pulldownpulluprs485tvs
I'm Using a THVD1451 for full-duplex RS-485 communication.
In Datasheet, for transient protection against surge transients, it's recommended to use the circuit below:
In this circuit, there is a pull-up resistor for 'receiver output' (R) pin, and also a pull-down resistor for 'driver enable' (DE) pin.
I have two issues here.
First, why there is no push-up/down resistor for 'driver input' (D) pin?
And the second one is that how to choose between pull-down and pull-up resistors for such pins?
This answer is not STM32 specific but is based on experience and many such discussions over many (many) years. Others can add to this - it covers the main points (I think) but may not be complete.
It's encouraging to see someone asking these simple but fundamental questions and showing an awareness how such "little things" can 'gang aglae' in real life.
ie "If the micro does not initialise properly ..." really reads " ... when the micro does not initialise properly ..." :-) - and it's obvious that you realise this.
So:
Use of external pullup or pulldown is essential for those really keen on getting a well defined result. This is the single biggest must-do here. All the rest is a bonus. ie Setting to inputs with internal pullxxx is a compromise which will almost always work.
BUT if "almost always" is not good enough for your design then you need external pull xxxs.
Pullup or down does not seem to have an overwhelming better result. It may vary between ICs but can be determined from the data sheet. All things being equal (as they may be) I'd favour pull-down as there is a potential for lower leakage currents to device external circuitry - but this is liable to be minimal in a conformally coated PCB and/or a benign environment.
You may wish to look at startup action if you really care. eg a pulled up pin will start low and transit high at some stage. A pulled down pin will probably stay low throughout. This is probably not important but is mentioned for completeness.
ESD susceptibility will be device specific, quite likely symmetric and on average over many processors probably favours pull down as drivers tend to sink better than source if asymmetric. If you care a lot about ESD then you may wish to use low outputs with pull downs - as a a low impedance path will (probably) offer better ESD protection. But if you care a lot about ESD you will want to design for it in other ways and not rely on in-IC protection as your main protection.
Re question 3 - external pullxxxs is desirable but it seems safe to use values which are at the limiting high end of proper design and then use internal xxx's in parallel if desired. However, as internal pull xxxs often have a 2:1 Reffective you can get largest R and smallest current by using external only. What you of course want to avoid is external pull ups and internal pull downs or vice bersa - but that's unlikely to be an issue.
When I say " ... limiting high end of proper design ... " I mean just that and not "past the limiting ...". ie the pin will have a specified value of resistance which allows the worst case Vin spec to be met. A larger resistor may take less current in the resistor but may start to very slightly turn on the internal switch. ie it may be that there is an Rpulldown_current versus lowest overall current tradeoff as the internal driver starts to see leakage current (which will be extremely small) increasing the current to the dirver and whispering it on very slightly.
If you use eg pulldown you may then find it lower power to set the pin to output and drive it low, but this is an option that can be decided on in due course.
Almost an aside - NEVER allow protection diodes to handle "any significant currents" at any stage during operation. Allowing them to do so can lead to totally inexplicable processor action. The less the current the lower the chance of things gong wrong - and the harder to find it when they do.
As per the schematic provided, if all 3 inputs were sharing a resistor, then any of the lines being pulled high via the switch would raise all 3 lines to high, countering the purpose of the design - the MCU would not know which switch position is selected.
A common way of reducing part count, not resistor count, for such designs is to use a common bus resistor network or array:
(from here)
These are available as through-hole SIP/DIP as well as SMD, in a variety of resistor counts, depending on your needs. The bus pin is connected to ground, and the other pins are connected to the respective MCU inputs as in your schematic.
(from here)
(from here)
Best Answer
D input has no resistor, because data input will be irrelevant if driver is disabled.
You will select pull-up or pull-down based on what is the preferred default action, for example when MCU is booting up and has not initialized yet. So one option is to have the default state to disable transmitter, but the other option is to enable the transmitter, and the bus will be jammed, and since the D input has no resistor, the bus will be jammed with random garbage.
The 1451 is full duplex and has no enable pins, so you don't need to worry about it.
The other devices that do have the enable pins do have internal 2 Mohm resistors to keep driver and receiver disabled, but that is still extremely high value and thus external resistors are recommended.
The resistance value is selected so that it is low enough to keep the pin at a defined voltage with all the leakage, and high enough not to consume too much current so that the output can still drive voltages high/low enough even with the resistor. Also sometimes it might be important how fast the resistor pulls to default state if some event happens that the output gets to high impedance state.