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  • Taximeter Using MC68HC908JL3

  • Created: Dec 18, 2015

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Description

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Summary

The design above is a taximeter based on the MC68HC908JL3 microcontroller (MCU). A taximeter is an electronic device installed on taxicabs used to calculate the fare of the passenger based on the travelled distance. This design is based on the NOM-007-SCFI-1997 regulation, which defines the taximeter regulations in Mexico and similarly all over Latin America.


The circuit is made to be compact and thin for it to be manufactured as a very portable electronic device. It features a six 7-segment display, five push buttons, a wheel turn indicator and a programming interface. The display segments are connected through the Port B of the MCU to display data. These display segments are powered through the Port A with the use of NPN transistors to supply the needed amount of current for the display to achieve 350uCD per segment. Also through Port A, the five push buttons are read. The push buttons are connected to port A before the transistors that lead to the common anode of the displays. The push buttons have pull-down resistors, so the microcontroller must enable its internal pull-up resistors while reading the push buttons. As the cab is running, the wheel turn indicator sends a pulse to the IRQ pin to measure the distance travelled for the calculation of the fare. To avoid high-frequency noise that could cause multiple IRQ requests within one revolution, a passive low-pass filter is included.


The circuit is made to be low-cost and open for development. Through the MON08 (JP2) interface, the microcontroller can be programmed. Therefore, this reference design can also be used as a development board. An expansion port in Port D (JP3) is also reserved for custom applications.