III.5 The BLINK Program

When BOOT-51 is customized and blown into an EPROM, and the target board is connected to the host PC, it would be nice to verify, whether the whole configuration is working together correctly. This can be done with a short test program that performs a well-defined action when started. For this purpose the tiny assembly program blink.a51 has been provided. It simply toggles the logic level of a port pin (roughly) once a second (great if connected to a LED) to verify, whether all the components in the chain

are working together correctly.
It can be adapted to your requirements with little efforts:

1. If your target system carries a LED that can be switched with a bit-addressable port pin, please change the BIT symbol LEDPIN accordingly. Then the program will make the LED blink.
    
   If your LED can only be switched with a non-bit-addressable port pin (say bit 2 of a port P6), simply replace the statement   CPL LEDPIN  by   XRL P6,#00000100B  or something like that.
    
2. Change the program start address START to the location, where user programs are usually loaded on your target system.
   The program code itself is position-independent!

If there is no LED on your target system, connect the port pin to a volt-meter. This may also do. Aside of P0 and P2 you may use every port with LEDs or spare outputs. Now the command

boot blink

should reset the MCS-51 board, assemble the test program, upload it to the target system, and finally start it.
If the LED is blinking, everything is now perfectly installed and ready for daily work. If not, you should read the next chapter!