In SX Microcontrollers, SX/B Compiler and SX-Key Tool, SteveW wrote:

>I also don't agree that you need 4 bypass caps for the SX48/52.... sounds like an overkill.

Err, your board reboots unpredictably... I can't quite understand your logic here. 

And yes, you absolutely do need to feed power and ground into all the pins.
If you've got access to a decent oscilloscope, you might like to have a look at the voltage across one of your 'unused' pairs of power/gnd pins. I bet you see horrendous noise - which is what that side of the chip is trying to use as a power supply.
Yes, routing power and ground to all the pins does take effort. However, it's not optional, as you've discovered.

If you think about it, the chip can't generate electricity from nowhere - if there's (say) half an ohm resistance (and inductance) on the power suply pin, if only 8 outputs switch simultaneously, delivering 30mA transiently into a capacitive load (a very likely scenario), then the power pin has to supply 8*30mA through half an ohm. That's 120mV drop, and is pretty much best case. The transient currents will actually be rather more than 30mA, and this is just a rough calculation of the power requirements from switching 8 pins - there's a lot more going on than that.

If you look at a modern CPU, you'll find hundreds of power and ground pins. The micro I'm currently working with has 357 pins, of which 57 are power and 60 are ground. They're certainly not there for my layout convenience :)

Steve

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