Help: First Robot Project - Motor Driver Breadboard Circuit

Hi

OK here's a challenge for you blokes!

This is my first robot/uC project. I am studying electrotechnology as a
student, but only at an early stage, so I don't have the skills to solve
this problem ( yet ? ).


Problem: fault finding electrical motor driver circuit connection/s on a
breadboard.

[ Now this problem probably needs me to take a photo of the circuitry,
or draw a diagram, but I thought ( vainly ) that it may be a grounding
problem or similar with my unused/floating pins/ports on the uC, so I
haven't done so until necessary. ]

Anyway, the first stage of my first robot project is trivial ( for
experienced 'bot heads ), but not for me - drive two small motors that
drive a Tamiya twin-motor gearbox using a Texas Instruments L293DNE
Half H-bridge driver using PWM modulation via two PWM modules on a
PIC16F876A.

OK, here's the thing. When the PIC is on a demo board ( Mikroelektronika
EasyPIC 2 ), and I use the set of IO pins for Port C and run jumper
wires ( including an Earth from breadboard to demo board Earth Port C IO
) from the demo board onto my breadboard circuit, the motors run
according to my test software OK no probs. NOTE: if I remove the Earth
off the demo board the motors stop, so I guess it's a
grounding/connection issue, because we KNOW the demo board is configured OK.

HOWEVER, when I place the PIC ( and crystal ) onto the breadboard
circuit ( minus jumpers from demo board, of course ), guess what
happens? - no movement in the motors. All unused pins are floating,
except MCLR, which is tied LO to Earth via 470 R resistor.

I have tried trouble shooting myself, thoroughly, and at one stage I got
the motors running by changing their power connections, but I think
there was too much noise ( everything on one power bus ), because the
motors were not following their program, so I changed to two power
buses, added some more caps, and now the motors work fine - but only
when the PIC is on it's ( wired properly ) demo board. I have done many
Google searches for photos of similar breadboard circuit also to attempt
to resolve the issue by myself.

I have tried tying all unused pins LO ( or was it HI ? or both ? -
confusing, isn't it with so many variables for a newbie ) using resistors.

*** Does Port A need special settings because it has AD/DA modules? ***

If anyone is game ( crazy ) enough to respond, your help is much
appreciated.

I plan to document my project on my website eventually to help other
beginner 'bot heads with similar problems.

|-]

Cheers

Dale