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MONITORING THE REVOLUTIONS

A Pedal Cadence Counter
by Matt Meerian

Start ý The Hardware ý Microcontroller Software ý Desktop Software ý Placement ý Happy Trails ý Sources and PDF

THE HARDWARE

The hardware is centered on the Atmel 90S2313 microcontroller, making the schematic almost a one-chip circuit (see Figure 1). For those of you who are not familiar with the microcontroller, it has a UART, an 8-bit timer/counter, a 16-bit timer/counter, 128 bytes of EEPROM, 32 general-purpose registers, 2 KB of flash memory, and several more internal features.

Plug JP1 is an in-circuit programming header. In-circuit programming makes updating software to the flash memory easy. JP1 mates with a programming dongle that comes with the STK200 development board. The desktop software that accompanies the dongle is available on Atmelýs web site.

The peripherals around the ý2313 consist of a Hall effect sensor, a LED, a MAX233 level converter, a power switch, a mode switch, and of course, a crystal for the microcontroller. A magnet on the shank of the pedal triggers the Hall effect sensor when the pedals revolve around once.

An output pin on the ý2313 can deliver 20 mA of current, which is enough to drive the LED. The LED blinks approximately once every second to allow you to see that the microcontroller is in Data-Gathering mode.

The MAX233 level converter chip converts the 0- to 5-V level of the microcontroller to the 10 and ý10-V levels for RS-232 serial communication. The DS1233, available from Dallas Semiconductor, holds the processor in reset if the power supply drops below 4.5 V. And, the power supply is a basic 7805 linear regulator. A fuse or polyfuse is a good idea when using power from a battery.

The power switch turns on the microcontroller. The mode switch allows the microcontroller to either gather data (take a survey) or upload to a desktop computer. Note that the mode can be changed only during powerup of the microcontroller. If the controller resets while gathering data, the contents of the EEPROM will not be erased.

The Hall effect sensor has an open collector output. The pull-up resister in the microcontroller is enabled. I wanted a little stronger pull-up than the internal resister, so I used a 39-kilohm resister in parallel with the internal pull-up.

The crystal frequency is 1.8432 MHz. The reason for this unusual value is that it is easily divisible to allow timer interrupts at one minute.

You donýt need a fast clock because not much is going on for the microcontroller.

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