Maxim Integrated Ultra-High-Speed Flash Microcontroller - Manual (Page 150)

13-2
Ultra-High-Speed Flash
Microcontroller User’s Guide
SECTION 13: TIMED-ACCESS PROTECTION
The ultra-high-speed microcontroller uses a protection feature called timed access to prevent accidental writes to critical SFR bits. These
bits could cause a system failure or prevent the watchdog timer from doing its job if improperly written. The timed access involves open-
ing a timing window during which the protected bit can be modified. If the window is opened correctly, it remains open long enough to
alter one protected register. This section explains which bits are protected, why, and how to use the timed-access feature.
Protected Bits
Bits that are protected by the timed-access feature are shown below. Only critical function bits unique to the ultra-high-speed micro-
controller are protected, ensuring code compatibility with the original 80C51 or 80C52. A full description of the function of each bit is
provided in Section 4:
WDCON.0 RWT Reset watchdog timer
WDCON.1 EWT Watchdog reset enable
WDCON.3 WDIF Watchdog interrupt flag
WDCON.6 POR Power-on reset flag
EXIF.0 BGS Bandgap select
ACON.5 PAGES0 Page mode select bit 0
ACON.6 PAGES1 Page mode select bit 1
ACON.7 PAGEE Page mode enable
ROMSIZE.0 RMS0 Program memory size select bit 0
ROMSIZE.1 RMS1 Program memory size select bit 1
ROMSIZE.2 RMS2 Program memory size select bit 2
ROMSIZE.3 RMS3 Program RAM enable
FCNTL.0 FC0 Flash command bit 0
FCNTL.1 FC1 Flash command bit 1
FCNTL.2 FC2 Flash command bit 2
FCNTL.3 FC3 Flash command bit 3
Protection Scheme
Each bit mentioned above is protected against an accidental write by requiring the software to perform a procedure before writing the
bit. Timed access requires the software to write two specific values to the timed-access register during two consecutive instruction
cycles. The values AAh, then 55h, must be written in consecutive instructions to the TA register at SFR location C7h. If the writes are
performed correctly, the write-access window opens for three memory cycles. During this window, the software may modify a protect-
ed bit. The suggested code to open a timed-access window is:
MOV 0C7h, #0AAh
MOV 0C7h, #55h
The procedure to modify a timed-access-protected bit begins by writing the value AAh to the timed-access register (TA;C7h). The value
55h must then be written to the timed-access register within three memory cycles of writing AAh. This opens a three memory-cycle win-
dow, after the write of 55h, during which any timed-access protected bits may be modified. Failure to complete any of the required
steps also requires the procedure to begin again, starting with the write of AAh to the timed-access register. Attempts to modify timed-
access-protected bits after the window has closed are ignored. This is regardless of whether any bits were modified. Figure 13-1 illus-
trates a number of examples of correct and incorrect use of the timed-access procedure.
Maxim Integrated