DS8921: DS8921 1PPS Reg.

Hi Deva,

So first thing I'd like to note on this device - it is a 5V device - while 3.3V LVTTL inputs should work alright - its output can > 3.3V as this device will need a minimum of a 4.5V supply voltage. There may also be a need for a level translator on the output.

With that being said - I also want to mention that all of our RS-422 exclusive devices (that is only made for RS-422) are all 5V devices. However, RS-485 is backwards compatible with RS-422 and we do have flexible supply options here. Most, if not all, of our RS-422(RS-485) devices will be able to pass a 1ms pulse once per second - the question becomes what is "low skew" in this application ? Do you have a skew budget that is allowed? Finally - do you know if there are any transition time requirements on the PPS signal as well?  

Ultimately - the device you have picked out could work for the application if:

1. You can supply at least 4.5V supply

2. The output can be level shifted back into 3.3V range (it could be closer to TRX VCC) 

3. This part is pretty low skew - in the range of tens of nanoseconds for the system (with our tested specs - longer busses are going to add more skew and jitter) but I'd like to confirm what the skew budget for your application is and how long the bus is anticipated to be. 

However we may also have other viable options if you can't meet the requirements outlined above - but if that is the case if you could provide the skew budget allowed, and any other requirements that you may have not listed (such as transition time / slew rate requirements/ anticipated bus length) and we should be able to find a good part to match.

Please let me know!

Best,

Parker Dodson

Hi Dodson,

Thanks for the above information.

As you mentioned, I have +5V supply in my module so powering wont be an issue. I'm concerned above the Level shifter that i need to add in my output as it may increase my Jitter further.

Low skew/Jitter in my application would be <5ns(lower the better) and It would better if a have a slew rate greater that 0.5V/nS.

1) Is there any RS422 to LVTTL part with low Skew/Jitter and high slew rate?

2) if not, Can you suggest level translator part with low jitter that i could use with DS8921?


It would be grateful, if you could also share other viable option so that i could check if i can work around my requirements.

Thanks in-advance,

Deva

Hi Deva,

1. So the closest device that we have that can work as a RS-422 to LVTTL part without additional translators is the THVD1424. It is a flexible RS-485 transceiver (it's compliant with RS-485 and RS-422 - so RS-422 applications 100% okay - RS-485 is the better interface standard from an electrical perspective) 

Some notes on how it would work:

1. RS-485 to LVTTL - the device has a separate logic supply so it can be directly connected to 3.3V so no translator is needed while getting the most out of the transceiver with its bus supply at 5V - no translator would be needed.

2. It can be configured for full-duplex operation - so it will meet the same configuration needed in your application.

3. With the Slew Rate Control turned off (it is off by default) the device has a through put of up to 20Mbps - its driver slew rate (appx. during transition region) is going to be typically 0.4V / ns but down to ~0.2V/ns  at worst tested case (max temp - RS-485 loading) and its receiver output (logic signal) is typically slightly above 0.5V/ns to slightly below 0.3V/ns at worst case (15pF loading) 

4. The biggest issue I see is the range of skew possible - typically the driver will add 1ns of skew (maxing out at 3.5ns for tested conditions) and the receiver can add a max of 4ns of slew - so around 7.5ns worst case without considering jitter/skew added by bus (which generally is going to greatly add to skew as length increases) 

5. It does include integrated terminations - they are disabled by default but could be utilized to reduce BOM size.

6. A slightly unrelated benefit is that this part can be utilized in a wide array of differential wired communication (RS-422/485) applications because of its flexibility of use in many different applications. 

This part also has the benefit of still being a smaller form factor - its in a 3mm x 3mm package - so it shouldn't be a large increase for a solution size here. 

Another possible option is the THVD1451 - its a simpler device than the THVD1424 - and has some tradeoffs with solutions.

1. It has a wide supply range - and can run on a 3.3V supply for bus and I/O - so this part can work at LVTTL voltage levels - however the tradeoff is that all the published specs (typical) use a 5V supply - so typical performance will most likely be worse than what we have listed (but should still conform to min/max specs at 3.3V) 

2. Its a full-duplex device and shares the same pinout as the DS device and is backwards compatible with RS-422

3. Its a 50Mbps device and has an appx. driver slew rate typically > 1V/ns and worst case slightly > 0.5V/ns (with RS-485 load) and the receiver has the same worst case (with 15pF load)

4. It is max 3.5ns on both the skew on the driver and receiver outputs - so a system addition of 7ns of skew without considering the bus effects on jitter (this is going to be a problem for every device)

You probably won't get much better on the skew than what is listed above for RS-485/RS-422 - the DS device will have a similar amount of skew intrinsically added to the system. 

Please let me know if one of these options may be able to work - if not I will hand this thread over to the level translation team as their expertise is going to be more valuable for a level translator selection than mine, but I do think it could be possible to work with one of the suggestions above - and with the parts listed above being newer the amount of information and support we offer is generally at a higher level as we have more tools to help - but the DS device is still valid.

Best,

Parker Dodson

Hi Parker,

Thanks for your valuable insight.

Parker Dodson said:

The biggest issue I see is the range of skew possible - typically the driver will add 1ns of skew (maxing out at 3.5ns for tested conditions) and the receiver can add a max of 4ns of slew - so around 7.5ns worst case without considering jitter/skew added by bus (which generally is going to

Here you seem to have taken the data from 6.9 Switching Characteristics_20 Mbps of Data sheet in which SLR pin is GNDed. But as i am going to use low speed (1Hz of varying duty cycle), Shouldn't i consider 6.8 Switching Characteristics_500 kbps of Datasheet in which SLR pin is pulled high?

Considering the above case, The Tr/Tf time for 6.8 Switching Characteristics_500 kbps seems to be large which will reduce my slew rate.

Thanks in-advance,

Deva