Kai:
I have stated before on this forum that the IDLE and PWRDN modes of various 8051/8052 derivative microcontrollers do not all work the same. Anf for this reason each of us must study our data sheets carefully.

If the processor you are using has a feature to allow wakeup from a power down via an Interrupt pin then I'll have to agree with you that my comment in the previous post is not going to work in that instance.

However there are two other factors to consider....

1) All of the 8051 flavors I have worked with only can use the INTR/ pins to leave a reduced power mode if that mode is the IDLE mode. I think I can safely say that virtually all of the derivatves define the IDLE mode as one where the oscillator is still running. And on many of these parts the IDLE mode can be exited by dragging the INTR/ line to a low level. I have done it just that way many a time.

2) If the specific processor we are talking about here is indeed one that can leave the PWRDN mode via an INTR/ input and that mode has, as is generally the case, the oscilaltor stopped. Then the resume activity will happen as you say. However once again, for a keypad application, there is no need to insert the one-shot component between the four input gate and the INTR/ pin. The processor wakes up on the switch press, the oscillator starts and the then normal operation resumes as the user takes their finger off the button.

3) The only use I can see that is legitimate for use of the one-shot would be in those cases where the processor is one of the types where the power down mode is exited via the application of a reset pulse. In these designs it is necessary to pulse the reset but at the same time allow the normal reset generator chip to do its thing. The use of the one-shot multivibrator can be useful in making the necessary pulse on the reset line. However I must say I have worked on an number of designs where the wakeup result pulse was simply coupled onto the reset line via a fairly good sized capacitor.

Michael Karas