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Motorola
DATA LINE PROTECTION DEVICE
Motorola Inc 03/01/01
United States
English (United States)
DATA LINE PROTECTION DEVICE
By David Lochrin and Cathal Gallagher
Background:
Data was being sent from an option board to a radio through a level shifting circuit. A 3.3V high on the option board was converted to a 5V high on the radio. At -30C for many hours, the level shifting circuit did not convert the 3.3V option board high to the 5V high on the radio. By changing a resistor value in the level-shifting circuit, the input level-shifter voltage level was decreased to get the 5V output. This enabled the level-shifter to consistently convert the input 3.3V to 5V at the output (at -30C). When data is not sent, there is a default 1.6V at the input of the level shifting circuit. By decreasing the input voltage level to achieve 5V at the output, the default 1.6V voltage was causing a 5V output at +60C. This caused a Radio EEPROM error. The radio was reading/writing to the EEPROM at this time.
Solution:
The solution is to isolate common data lines, used in two different devices, from each other. In the above example, the option board microprocessor knows when it is sending data to the radio microprocessor. Connect the 5V output of the level-shifting circuit to a bipolar collector. The emitter goes to the radio data line. The bipolar transistor is controlled by 3V at the base. The 3V at the base is controlled by the option board microprocessor. When data is being sent, this transistor switch is closed by the option board microprocessor and high level data flows to the radio. When there is no data sent, this transistor switch is open and no high level data is sent. So in the example above if the default 1.6V at the input of the level shifting circuit caused a 5V high at the output, it would not flow to the radio and collide with radio data lines.
Conclusion:
Implementing this solution involves an additional transistor, two bias resistors and software control. It ensures definite operation of the level-shifter at -30C and also at +60C. This solution offers software control of the transistor via the microprocessor and thus software protection of the radio data lines from the option board data line over temperature extremes.
[See accompanying PDF for drawing]
