Design Ideas: March 1, 1996

The advantage of in-system programming is to provide software updates or changes in system configurations without any component changes. Figure 1 shows an approach, which includes the 8051 µC, programmable memory device (EEPROM, or flash memory, for example), RS-232C interface, and PAL. The PAL provides proper control signals to the programmable device in both run and program modes. In run mode (S₁ is open), the EA input of the µC is at logic "0," and the PAL routes the PSEN signal to the memory device's RD input. The controller fetches code from the external memory device and executes.

In program mode (with S₁ grounded), the EA input is at logic "1." The PAL routes the read and write signals from the µC to the programmable device's RD and WR inputs, respectively. The µC executes instructions from its internal ROM. The ROM contains bootload program code that receives data from the RS-232C interface and programs the programmable memory device. Listing 1 shows the PAL equations. (DI #1834)

Listing 1 —PAL equations for in-system programming