Use a PC's parallel port to program a clock source

This Design Idea shows how you can use Linear Technology's LTC6903 programmable oscillator as a clock source for direct-digital synthesis, data conversion, switched-capacitor filtering, clock, and voltage-controlled oscillator circuits. The LTC6903 operates from 2.7 to 5.5V with modest power consumption and can produce clock signals at frequencies of 1 kHz to 68 MHz. Typical frequency error and resolution over the range are 1.1 and 0.1%, respectively.

You can control the programmable oscillator circuit in Figure 1 via an IBM-compatible PC's parallel port, which also provides power to the circuit. Resistors R₁ and R₂ limit power-supply current drawn from parallel-port data bits DB₃ and DB₄, and resistors R₃ through R₅ isolate programming bits DB₀ through DB₂. A precision micropower voltage reference, IC₁, provides 4.096V of stable power to IC₁ and IC₂. For optimal performance, minimize the lead lengths of bypass capacitors C₁ and C₂ with respect to IC₂'s power and ground connections. High-speed buffer IC₃ isolates IC₂'s output and prevents frequency pulling due to load variations. Listing 1 translates a user-supplied input into a 16 bit, SPI-compatible data stream that programs IC₁'s output frequency. The LTC6903's output frequency depends on two control coefficients, OCT and DAC. The program derives the closest values for OCT and

DAC by solving the equation: f=(2^OCT)×2078/(2–(DAC)/1024). At initial application of power, IC₂'s output frequency defaults to 1.039 kHz.

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