Numeric Foil-Keypad Calibration V0.04

with LCD Display for Microchip PIC16F77

Table of Contents [Toc]

Concept
   How the numeric foil-keypad works
   Specifications
   Parts order information
Project Resources
   Available Microchip PIC Assembler Source Code
   Schematics, Data Sheets, Pinout

Concept [Toc] [Top]

This application has entirely the same setup as the AT Keyboard Box V2.05.

This program refers to the calibration of the numeric foil-keypad of the AT Keyboard Box. The analog foil-keypad decoding approach deserves dedicated design and calibration: If a key on the foil-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). For a detailed description, please refer to the section 'How the numeric foil-keypad works' and the schematics below.

Numeric foil-keypad
Connection topology: 1x12

AT Keyboard Box V0.04 - Debug and Calibration
Debug and calibration for numeric foil-keypad on PIC16F77. Dedicated output on the second line: Acquired A/D value from numeric foil-keypad and corresponding symbol from look-up table (# has been hit).

*AT Keyboard Box V0.04 setup* with numeric foil-keypad, dot matrix LCD display and AT keyboard		*Interrupt generator* for numeric foil-keypad: Whenever a key is hit, a key-specific analog voltage is put on the first line to the A/D converter. At the same time an interrupt is generated by this comparator circuit and put on the second IRQ line.

*AT Keyboard Box V0.01* (initial version with PIC16C74A)		*Front view* Microchip PIC16C74A microcontroller and piezo-beeper on the left side.

*Screen shot of the HyperTerminal Program* showing the start-up message of the AT Keyboard Box V0.04 and some keyboard and keypad entries

How the numeric foil-keypad works [Toc] [Top]

Here is 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.

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

Specifications [Toc] [Top]

Processor:	PIC16F77 (PIC16C74A)
Clock Frequency:	14.745600 MHz (HS mode)
Throughput:	3.7 MIPS
RS232 Baud Rate:	Customizable by user (BRGH = 0), any setting from 1200 baud - 115200 baud
Serial Output:	default setup: 9600 baud, 8 bit, no parity, 1 stopbit
Keyboard Routine Features:	Capability of bi-directional communication between controller and keyboard
Numeric Keypad Features:	Interrupt-based acquisition, direct 8 bit A/D conversion
Acquisition Methodology:	Preemptive, interrupt-based keyboard scan pattern acquisition, decoding to ASCII characters during normal operation mode activated by ready flag (including LCD display and RS232 activities)
Code Size of entire Program:	1231 instruction words
Required Hardware:	AT keyboard, PS/2 connector, MAX232 level shifter, dedicated foil-keypad, LM393 comparator circuity for interrupt generation, HD44780 compatible dot matrix LCD (2x16, 2x20 or 2x40 characters)
Optional Hardware:	Piezo beeper with decoupling capacitor
Required Software:	RS232 terminal software (or Excel 97 RS232 Debug Interface)

Parts order information [Toc] [Top]

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

Project Resources [Toc] [Top]

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

Main File	Main Keyboard Decode Lookup Table	SHIFT Keyboard Decode Lookup Table	HEX Files
Debug and calibration version for numeric foil-keypad V0.04: box_dbg.asm	English 'codepage' (QWERTY) View: eng_main.html Download: eng_main.asm	English 'codepage' (QWERTY) View: eng_shif.html Download: eng_shif.asm	QWERTY 'codepage': box_dbg_eng.hex QWERTZ 'codepage': box_dbg_sg.hex
	Modified Swiss German 'codepage' (QWERTZ) View: ger_main.html Download: ger_main.asm	Modified Swiss German 'codepage' (QWERTZ) View: ger_shif.html Download: ger_shif.asm
The above programs need additional include files (modules) to get successfully assembled: m_bank.asm, m_wait.asm, m_lcd_bf.asm Important: Due to bi-directional communication between controller and keyboard as well as between controller and LCD display, the above programs only work if both components are connected and are working properly!

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>.

Last updated: 2005/06/06

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