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TCA8418: Debounce time specifications clarification

Rajesh Kumar72
Intellectual 500 points
Part Number: TCA8418
Other Parts Discussed in Thread: LM8330, LM8333

Hi,

We are planning to use TCA8418 to connect a 4 x 4 membrane keypad matrix switches.

With respect to debounce logic implementation, we have following observations:

1. First page of the device datasheet under feature section "Integrated Debounce Time of 50 μs" is specified, whereas in section 6.9 Keypad Switching Characteristics, maximum debounce time is mentioned as 60ms. So what is difference in these two debounce timing specs. please clarify?

2. As of now, we don't have the final specs of 4x4 membrane keypad switches, so in general, can we able to detect the membrane keypad switch cleanly if their debounce time is less than 60ms? In general, do you have idea what is the debounce time required for the membrane switches?

3. Do you have any other keypad scan IC to interface membrane matrix switch with adjustable/programmable debounce time? 

4. Can we use external discrete components, usually a lo pass filter combination of R & C to increase the debounce time of the TCA8418?

Thanks & Regards,

Rajesh Kumar

  • 0
    Guru 243980 points

    The 50 µs value refers to the interrupt deassertion (see sections 8.3.5.1 and 8.6.2.1).

    I've seen membrane switches with specified worst-case bouncing of 5, 15, or 20 ms.

  • 0 in reply to Clemens Ladisch
    Intellectual 500 points

    Hi Clemens Ladisch,

    Thanks for the clarifications.

    So in that case we shall not have any issue in using TCA8418. We require couple of clarifications as below before we finalize on TCA8418:

    1. Can we use remaining rows and columns of TCA8418 as GPI and read the event. Like in our case, we will be using only 4 rows and 4 columns, so we want to use remaining rows and columns as additional GPI purpose.

    2.   We have requirement of interfacing a 7 switch keys apart from 4x4 matrix keypad switches. So basically we have total of 16 +7 = 23 switches to be connected to TCA8418. We would like arrange both types of keys/switches in 5 x 5 matrix configuration. In that case, we would like you to review our keys arrangement and connections to the TCA8418. Could you please do it offline, as our schematic is confidential.

    Best Regards,

    Rajesh

  • 0 in reply to Rajesh Kumar72
    Guru 243980 points

    1. Yes; see sections 8.3.1.1/2.

    2. As long as the switches connect a row pin with a column pin, it will work.

  • 0 in reply to Rajesh Kumar72
    TI__Mastermind 22835 points

    Rajesh,

    Rajesh Kumar72 said:

    Do you have any other keypad scan IC to interface membrane matrix switch with adjustable/programmable debounce time?

    Take a look at LM8330 and LM8333 and see if they fit your needs.

    Rajesh Kumar72 said:

    In that case, we would like you to review our keys arrangement and connections to the TCA8418. Could you please do it offline, as our schematic is confidential.

    I'd love to take a look at the schematic and give feedback. You can reach me via email at d-bacic@ti.com (or by clicking my E2E name here), and we can work offline.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    Intellectual 500 points

    Thanks Danny.

    We will share you the keypad interface circuit offline for your review by next week.

    Regards,

    Rajesh Kumar

  • 0 in reply to Clemens Ladisch
    Intellectual 500 points

    Hi Clemens Ladisch,

    Thanks for the clarifications. I have gone through the datasheets and found section 8.3.1.1 really useful. We will get it reviewed by Danny in couple of weeks once circuit is ready.

    1. Regarding below paragraph under section 8.6.2.15

    "The debounce time for the keypad is for the columns only. The minimum time is 25 ms. All columns are scanned once every 25 ms to detect any key presses. Two full scans are required to see if any keys were pressed. If the first scan is done just after a key press, it will take 25 ms to detect the key press. If the first scan is down much later than the key press, it will take 40 ms to detect a key press."

     Trying to understand the keypad scan algorithm and we think that 40ms = 25ms + 15ms, i.e., if we press the second key after a gap of 15ms, then TCA8418 will take 40ms to detect that key press. Or, does it mean that worst case key press detection is 40ms? Could you please elaborate that how does it take 40ms?

    2. Also referring to section 6.9.1 - Keypad Switching Characteristics:

    What to you mean by max debounce of 60ms? Does it mean by the keypad switch bounce time shall not be more than 60ms to be detected? Otherway, what is the minimum debounce time assured by the TCA8418 for successful key press detection. This understanding is required to select a suitable keypad switches. Request you to elaborate on debounce specs of the TCA8418.

    Best Regards,

    Rajesh Kumar

  • 0 in reply to Rajesh Kumar72
    Guru 243980 points

    I think that 40 ms should be 50 ms. After the key is pressed, a random time between 0 ms and 25 ms elapses until the first scan happens, then 25 ms later the second scan.

    The specification in section 6.9.1 says that the TCA8418 can handle any bounce time up to 60 ms.

  • 0 in reply to Rajesh Kumar72
    Intellectual 500 points

    Thanks for all the clarifications.

    We will go ahead with implementation and get it reviewed by Danny.

    Best Regards,

    Rajesh Kumar