mahmood Elnasser wrote:
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why don't chip manufacturers include a decoupling capacitor between VCC and ground on the chip itself


The answer to this is two fold.....if the manufacturer could provide for this capacitor on chip I can assure you that they would do so....but here is the rub....

1) It is not possible to build capacitors with sufficient capacitance into a piece of silicon and still keep chips as small as possible. A capacitor built on a hunk of silicon tends to be a one layer capacitor whereas the types that are made for us to buy and use have many many layers permitting huge equivalent surface area.

2) If the manufacturer was able to package the part with a capacitor (such as molding an 0603 ceramic multi layer part) right into the plastic of the IC then that bypass would be applicable to a narrow range of possible applications for the ICs. Different PC boards, layouts, frequencies, loading, power sources, adjacent parts etc etc etc all have an effect on the type and size of ideal bypass capacitor.

Note there are very many parts that have a requirement for multiple bypass capacitors these days. The better versions of these parts, such as the Cygnal C8051F124, in surface mount packages put the power and ground pins on the chip and correspondingly on the package lead frame on the shorter lead frame legs. It is wise to connect the bypass capacitors to the package with the shortest possible copper length and place them so that there is a VDD-copper-CAP-copper-GND path without interruption due to via holes. The following picture shows a suggested bypass technique for a C8051F124 in a TQFP-100 package with 0603 sized bypass capacitors on a multilayer PC board with internal power and ground planes.



Michael Karas