Dear TI Team,
we are using an ADS7846 in one of our products.
We have observed a number of units exhibiting 'sticky' button performance, where the soft buttons on the touch screen appear to get stuck down.
Further investigation has shown that the N_PENIRQ line ( which is pulled up to the +3V3 rail with a 100k resistor ) appears to 'snap' low on a touch, interrupting the microcontroller. We then see a sequence of 4 x SPI reads, which occur every 10ms to recover the X, Y and Z data as defined by our code, superimposed on the N_PENIRQ line.
However, when the touch is removed, we are seeing a slow exponential rise of the N_PENIRQ line back up to the +3V3 rail, not a sharp rise as expected: The apparent 'time constant’ on the machines tested has shown a range of times from 8ms to 25ms. The longer of these times is apparently what is causing us a problem, since the microcontroller sees the N_PENIRQ line as still being asserted and is therefore recalling the interrupt routine, hence, the code gets stuck in a loop…
Of course we can code around this / reduce the value of the pull up resistor ( which has been shown to shorten the time constant ), however, these solutions are not practical for the installed base.
Before we do anything, I would welcome an explanation as to why, what I would assume to be a digital signal, seems to exhibit analogue behaviour… Also, might I recommend that something about this is included in the datasheet, to make designers aware of this behaviour.
Many Thanks,
Stewart