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SMS Controller, Part 2

Second article tells you how to complete the circuit checks and describes how the unit is used.

By Peter Smith

Having carried out the power supply checks described last month in Part 1, the next step is to check out the serial interface and the microcontroller.

First, disconnect the power and insert IC1 and IC3 into their sockets. If the microcontroller needs to be programmed, then you should do that next. Refer to the "Microcontroller Programming" panel for more details on this.

Click for larger image
On the 3310 model, the serial interface is accessible through a hole in the rear of the case, underneath the battery. The data cable is terminated with a plastic head assembly that includes a set of spring-loaded contacts as well as tabs to retain the battery that it partly displaces.

Next, install jumper shunts on JP4–JP7. These should always be in place when the inputs (IN1 – IN4) are not connected, otherwise the micro’s port pins will be "floating" in an indeterminate logic state.

Conversely, remove all jumper shunts from JP1–JP3 if installed earlier and apply power. The "Comms Error" LED (LED1) should illuminate, while all other LEDs (except the "Power" LED) should be off. This indicates that the micro cannot communicate with the phone, which of course isn’t connected yet. However, it does tell us that the micro is doing what it should.

Note: the very first time you apply power, all red LEDs may come on for one second and then go out, with just the "Comms Error" LED remaining on. This sequence indicates that the micro has automatically erased its on-board EEPROM, ready for programming.

If you get a different result, the problem is most likely due to one or more pins of the micro having missed their sockets and bent underneath the chip.

SMS Controller Kit

This kit is available here at AutoSpeed

If bent pins aren’t the problem, then check out the oscillator circuit, consisting of crystal X1 and the two 22pF capacitors. If you have access to an oscilloscope, you can observe the operation of the oscillator on pin 18. In addition, check the voltage on the micro’s RESET input (pin 9). This pin should measure close to +5V during normal operation, going low only during power up and power down.

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