LM1815: pin 10 is not ensured to drive TTL load

Jonathan Navor

Part Number: LM1815

Hi Team,

We need to ask your assistance to check our customer's schematic design.

Customer's inquiry below:

Regarding LM1815 IC, I have some doubts
In page no. 9 of datasheet, it is given that pin 10 is not ensured to drive TTL load.
In our design pin 10 of LM1815 is directly connected to Input capture pin of MCU.

See attached schematic design

We are looking forward for your response. Thank you in advance for the support.

Best Regards,

Jonathan

over 4 years ago

0 Josh Wyatt over 4 years ago

TI__Guru* 91206 points

Jonathan -

is the MCU you are using TTL or CMOS?

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Hi watt,

MCU we are using is STM32439ZIT6 in which all I/Os are both TTL and CMOS compatible.

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

OK - so then you are all set then, correct? page 139 (?)

https://www.st.com/resource/en/datasheet/stm32f437vg.pdf

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Josh,

So that pin should not be connected to MCU directly?

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

Santhosh -

I think normally when MCU says they are TTL and CMOS compatible, then it should be OK. The TTL is a higher level of drive vs. CMOS, so that means the MCU should be able to handle the lower thresholds, which are ratios to the operating voltage vs TTL which is usually in the +5VDC range. Since the MCU GPIO is being used as input here and that MCU claims support for both levels, this is probably not an issue.

in that MCU datasheet, on page 137, this note is given, which I think gives 100% confidence for you.

All I/Os are CMOS and TTL compliant (no software configuration required). Their characteristics cover more than the strict CMOS-technology or TTL parameters. The coverage of these requirements for FT I/Os is shown in Figure 35.

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Josh,

Thanks for your answer.

I have one more doubt. The output I am getting at pin no. 10 of LM1815 is ranging from -V to +V. But TTL or CMOS device doesn't support this level right?

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

Santhosh -

There are CMOS technologies that support up to 18VDC - that is not what differentiates CMOS and TTL, however - it is the thresholds of what they consider to be high or low signals. Here you are operating the part from +3.3VDC, which is commonly supported range of MCU - and also, if you still have questions about the input capabilities of that MCU, I would kindly suggest you ask STM.

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Ok Josh.

But can i make something to get output signal voltage ranges from 0 to +V for an Input sine wave signal ranges from -V to +V with the circuit i have attached?

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

Santhosh -

Perhaps - I would recommend that you arrange your circuit the way that it is shown in the datasheet.

For example, it appears you want to use mode 3, but you used 10k pull down instead of grounding directly. With that pin grounded, then the arm threshold will be set to 0V and positive crossing will arm the IC, the next negative crossing triggers it.

Next, the one shot timer does need RC, which you have pads for, but need to populate components and the reference pulse out pin needs a pullup and this is the signal that you are looking to use from the part. That pin does not look to be connected to anything in your schematic.

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Josh,

Thanks for your suggestion. I will arrange my circuit as you suggested. After testing, I will come back to you.

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Hi Josh,

1. I have removed 10k pull down resistor and directly connected Mode pin to ground.

2. As we don't require RC timing, We connected it to ground as suggested in the datasheet.

3. Since we are not using pin 12 Reference pulse out and it is output pin. we just left it floating.

No i tested the circuit again, but its output is same i.e. square wave with voltage range -V to +V.

Please let me know if it is possible to get 0 to +V pulse output by using LM1815 with sine wave input.

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

Santhosh -

do you have a capture of this on o'scope that you could share?

0 Santhosh Kumar S over 4 years ago in reply to Josh Wyatt

Intellectual 500 points

Hi Josh,

I got my output pulse. It was because i had selected AC coupling in oscilloscope. When i changed it to DC coupling, I got positive pulse.

Thanks for your support

0 Josh Wyatt over 4 years ago in reply to Santhosh Kumar S

TI__Guru* 91206 points

Santhosh -

Great news! Glad to help out.

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LM1815: pin 10 is not ensured to drive TTL load