HCS12C32 SCI inferface with GPS Module

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HCS12C32 SCI inferface with GPS Module

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Deadstar
Contributor I
Hello,

I am interfacing with a GPS module which requires a LVTTL, 4800 baud, no parity, one stop bit signal.

LVTTL level is 0-3.5V. I have two questions:

First, do I need to boost this voltage range to 0-5V to interface.

A problem I have is that when I connect the Tx of the GPS module to the SCI Rx on the C32, the output jumps from pulses between 0-3.5V to a steady 4V with the pulses with a peak to peak of about 400mV on top of this. Do I need some sort of circuitry to correct this?

Thanks,

Colin
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mculater
Contributor III

Colin, the 9S12C32 is capable of operating at 3.3V, so if you could do that, it would solve your problem.  Otherwise, you'll likely need some sort of level-translation circuit.  It could be as simple as a transistor plus a couple of resistors...

Best regards,
Carl Barnes

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Deadstar
Contributor I
Thanks for the reply.

I have tried running the C32 at 3.5V but I still suffer from the same problem where the pulses appear 'on top' of a constant voltage when the SCI RX is connected. It is like the GPS module cannot pull the line down to 0V.

Do you understand what I mean? I will try and get some pictures tomorrow.

Thanks,

Colin
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bigmac
Specialist III
Hello Colin,

Is there any possibility that there is an external pull-up resistor on the MCU pin?  If so, the symptoms would indicate that it may be substantially too low in value, perhaps a component error on the board.

What level of sink current can the GPS unit provide at its output pin?

Regards,
Mac

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Deadstar
Contributor I
Mac,

Thanks for the advice. I cut the traces between the MCU and the on-board DB9 connector and voila it works.

Thanks a lot; these solutions always seem simple one you know about them.

Colin
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