Frequency Meter and Pulse Generator with AT902313

Power input is to a 7805 5 volt regulator. A pair of LEDs is connected between the 5 volt supply and ground, with current limiting resistors in series and one pin on the AT90S2313 shunts the current through one LED or the other. When one LED is on, the device is `ready`, that is not generating pulses on the output and not measuring input frequency. When the other LED is on, the devise is actively generating pulses or measuring frequency. As a consequence, one of the LEDs is on at a time allowing quick visual confirmation that power is applied and the state of the device. The serial interface is the AT90S2313's internal UART connected to the outside world via RS-232 using a Maxim MAX232 interface chip, which derives RS-232 drive voltages from the 5 volt supply.
Frequency Meter and Pulse Generator with AT902313 - schematic

The output of the pulse generator is via a resistor ladder network. I chose the values I did so that I would have a few low-amplitudes to select from and still be able to drive 5 volt CMOS with the maximum level pulses. If I were to do it over, I would probably just use an R-2R ladder network. Pin 9 doubles as an input and output pin, thus it is connected as the MSB of the ladder network so that it can be exposed to the full amplitude input signal (without attenuation). To provide a measure of protection against electrostatic discharge and clumsy handling of test leads, a reverse biased emitter-base junction of a bipolar transistor is used. The junction will avalanche when the voltage exceeds at high positive voltages and enters forward conduction when the voltage swings negative. The collector of the transistor is left floating. I trimmed the frequency meter timing loops by measuring the output of a frequency reference I made with a 74HCT4060 and a 4 MHz crystal and a 74HCT4040 12 stage divider. The fifth harmonic of the '4060 was zero beat with NIST station WWV by padding the capacitance from the crystal terminals to ground. The frequency meter was consistently within two counts of the output of frequency reference that was being measured. You can build this without needing a printed circuit board, though it you really are anxious to, it sure would look neat. Layout isn't really critical, except that its a good idea to...

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