The design of a microcomputer-controlled automotive AM/FM radio is discussed. The system features, and gives priorty to, a clock display. However, the 3-1/2-digit LED display can also serve as a 4-year calendar, elapsed-time counter, or frequency channel, on request. The tuning process automatically activates the frequency display.
Basic aspects of real-time clocks and frequency measurements in microcomputer systems are discussed, as is the ability of the COSMAC processor to permit a software-generated clock to be obtained with low cost and high accuracy.
While the ignition switch turns the display on or off, the CPU and clock run continuously. The use of CMOS circuitry in the system keeps the power drain below 5 milliamperes and assures months of system operation, even when the vehicle (battery) is not used.