you do not state which derivative you use and most flash based derivatives do, in fact require a supervisor chip to work "forever"

The capacitor based reset shown in your download rely on some internal resistance, check the datasheet for your derivative to see if an external resistor is required if you absolutotally have to not use a supervisor.

Whoever posted the circuit does nor read datasheets, the capacitors on the MAX232 are all wrong.

Also, why on earth use an external oscillator, all derivatives has a perfectly good (not true for some Arghmels, I have read) oscillator inside and just require a crystal and a couple of capacitors.

The problem with downloading schematics you do not understand from the internat is that you are forced to use the posters (possibly bad) choices.

If you are "blank" on hardware, buy a devboard, there are some very cheap with NXP LPCs, and some excellent with SILabs chips. I will, to any beginner, recommend a SILabd devboard (if your budget is zero, look at a SILabs toolstick [<$25]) since the built in "ICE" gives you the opportunity to "look under the hood"

Erik