GPS is TTL and host is active low CMOS, will it work?

GPS is TTL and host is active low CMOS, will it work?#121451

By blambi - Sun Feb 27, 2011 5:26 pm

- Sun Feb 27, 2011 5:26 pm #121451

I have a host system that has UART lines that are active low CMOS 0-2.7v. But my GPS is TTL. The GPS datasheet says it is a 3.3v input with UART (TTL). My host is powered by a 3.3v input also. It does work, I hook them up and my device can read data from the GPS. I can't tell if sending commands to the GPS is working or not.

My problem is that after some time (a few days of testing), the GPS stops working. I'm on my second GPS and it has stopped working also. Am I damaging the GPS by hooking up TTL to CMOS?

I'm using the D2523T GPS, http://www.sparkfun.com/products/9566

Re: GPS is TTL and host is active low CMOS, will it work?#121498

By itikhonov - Mon Feb 28, 2011 5:58 am

- Mon Feb 28, 2011 5:58 am #121498

Try adding pull up resistor on the line from TTL to CMOS.

http://www.interfacebus.com/Design_Translation.html#c

this example is for 5v but i believe the same apply to low voltage versions.

Re: GPS is TTL and host is active low CMOS, will it work?#121514

By blambi - Mon Feb 28, 2011 9:07 am

- Mon Feb 28, 2011 9:07 am #121514

I'm going to use one of these instead:

http://www.sparkfun.com/products/8745

But I don't see how this could damage the GPS. Especially since the GPS is 3.3v and the CMOS is 2.7v. I'm just trying to figure out if I'm damaging the GPS units or if I'm just having bad luck with them.

Re: GPS is TTL and host is active low CMOS, will it work?#121520

By itikhonov - Mon Feb 28, 2011 9:43 am

- Mon Feb 28, 2011 9:43 am #121520

blambi wrote:I'm going to use one of these instead:

http://www.sparkfun.com/products/8745

Probably this won't work. You said that there is 3.3v on both sides. Equally powered ttl have troubles to talk to cmos because they use different levels do represent things in this 3.3v band. Pull-up resistor shifts ttl level into cmos domain.

blambi wrote: But I don't see how this could damage the GPS. Especially since the GPS is 3.3v and the CMOS is 2.7v. I'm just trying to figure out if I'm damaging the GPS units or if I'm just having bad luck with them.

Firstly i thought that it was due to heating chip shifted ttl output voltage levels. But since you don't mention gps working again after cool down may be it's initial burn-out (like fresh cars have different characteristics during first 3000 miles).

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