Nuts & Volts - February 2006

oscillator.

A fuse bit programming facility is not included in the AVRHVP-1 programmer since there is not enough program memory to support this. For this reason, another special programmer AVRHVP-2 has been developed, which handles only fuse bit programming, and as well, writing the “lock bits” to hide the binary code functions.

Listing 2 shows the software. To generate the accurate 440 Hz frequency tone, an infinite loop of 1,136 μS high and low voltage is alternately applied to the speaker (1,136 μS is the half period corresponding to 440 Hz.)

The key routine here is the DL1132 μS delay routine, but every other instruction counts as well. For example, setting pin PB1 high or low takes 2 μS each, and the call overhead to DL1132 also takes 2 μS. Adding them all up gives us exactly 1,136 μS.

With such meticulous calculations, the circuit output tone is exactly 440.0 Hz when measured by a frequency meter. As a comparison, if you don’t re-program the fuse bits and use the built-in RC oscillator, the frequency meter will show quite different readings, such as 495.7 Hz.

PARTS LIST

➦ C1 — 100 μF 16 volt radial electrolytic capacitor

➦ C2 — 10 μF 16 volt radial electrolytic capacitor

➦ C3, C4 — 0.1 μF ceramic capacitor

➦ R1 — 210Ω 1/8 watt metal film precision resistor, 1% tolerance ➦ R2 — 1.82K, 1/8 watt metal film precision resistor, 1% tolerance ➦ R3, R4, R6 — 10K, 1/4 watt carbon resistor ➦ R5 — 330Ω 1/4 watt carbon resistor ➦ U1 — RS-232 transceiver DS275 from Dallas Semiconductor ➦ U2 — AT89C4051 Flash microcontroller (programmed) ➦ Q1 — PNP transistor 2N2907 ➦ Q1 — NPN transistor 2N3904 ➦ BR1 — Bridge rectifier DF01 ➦ VR1 — 317LZ voltage regulator ➦ VR2 — 78L05 voltage regulator ➦ D1 — Schottky diode 1N5817 ➦ LED1 — General-purpose light emitting diode ➦ J1 — Power jack ➦ J2 — DB9F connector ➦ CR1 — 3. 58 MHz ceramic resonator ➦ Miscellaneous: Eight-pin machined socket, 9VAC or 12.5VDC power supply, etc.