digital-logiclevelmicrocontroller
Convention?
Easier to implement?
Another reason?
Is there a reason things like MCLR or RESET on microcontrollers are active-low, that is, you have to pull them down to reset the IC, and pull them up to "run" the IC.
I'm just curious because this causes me some problems. If it were active high, I could avoid the capacitor on MCLR required in some instances and deal with just a pull-down resistor. It seems only to add to complexity.
Essentially, either works; A is probably better, B is more flexible.
A will drive the reset line to '1' faster, which could be an issue on a very large board.
B allows the option of adding other means of driving reset - hidden switch + paperclip, brownout monitor, watchdog timer, etc. It probably doesn't even cost the board space for a resistor, because a lot of microcontrollers have software configurable pullup resistors on each pin.
Your circuit needs to be started in a known state, and if the default is to be with the 4013's Q low and both 4017s' Q0s high, then to that end I've taken the liberty of redrawing your circuit, below, to include the Power-On-Reset using diodes to make it compatible with the rest of your diode logic.
Note that your R5 isn't needed since the diodes are being driven by a CMOS totem pole which never floats.
Just for fun I've also added in some arbitrary clocks - to make it run - and, below, included a version of your circuit - with the same functionality but simplified by eliminating all the diodes and their associated resistors and replacing them with CD4071 OR gates. The LTspice files needed to play with the circuits or run simulations, if you want to, are here
Best Answer
Look at what happens during power-up: Vcc rises to a point where it's high enough to make everything work properly. However, that point isn't clearly defined and may vary from device to device. It makes sense not to use this voltage to reset the controller.
It's easy, however, to keep a level low regardless of Vcc. After all, Reset is already active the instant you switch power on, since at that moment everything is at a low level.
edit
The graph below illustrates how the output voltage of the reset controller (i.c. an MC34064) remains low until Vcc is high enough to have the complete microcontroller stable.