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Author Topic: TTL to CMOS level conversion for PPS  (Read 1711 times)

CAG

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TTL to CMOS level conversion for PPS
« on: November 12, 2012, 05:01:55 pm »

Hi,

I want to connect the PPS output of the EM-406A to the clock input of a 74HC390 decade counter operated from a 5V supply rail. Can I use the standard method of interfacing TTL to CMOS logic levels, viz by using a pull-up resistor from the connection to +5V, so preserving the noise immunity of the signal?

Many thanks,
CAG
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GS_TECH01

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Re: TTL to CMOS level conversion for PPS
« Reply #1 on: November 13, 2012, 07:14:51 pm »

Hello CAG,

Welcome to the forums! Just a couple of questions:

Can you please provide some references, datasheet, links, etc. about the 74HC390 decade counter?
Also, what do you want to connect the PPS output to this counter? Can you please further explain?
Please provide more details about your user case scenario.
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CAG

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Re: TTL to CMOS level conversion for PPS
« Reply #2 on: November 18, 2012, 05:28:23 pm »

Hello USG_TECH10,

Thanks for your reply.

A datasheet for the 74HC390 decade counter can be found on the Texas Instruments website at:
http://www.ti.com/lit/ds/symlink/cd74hc390.pdf

My intention is to drive the clock input of one of the divide-by-2 circuits in the '390 from the PPS output of the EM-406A to produce a very precise 0.5Hz square-wave which can be used to control the 1s gate of a frequency counter. From the EM-406A datasheet it seems that the maximum output voltage of the PPS signal (high-state) is 2.85V. The minimum input voltage for the '390 to recognise a high-state signal is, however, around 3.5V for a supply voltage of 5V.

To bridge this gap, a pull-up resistor is usually recommended to be connected at the input of the HCMOS circuit. A reference to this technique can be found on pages 19 to 21 of the TI note on 'HCMOS design considerations' at:
http://www.ti.com/lit/an/scla007a/scla007a.pdf

The question seems to be whether the output of the EM-406A can sink sufficient current to allow this technique to be used.  Do you have any thoughts on this?

Many thanks,
CAG.
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GS_TECH01

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Re: TTL to CMOS level conversion for PPS
« Reply #3 on: November 19, 2012, 01:12:56 pm »

Thanks for providing the links and your user case scenario.

Yes, the maximum voltage rating that can be applied to PPS is 2.85V. Regarding the 3.5V, perhaps, you're referring to the TX and RX pins, the maximum output voltage is 2.85V as well and the maximum input voltage is 3.6V.

I will go ahead and for your inquiries to our engineers. Will keep you posted.
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GS_TECH01

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Re: TTL to CMOS level conversion for PPS
« Reply #4 on: November 21, 2012, 12:06:16 pm »

Hello CAG,

After checking with our engineers, they are not familiar with this decade counter; thus, they're unable to provide you some answers. Moreover, they are currently occupied on some projects and working on them right now.

Please test it out to see if it works and let us know how it goes.
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