of the system and its controlling whole system, for develop this device PIC16F877A microcontroller is selected.
It’s belongs to 16-bit MCU family and it has 33 I/O peripherals, these requirement more than enough to implement the design.
Mainly i used capture and interrupt functions available in this microcontroller to develop this system.
The pin diagram of the PIC16F877A microcontroller given below in Figure 1,
LCD Module (HD44780)
Dot-matrix LCDs produces in two formats:
1) Fully functional, Alphanumeric Modules.
2) fully-populated Graphic modules.
For this system fully functional, Alphanumeric LCD Module selected.
Alpha-numeric modules display, characters, numerals, symbols and some limited graphics. Interface is achieved via a bi-directional, parallel ASCII data bus. Necessary features such as Character Generation, Display RAM Addressing, Cursor Scrolling, Blanking, and Handshake are all included. These LCD modules range from 8 to 80 characters per line. One, two or four character lines may be chosen.
In summary, these modules are the simplest and most economic means to communicate meaningfully between any micro-system and the outside world.
To develop this system 16×2 hd44780 LCD module selected.
Pin Description and LCD registry instruction menu shown in Table 1 and Figure 2respectively.
Table 1 : LCD Pin Description
| Pin No. | Symbol | Signal Description |
| 1 | Vss | Power Supply (Vss=0) |
| 2 | Vdd | Power Supply (Vdd=5 V) |
| 3 | Vo | Operating voltage for LCD (variable) |
| 4 | RS | Register Selection input |
High = Data register
Low = Instruction register
5R/WRead/Write signal input is used to select the read/write modeHigh = Read mode
6EStart enable signal to read or write the data 7 -10DB0 – DB3Four low order bi-directional three-state data bus lines.Used for data transfer between the MCU and the LCD module.
11 – 14DB4 – DB7Four high order bi-directional three-state data bus lines.Used for data transfer between the MCU and the LCD module.
15A Vdd( backlight) 16K Vss
Figure 2 : LCD registry instruction menu
PS/2 Keyboard
History
IBM company first designed the keyboard called “AT keyboard” which refers “Advanced Technology “and then they developed “PS/2 Keyboard” which refers “Personal Systems /2 “keyboard that also compatible with AT keyboard technology.
So actually PS/2 means not the port the port is called as Mini-Din 6 port but as PS/2 port its more popular in society.
To design this system PS/2 keyboard with 104 keys is selected. A unique SCAN code is assigned for every key as shown below in Figure 3
Figure 3 :PS/2 Keyboard Layout
Figure 4 : Mini-Din-6 Female Port
Methods
The Algorithm that used to interface PS/2 keyboard to LCD Module;
1. Initialize the Capture module for capture every falling edge of the keyboard clock.
2. Initialize the Timer1, enable 16 bit timer 1, pre-scale set to 1:1 and sync enable.
3. After first falling edge clear interrupt flag (to avoid processing unwanted interrupts).
4. Declare the interrupt Handler on ISR ()
5. Read the data from keyboard data line (connected to RDO pin) and collect it in a variable
6. Get the 8 data bits
7. Compared against case statements
8. Pass the Output to LCD.
Keyboard To Host Protocol
Keyboards consist large number of keys and it all connected to each other like a matrix of keys, all of these keys are continuously scanning by keyboards internal processer to identify whether a key is being pressed, released, or held down, and if any key is pressed, released, or held down, the processor send appropriate scan code to the host, to identify which key is being, released, or held down. This processor have 16-byte buffer and it can handle any scan code with it.
There are two different types of scan codes,
1. Make codes
A make code is sent when a key is pressed or held down. Make codes contains only one hex code,
I. Extended make codes,
It contains two hex codes, E0 hex common to all extended codes; keyboard sends extended code with E0 hex code and along with another
hex code.
2. Break codes
A break code is sent when a key is released. Break code contains two hex codes, F0 hex code common to all break code. Keyboard sends break
codes with F0 hex code and along with releasing key make code.
I. Extended break codes,
It contains three hex codes, E0 hex is common and keyboard sends extended break code with E0 and along with its releasing key break code.
Every key in keyboard is assigned to its own unique make code and break code so the host can determine exactly which key is active by looking at the corresponding scan code.
The set of make and break codes shows in the Example in Table 2
Table 2 : Scan code set example (A-C)
|
KEY |
MAKE |
BREAK |
|
A |
1C |
F0,1C |
|
B |
32 |
F0,32 |
|
C |
21 |
F0,21 |
|
Extended codes |
||
|
Up ARROW |
E0,75 |
E0,F0,75 |
|
Down ARROW |
E0,72 |
E0,F0, |
See Attachments for complete scan code table
Keyboard using bi-directional communication protocol, but the host has the ultimate controlling power to select the direction of communicating because host controlling the keyboard clock and data line states. As in Figure 5 host normally given both lines to high states using pull-up resistors and making C and D port low, if host want to control the lines states it can be done easily by changing C or D port states.
Figure 7 : practical transmission signal of, ‘a’ key scan code (‘1C’ hex code)
The keyboard clock frequency is 10-16.7 kHz. The time from the rising edge of a clock pulse to a Data transition must be at least 5 microseconds. The time from a data transition to the falling edge of a clock pulse must be at least 5 microseconds and no greater than 25 microseconds.
The host may inhibit communication at any time by pulling the Clock line low for at least 100 microseconds. If a transmission is inhibited before the 11th clock pulse, the device must abort the current transmission and prepare to retransmit the current “chunk” of data when host releases Clock. A “chunk” of data could be a make code, break code, device ID, mouse movement packet, etc. For example, if a keyboard is interrupted while sending the second byte of a two-byte break code, it will need to retransmit both bytes of that break code, not just the one that was interrupted.
If the host pulls clock low before the first high-to-low clock transition, or after the falling edge of the last clock pulse, the keyboard/mouse does not need to retransmit any data. However, if new data is created that needs to be transmitted, it will have to be buffered until the host releases Clock. Keyboards have a 16-byte buffer for this purpose. If more than 16 bytes worth of keystrokes occur, further keystrokes will be ignored until there’s room in the buffer. Mice only store the most current movement packet for transmission.
Capture Method
The CCP modules are identical in operation, so it can precisely capture when event occurs. Capture module contains a 16-bit register which can operate as a 16-bit capture register, see Figure 8
In Capture mode, CCPR1H:CCPR1L registers captures the 16-bit value of the TMR1 register when an Event occurs on pin CCP1. An event is defined as:
• Every falling edge
• Every rising edge
• Every 4th rising edge
• Every 16th rising edge
For this system event is defined as capture every falling edge on the keyboard clock line. An event is selected by control bits CCP1M3:CCP1M0 in CCP1CON register, Figure 9 shows the CCP1 capture mode in block diagram.
Figure 9 : CCP1 capture mode block diagram
When a capture is made, the interrupt request flag bit, CCP1IF, is set. The CCP1IF bit must be cleared in software. If another capture occurs before the value in register CCPR1 is read, the previous captured value will be lost. So to avoid occurring unwanted capture interrupts, after capture is made CCP1IE and CCP1IF should cleared in software.
High Level Design
PS/2 Keyboard clock line connected to RC1/CCP1 pin and data line connected to RD0 pin, RD1-RD3 connected to RS, RW, E in LCD module and RB0-RB3 connected to high nibble(D4-D7) of the LCD module.
So the basic idea of the software design is, it should read the fall down edge of the clock line in the same time it should assigned the data value from RD0 pin to a variable. Then variable value must scan with the keyboard hex codes an pass the output to LCD module to display which key is being pressed or held down.
Hardware construction
Motherboard
This motherboard contains the PIC16f877a microcontroller and all the requirements to get it in to working condition, with all port outs and VDD and VCC outs.
Also this contains serial port which allows programming the PIC using serial communication and ICSP port which allows programming the PIC using USB.
Figure 11: Mother board pcb layout
Figure12 : Mother board pcb layout with components
Figure13 : Complete Mother board
Daughter Card
This is used to connect LCD module and PS/2 Port from the keyboard to motherboard, daughter card getting power from the mother board and then it supplies power to LCD module and to the keyboard.
Figure14 : Daughter card PCB layout
Figure15 : Complete Daughter card with LCD module
power
12 V DC supply provided to Mother board, mother board contains internal 5 V regulation circuit, it will provide the input voltage to the all components. 500 mA current supply provided to mother board to get all components to a perfect working condition.
PIC16F877A microcontroller
Input voltage = +4.5 V to +5.5 V
Current = 300 mA (max)
LCD module
Input voltage = +2.7 V to +5.5 V
Current = 50 mA (with back light on)
PS/2 Keyboard
Input voltage = +4.5 V to +5.5 V
Current = 275 mA (max)
Results
It works fine, the device can display any characters, symbols, and perform well when combined with shift keys, Caps lock and Number lock functions working perfect.
Also Delete,Backspace, Arrow keys, Enter functions working well.
Figure 16 : Complete Full device circuit
Figure 17 : Final Enclosed device with keyboard in working condition
This is a video example of working device.
YouTube Video
Improvements and Suggestions
- 20 MHz oscillator is not good enough to get keyboard scan code quickly.
- If statement is powerful but it contributes poorly when compiler optimization starts.
- Switch statement not powerful as if statement but it contributes to get the fastest compiler optimizing execution.
- Can develop a host to device communication so then we get the data from LCD module.
- Save it in a external memory card module (MMC, Micro SD) or external EEPROM IC.
- Then it can be use as a real time data logger.
- Recreate the same project using PIC 18F series microcontroller (ex: PIC18F452 or PIC18F458) instead of PIC 16F877A then using higher oscillator (40 MHz) will give the perfect results with very less delays.
- Also in this project scan codes are not saved in the EEPROM because there’s not enough memory to save all those scan code corresponding to all 104 keys, so as a suggestion if we interface external EEPROM and save the scan codes over their may be we can hope little bit of fast execution on the code, so we can type key after key faster than in this project with less delay, then it can use as a real time faster data logger.
Conclusion
High speed scan code decoding is not impossible, its just we need to know the protocol exactly because all these devices are logical, when we identified the protocol exactly and when we have a good understanding of logical states in the device, it is the key for catching and decoding any high speed code and it can interface easily.
Appendix
Appendix 1: Mother Board components list
| Component | Quantity |
| PIC16F877A | 1 |
| 40-pin IC base | 1 |
| Max232a | 1 |
| 16-pin IC base | 1 |
| 20 MHz crystal oscillator | 1 |
| 22 pF capacitor | 2 |
| LM7805 regulator | 1 |
| 0.1 µF capacitor | 4 |
| o.1 µF electrolyte | 4 |
| 220 µF electrolyte | 2 |
| 10 k Ω resistor | 1 |
| 330 Ω resistor | 2 |
| LED – RED | 2 |
| LED – GREEN | 2 |
| On-Off switch | 1 |
| 2-way dip switch | 1 |
| Push button | 1 |
| 8-way wire connector | 3 |
| 6-way wire connector | 2 |
| 3-way wire connector | 1 |
| 2-way wire connector | 1 |
| DB9 –female connector | 1 |
Appendix 2 : Daughter Card component list
| Component | Quantity |
| 50 Ω resistor | 1 |
| 5 kΩ variable resistor | 1 |
| 16 – way connector | 1 |
| 10 – way wire connector | 1 |
| 4 – way wire connector | 2 |
Appendix 3 : PS/2 keyboard Scan code list
Code listing
Schematic diagrams
Shematic diagrams of mother board, Daughter card and Full schematic can be downloaded from attachments.
References
PS/2 Keyboard Protocol
PS/2 keyboard interface to LCD.
16×2 LCD interfacing
16×2 LCD module interface to pic16f877a.
Capture module
Interrupts
Coding
how do i use interrupts in hitech c.
Speed Test of If,Else,Switch statements.
General references
Data Sheets and Reference manuals
HITACHI. HD44780U (Dot Matrix Liquid Crystal Display Controller/Driver).
MICROCHIP TECHNOLOGY INC. 2003. PIC16F87XA Data Sheet.
MICROCHIP TECHNOLOGY INC. 2003. PICmicro™ Mid-Range MCU Family Reference Manual.
Attachments
by Lahiru Hettiarachchi :dn.lahiru@gmail.com
