Hallo Amigo (real name?),

power supply for 89C51 system is not so difficult to realize. Just a normal 5V regulator is enough in most cases. But some details are important:

- A good car battery is allowed to go down to about 8V at engine start. So a normal 5V regulator is ok in simple cases. When you need stable 5V even in situations when battery voltages is lower, a low dropout regulator like LM2941 is needed.

- Good decoupling is essential. Just low dropout regulator are very sensitive to poor decoupling! Besides standard decoupling capacitors arround regulator a good low pass filter at input of regulator is neccessary. Take two (or more) BL02RN2 ferrite beads (each about 2.2µH) in combination with 100µF/35V aluminium electrolytic for achieving a corner frequency of about 8kHz.

- Overvoltage protection is needed. Take a transzorb diode like SMBJ16 across electrolytic in combination with additional fuse.

- Voltage regulator should be switched to car battery via relais (ignition switch controlled), because even small idle current can make the battery loose its charge.

- Extremely important is the use of good reset controller. Then undervoltage situations or dips cannot interfere microcontrollers operation. Take e.g. MAX1232.

- Good grounding management and use of screended cables is also important.Try to configure a complete Faraday cage for sensible microcontroller circuit. All cables must leave microcontroller module at only one entry, a small hole in metallic back plane (high frequency backplane). All cable screens and system ground have to be connected to this 'point'. Try to position all cables as next as possible to metallic chassis or motor parts. Especially spark plug back currents should see an as short as possible trace back to ignition driver. Big current loops must be avoided!

- Keep in mind that temperature can be quite high in engine environment. Use chips designed for automotive temperature range (-40°...+85°) and have good cooling (metalic contact of enclosure to chassis).

Bye,
Kai