In S series the main difference is that you can program the "S" verison through ISP (in application) or using very simple programmers, while the "C" version still needs "full" parallel programmer . (ISP stands for In System Programming).
89s51 has more new feature than 89c51 such as
- ISP programming ( In system programming).
- Watch dog timer.
- Dual Data Pointer(2 Data Pointers).
-cheaper than c51.
Pin function: Pins almost identical changes were, in the AT89S51 in P1.5, P1.6, P1.7 have a secondfunction, that is, the second 3-pin serial ISP function formed programming interface.
Voltage in the programming area: AT89C51 in addition to normal programming voltage of 5V, another12v is needed for Vpp. But AT89S51 just need to be 4-5V.
Endurance: AT89C51 flash memory can be programmed upto 1000 times while the AT89S51 can beprogrammed from 1000-10000 times.
Frequency: AT89C51 limits the operating frequency is 24MHZ, and AT89S51 maximum operatingfrequency is 33MHZ, (AT89S51 chip in two models, to support the maximum operating frequency of 24MHZ and 33MHZ) in order to have a faster computing speed.
Power Supply: AT89S51 operating voltage range is up to 4-5.5V, while the AT89C51 at the end of the 4.8V and 5.3V.