AT Keyboard Box V2.05

with Dot-Matrix LCD Display and numeric Keypad for Microchip PIC16F77
 

Table of Contents [Toc]

Concept
   How it works
   Specifications
   Parts order information
   Features
   Limitations
Project Resources
   Available PIC Assembler Source Code
   Schematics, Data Sheets, Pinout
   User-specific Customization
 

Concept   [Toc] [Top]

This implementation contains the complete fetch and decoding of AT keyboard scan patterns as well as RS232 transmission and reception of ASCII characters to and from the remote RS232 client. This microcontroller application also features an interface to a dot matrix LCD display to visualize the the data received from the RS232 client on the first line, and the characters typed on the locally attached keyboard on the second line. Further, the application has also a small numeric foil-keypad and a piezo-beeper for acoustic feedback.
Dynamic configuration of RS232 baud rate setting at start-up (user-customization with 1200 baud - 115200 baud), with 12 seconds inactivity time-out. In case the time-out applies, the user-customization process terminates with the current setting. Default setting after power-up is 9600 baud.
 

How it works   [Toc] [Top]

Basically it works in the same way as the AT Keyboard Interface V3.xx.

Here is only the description of the additional small numeric foil-keypad. The numeric foil-keypad is equipped with a specific resistor cascade to decode the values through direct 8 bit A/D conversion using the PIC-internal A/D converter. The advantage is a very low pin usage: Only two pins are necessary for proper detection and decoding of all keypad entries. One pin provides the analog value, the other pin serves for interrupt generation whenever a key of the keypad is touched. The interrupt is used to start the A/D conversion.

During the interrupt service routine, only a short busy wait (analog settling time) and the A/D conversion - using the internal RC oscillator - is carried out. Before leaving the ISR, the 8 bit A/D result is stored in a specific register and a dedicated flag is set.

Decoding of the A/D value is done during normal operation (activated by the flag) using two look-up tables. The first look-up table (LUT1) contains the expected 8 bit values of the keypad to check for valid entries. A numeric window of ±3 allows for slight analog deviations during matching. The matching algorithm just scans the entire LUT1 until the received keypad A/D result matches a LUT1 entry. The amount of loops carried out in LUT1 determines the position of the corresponding symbol/character in LUT2. At the end, RS232 transmission and LCD display update are carried out.

Dynamic configuration of RS232 baud rate setting at start-up (user-customization with 1200 baud - 115200 baud). A watchdog timer implemented using TMR1 checks for inactivity during the customization process. After 12 seconds of inactivity, the user-customization process terminates with the current setting. At power-up, the default setting is 9600 baud, which will be configured after the time-out - unless no user-customization takes place.

This setup works also without attached foil-keypad, even if the corresponding code is assembled and loaded into the microcontroller.

Specifications   [Toc] [Top]

Note that every change in microprocessor clock frequency needs a re-calibration and/or re-design of the analog foil-keypad decoding circuitry.
 

Parts order information   [Toc] [Top]

Numeric foil-keypad order information and technical specifications: www.conrad.de: FOLIENTASTATUR 1x12, Part-Nr. 709948-14

Features   [Toc] [Top]

• Dynamic configuration of RS232 baud rate setting at start-up.
• Bi-directional communication between microcontroller application and remote RS232 client.
• Bi-directional communication between microcontroller and keyboard.
• Bi-directional communication between microcontroller and LCD display.
• Supports foil-keypad input through direct 8 bit A/D conversion and look-up table.
• Piezo-beeper for acoustic feedback of keypad entries.
• Visualization of received and transmitted characters on local LCD.
• Parametrizable LCD display width: constant 'LCDwidth'
• Support for all keyboard characters typed with shift button active and inactive.
• English and modified Swiss-German 'codepages' available (QWERTY and QWERTZ)
• Caps Lock implemented
• Num Lock always active
• Support of ASCII conversion from direct ALT-DEC entries, e.g. ALT + 6 + 4 = @ (ALT + [1..3] numbers)
• Support of ASCII conversion from direct CTRL-HEX entries, e.g. CTRL + 3 + F = ? (CTRL + [1..2] letters/numbers)
• ALT-DEC and CTRL-HEX features work for both, keypad and keyboard numbers, as well as with upper and lower case letters [a..f]
• Possibility to implement short-cuts or user defined characters for 'Esc', 'Num Lock', 'Scroll Lock' and 'F1' - 'F12' keys.

Limitations   [Toc] [Top]

Basically the same limitations as for AT Keyboard Interface V3.xx.

The analog foil-keypad decoding approach deserves dedicated design and calibration: If a key on the keypad is hit, an interrupt is generated to start the A/D conversion. The analog value built by the keypad resistor cascade needs some settling time until stable and reproduceable A/D values can be read out by the PIC microprocessor (overshoots, undershoots).

This means that the PIC microprocessor clock frequency (related to the A/D conversion speed), output drive strength of the keypad resistor cascade and both debounce capacitors on IRQ line and analog value pin affect the proper function of the keypad circuit. Whenever the PIC clock frequency is changed, (slight) adaptations on the analog circuitry may have to be expected.
For calibration of the numeric foil-keypad, please refer to the page 'Numeric Foil-Keypad Calibration V0.04'.

Project Resources   [Toc] [Top]

Available Microchip PIC Assembler Source Code   [Toc] [Top]

Schematics, Data Sheets and Pinout   [Toc] [Top]

AT Keyboard Specification (PDF, 189 kB)

The schematics of the AT Keyboard Box using the PIC16F77:

• Top view: kbd_box_top.pdf      (with outdated 4 MHz crystal, now 14.7456 MHz)
• LCD connection: kbd_box_lcd.pdf
• Numeric foil-keypad: kbd_box_keypad.pdf
• RS232 circuitry: kbd_box_max232.pdf
• Power supply: kbd_box_power.pdf
• Simulation of interrupt generation circuit (positive IRQ pulses): kbd_box_sdf.pdf

You don't know how a dot matrix LCD is working? Have a look at my data sheets page.

Download ASCII Character Map: ASCII-Map.pdf

You can get the description of the various keyboard connectors <here>.
 

User-specific Customization   [Toc] [Top]

For a high level view, please refer to the section 'How it works' above.
Basically the same customization as for AT Keyboard Interface V1.xx applies to this implementation.

If you apply changes to the existing code, you may need to change the ORG directives in order to realign the assembler code properly.

Last updated: 2004/12/26