Portable devices exist that utilize mechanical tuning for radio frequency (RF) receivers, such as mechanically tuned radios for tuning and receiving AM and FM terrestrial audio broadcast channels. Many prior mechanically tuned radios utilize mechanical tuning wheels where one or more local oscillators and/or an analog filter are directly adjusted by the mechanical tuning mechanism. This tuning wheel, therefore, is directly used to select a tuning frequency for the mechanically tuned radio.
Another type of mechanically tuned radio that has been considered is one in which an analog value associated with a mechanical tuning mechanism is digitized and then used to select a tuning frequency for the mechanically tuned radio. Such a digitally-controlled mechanically-tuned radio is described in U.S. patent application Ser. No. 12/231,184, entitled “MECHANICAL TUNING OF A RADIO,” and filed Aug. 29, 2008, which is hereby incorporated by reference in its entirety. As described therein, detection of the setting for the mechanical tuning mechanism is achieved by receiving an analog signal from the mechanical tuning mechanism and then digitizing this analog signal within a receiver integrated circuit. The resulting digital value is then used to select the tuning frequency for the radio. As such, the mechanical tuning mechanism indirectly selects the tuning frequency for the radio.
While this use of an analog-to-digital converter within a receiver integrated circuit is a viable solution, it is also desirable to detect a setting for a mechanical tuning mechanism without utilizing such an analog-to-digital converter.
Some prior devices have used input pins for an integrated circuit to detect rise time associated with multiple selectable resistors in combination with a capacitor. For example, a user can select a button from 3 to 4 buttons that are each associated with a different resistor value. The selected resistor value becomes part of a completed circuit with the capacitor. Further, the resistor values are typically selected to have a non-linear, geometric relationship. In operation, a voltage rise time associated with this circuit is used to determine which button the user has selected. Unfortunately, this technique typically provides about 2-3 bits of precision. This level of precision is far below the 8-bits of precision needed for a mechanically tuned radio solution.