MON51: Using With Devices With External Program Lock Bits

Information in this article applies to:

• C51 All Versions

SYMPTOMS

I have a problem with Monitor-51 on an Atmel AT89C51CC01 board. I have installed the Monitor according the instructions in Application Note 152: Installing and Using the Keil Monitor-51.

When starting the debug session, the µVision debugger connects to the target but instead of the version number I see only letters displayed. The version number is missing!

My application seems to be downloaded. However, I do not see the correct hex values for my code in the memory window. After I try any code execution command (step or run) the debugger loses communication with the hardware. What can be be reason for this behavior?

CAUSE

The Atmel T89C51CC01 (and several other Atmel parts like the T89C51RD2, T89C51CC02, and T89C51AC01) have security bits that disable external code accesses.

RESOLUTION

You must make sure that the three lock bits in the HARDWARE SECURITY BYTE are inactive (cleared). They must be cleared with a parallel programmer before the chip can execute code from external memory. The lock bits of the HARDWARE SECURITY BYTE cannot be modified with ISP software like the Atmel FLIP utility.

SEE ALSO

• Application Note 152: Installing and Using the Keil Monitor-51

Last Reviewed: Thursday, February 25, 2021