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How to connect unused LF pins?

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Newbie level 2
Jul 21, 2015
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I designed TPMS sensor on freescale FXTH87 and I don't want to use LF receiver, so I have to configure LF pins to avoid a floating input. What I have to do? leave pin not connected or connect to Vss, or something else?

from datasheet:
Each LF[A:B] pin will always have an impedance of approximately 500 kOhm to VSS due to the LFR input circuitry. The LFA/LFB
pins are used by the LFR when the LFEN control bit is set and are not functional when the LFEN control bit is clear.

Really appreciate your help.

Just to have a guess... I couldn't see the datasheet as you have to register and sign in, and I probably wouldn't understand it anyway, but I noticed there are different models of the FXTH87, might help to let people know which specific one you're using if that is important.

Anyway, just to have a really clueless guess, if it says "The LFA/LFB pins are used by the LFR when the LFEN control bit is set and are not functional when the LFEN control bit is clear."
- Does it mean that if LFEN is 1 (high/set?) they are functional, but if it is 0 (low/clear?) they are in a high impedance state, so their state 1/0 is unimportant, what matters is the LFEN state? It sounds like LFA/LFB are dependant on LFR which is dependant on LFEN. Like a bilateral switch/enable pin function?


The main problem is that device works from battery. So need design a circuit and avoid excessive supply current. Of course I disable LFEN, but what happen on PIN? I don't know. Maybe better connect pins to ground or leave floating input. If it will be a common gpio CMOS input like on microcontrollers, I know what to do, but here a specific input pin, and I just don't know better way.

I attached datasheet.View attachment FXTH870xD.pdf


Thanks for uploading the datasheet. Don't consider my opinion an informed one, please, just want to help if I can.

From your last post, and - if this applies to LFA and LFB - reading what it says in section 6.1, I'd opt for configuration #2 (not connected) "2. Configure the general purpose I/O pin as an input (PTxDDn = 0) with the internal pullup activated (PTxPEn = 1) and leave the pin disconnected."

Figure 66 on page 96 makes it look like when LFEN is disabled it won't interact with anything, if this is so then I assume it won't affect LFA/LFB (which I think are high impedance pins anyway, so not understanding much, don't see how they would draw current if left floating).

Are there no Freescale simulation tools to capture what happens on those pins in different states and connection configurations? Not that that would necessarily give you an accurate answer anyway.

My clueless answer would be to leave them floating - if they are high impedance they won't know the difference between floating and connected anyway, and if floating LFA and LFB have no path through which to drain the battery; or use config. #1 in 6.1 if you see mistakes in my reasoning.

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