Maxim Integrated High-Speed Microcontroller - Manual (Page 153)

High-Speed Microcontroller User’s Guide
Rev: 062210 153 of 176
13. TIMED-ACCESS PROTECTION
The 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 opening 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
bit. This section explains which bits are protected, why, and how to use the timed-access feature.
13.1 Protected Bits
Bits that are protected by the timed-access feature are shown below. Only critical function bits that are
unique to the high-speed microcontroller family are protected, assuring code compatibility with the
original 80C51 or 80C52. A full description of the function of each bit is provided in Section
4 .
EXIF .0 BGS Bandgap Select
WDCON .6 POR Power-On Reset Flag
WDCON .1 EWT Watchdog Reset Enable
WDCON .0 RWT Reset Watchdog Timer
WDCON .3 WDIF Watchdog Interrupt Flag
TRIM .7 E4K 4096Hz RTC Output
TRIM .6 X12/ 6 12pF/6pF Crystal Select
TRIM .5 TRM2 Capacitance Trim Bit 2
TRIM .4 TRM2 Inverse Capacitance Trim Bit 2
TRIM .3 TRM1 Capacitance Trim Bit 1
TRIM .2 TRM1 Inverse Capacitance Trim Bit 1
TRIM .1 TRM0 Capacitance Trim Bit 0
TRIM .0 TRM0 Inverse Capacitance Trim Bit 0
ROMSIZE .2 RMS2 ROM Size Select Bit 2
ROMSIZE .1 RMS1 ROM Size Select Bit 1
ROMSIZE .0 RMS0 ROM Size Select Bit 0
RTCC .2 RTCWE RTC Write Enable
RTCC .0 RTCE RTC Enable
13.2 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 will open for three machine cycles. During this window, the software
may modify a protected bit. The suggested code to open a timed-access window is:
MOV 0C7h, #0AAh
MOV 0C7h, #55h
The procedure to modify a timed-accessprotected 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 machine
cycles of writing AAh. This opens a three-machine cycle window, after the write of 55h, during which
any timed-access protected bits may be modified. Failure to complete any of the required steps will also
require 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 will be ignored. This is regardless of