I would like to set a (fuse?) bit to select the external Flash/PEROM as program memory instead of default Flash internal to the AT89C51. Is the BLJB bit the one I want to set? How do I set it?
The BLJB bit is in the Hardware Security Byte; ergo, you must use a programer option to set it. Is that not clear? No, it is not clear. Elsewhere in the data sheet, an API call is listed to program both the BLJB fuse, and the HSB ! See Table 76 of doc4235. So it may be possible that this can be accomplished with FLIP.