HCS12C32 SCI inferface with GPS Module

[Deadstar]

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

[mculater]

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

[Deadstar]

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

[bigmac]

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

[Deadstar]

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