If at all, then only by a self built circuit. But this is not at all a standard procedure!

Why?
AT89C51 can only be programmed in parallel mode, means where all address lines and data lines are applied. And unfortunately some of these lines, which are used for normal operation are not identical with those, that are used with programming mode: A8-11 is connected to P2.0-3 and D0-7 is connected to P0. That's identical. But A0-7 is connected to P1, which is different! Also, P3.6, P3.7 formerly /WR and /RD lines, and P2.6, P2.7 formerly A14 and A15 lines are now used differently.

So, you would need an extreme amount of additional circuitry to provide this line switching and, not to forget, additional control logic to prevent bus contention, means to prevent the situation, that more than one output driver accesses the same line and eventually fabricates a destroying short circuit!

Well, this methode would be so uncomfortable, that it's never used, at least to my knowledge.

A much better way to program the code memory is either to use a standard parallel programmer, or to use AT89S51, which supports serial programming: You can download the code from PC into flash code memory of AT89S51 via a cable link. Have a look at Atmel's website. Look for datasheet of AT89S51.

By the way: AT89C51 is still available, but 'not recommended for new designs' by the manufacturer! This means, that the AT89C51 will suddenly be no longer available one fine day. This can be next year, next month, but even tomorrow! Then you will have to re-design your board anyway. So, it's better to switch to the AT89S51 (or to another brand, of course) right now.

Kai