Board Descriptions

68HC11EVB This is the Motorola 68HC11 EVB with +12 volts wired out to pin 59 and with -12 volts wired out to pin 57, on J1.
INTERFACE BOARD This board interfaces to the other four boards and the keypad and the three BNC connectors.

The output of the 741 op-amp drives the voltage tuned analog filter. The SPI bus drives the 74HC595, the DAC-08, and the 741 op-amp. Sending "0E" followed by "0F" to port B causes the 74HC595 to latch data.

The 74HC138 selects which device is driven by the SPI bus. Writing a "02" followed by a "03" to port B causes data to be latched into the square wave synthesizer. Writing a "0C" followed by a "0D" to port B causes data to be latched into the digitalker.

Six transistors compose the circuit that sets the amplitude of the square wave output to either 5 volts or 12 volts.

Relays K1, k2, K3, and K4 select the filter mode. Relays K5 and K6 select the gain of the filter. Relays K7 and K8 select whether the filter is driven externally or internally.

DIGITALKER BOARD This is a standard digitalker circuit. Words are stored in ROM and accessed by sending a 9 bit word to the MM54104 speech processor. An interrupt line (pin 6) on the MM54104 lets the program know when digitalker is finished speaking a word.
SQUARE WAVE FREQUENCY SYNTHESIZER BOARD This board produces square waves from 1 Hz to 1 MHz. It uses a 1 KHz reference frequency derived from a 1 MHz CMOS oscillator. A 4046 PLL chip produces an output frequency of 100KHz to 1 MHz. The two 74HC595 IC's preload the 74HC192 counters with the number to be synthesized. A divide chain of five 74C90 IC's provide a selectable frequency of 1 Hz to 1 MHz.

Bandswitching of capacitors is done from the A0, A1, and A2 lines on the synthesizer board.

The filter will only work up to 100 KHz even though the square wave synthesizer can be set up to 1 MHz.

FILTER BOARD The filter board consists of a two pole state-variable filter. Bandswitching is done by switching capacitors. Tuning the filter over any particular decade is accomplished from lookup voltages stored in EEPROM driving the VCF pin through the D/A converter on the interface baord.
FREQUENCY COUNTER BOARD The frequency counter baord also contains the circuitry for the inductance and capacitance testers. The frequency counter consists of a FET input with a divide-by-100 prescaler. The prescaler is activated only for input frequencies greater than 100 KHz and feeds directly into the plse accumulator input on the 68HC11. Each overflow of the pulse accumulator is equivalent to 256 input pulses. Output compare 5 is used to precisely control the measuring time to 1 second.

The analog portion of the capacitance tester and the inductor tester are fully described on separate pages. These circuits use a capacitance to frequency converter and an inductance to frequency converter. One cycle of the output of the CA3100T op-amp is used to gate pulses from a 1 MHz oscillator into the pulse accumulator input. Thus the pulse accumulator count will exactly equal the value of the capacitance (or inductance) under test.

Summary of 68HC11 features used in this project

Calibration Tables Digitalker words are stored in EEPROM.
To Tailor Features of the End Product Level 2 keypad configuration contains features which may be selected and stored in EPROM.
Program Patches Words stored in EEPROM for digitalker can be changed. Level 2 keypad interrupt service routine can be intercepted at EEPROM location B620 to change the definition of a key.
For Self-Adaptation to Changes Calibration routine adjusts lookup voltages in EEPROM locations B701 to B751 for any changes in components due to aging, replacement of a component, power supply variation, etc.
Low Power Use of wall transformer and power supply demonstrates value of HCMOS for low power.
A/D The A/D is used in the adaptive calibration circuit to monitor the output of the filter and changes the EEPROM table accordingly.
SPI The synthesizer board, filter board, frequency counter board, and digitalker board are connected to the 68HC11 by the SPI bus.
Input Capture Input capture 3 is used by the digitalker to tell the 68HC11 when digitalker is finished speaking a word.
Real Time Interrupt The keypad uses the real time interrupt.
COP Watchdog COP is used. Memory 103F is set to 09 to activate COP. COP vectors to C000 if timer times out.
Output Capture Output capture 5 is used to configure a time of exactly 1 second for use in counting frequency.
Pulse Accumulator The frequency counter, capacitance tester, and inductor tester use the pulse accumulator as the primary input.

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