A Security Lock Byte offers
 protection of the EPROM program
 memory from access (reads, writes, or erases)
 by unprotected code or the C2 interface.
 Bits 3–0  of SLB - Read Lock: 
 Clearing any of these bits to logic 
 0 prevents all code memory from being
 read across the C2 interface.
 Bits  7–4  of SLB  Write Lock:
 Clearing any of these bits to logic 0
 prevents all code memory from being
 written across the C2 interface.
 
Important Note: Once the security byte
 has been written, there are no means 
 of unlocking the device.
 Locking memory from write access 
 should be performed only after
 all other code has been 
 successfully programmed to memory.