Hi Ricardo,

the standard pinout of 80c51 in DIP40 package has some disadvantages:

The DIP40 package is huge and the GND and VCC pins sit at opposite sides of the package. Even if you place the decoupling cap directly at the VCC pin and use a solid ground plane for the connection to the GND pin, you will result in huge ground-bounce and supply-bounce, because the die is far away from the package pins and huge voltage spikes will drop accross the inductances of lead frame. This ground-bounce and supply-bounce is superimposed to all I/O signals of 80C51.

In the standard DIP40 pinout the XTAL pins of internal Pierce oscillator are located next to the GND pin and are also far away from the die. This will increase the ground-bounce.

Ground-bounce and supply-bounce can drastically reduce the noise margins. And it will cause lots of radiation, the more the longer the port lines are!

So, if you plan a mixed analog digital application or an application with radio waves (Bluetooth, WLAN, ...) I would recommend the use of a much smaller package. TQFP44, e.g., is smaller in all dimensions by a factor of 4 compared to DIP40! And by the same factor ground-bounce and supply-bounce is smaller, too. Radiation is even still much smaller!

With the TQFP44 package you can easily use lower supply voltages, because you will have enough noise margin. You can easily use higher clock rates, because ground-bounce and supply-bounce are reduced. And you can fully profit from the benefits of a multilayer board with massive ground plane, because the die sits much closer to the PCB and the lead frame connections are much shorter. Your application will produce much less unwanted radiation and will be much more immune to external radiation and noise.

Kai