Maxim Integrated Secure Microcontroller - Manual (Page 22)

Secure Microcontroller User’s Guide
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When the partition is at 3000h and the range at 32kB, program memory below 3000h is accessed on the
bytewide bus. Program memory at or above 3000h is directed to the expanded bus or Ports 0 and 2. When
the partition is at 5800h and the range at 32K, data memory at 0000h is accessed on Ports 0 and 2. Data
memory at 6000h is located in NV RAM on the bytewide bus. When the partition is at 1000h and the
range at 8kB, all memory access above 1FFFh is on the expanded bus. The partition rules apply when the
range is below 8kB.
Important Application Note
The MCON register is an SFR unique to Maxim microcontrollers that contains nonvolatile memory
configuration information. This register should be set to the desired value before loading the device via
the bootstrap loader. Failure to correctly configure the MCON register can cause the device to operate
incorrectly, including symptoms that appear similar to a defective device. Because this register is
nonvolatile, incorrect memory settings will be preserved when power is removed. The DS5001FP,
DS5002FP, DS2251T, and DS2252T store additional memory configuration information in the RPCTL
register, which should also be set to the desired value before loading the user program via the bootstrap
loader.
Figure 4-3 illustrates the typical operation. There are two conditions that can modify this memory map.
The first is the EA pin. The second is the security lock. When the EA pin is grounded, the DS5000 forces
all memory access to the expanded bus. This causes the DS5000 to behave like an 8031, regardless of the
partition, range, or ECE2. The
EA should be pulled to +5V for normal operation. The second modifier is
the security lock. When set, the security lock prevents the bootstrap loader from reading the contents of
the NV RAM. For security purposes, it also prohibits program memory access on the expanded bus.
Thus, all program fetches must be restricted to the bytewide bus when locked. The security lock overrides
the condition of the
EA pin as well.
These memory map controls provide unprecedented flexibility to configure a system. However, it is
possible to select contradictory settings. The partitioning function allows a user to select the quantity of
program and data memory. It is possible to select all data and no program in NV RAM by choosing a
partition of 0000h. This is a valid selection, except when the security lock is set, as it simultaneously
configures and prohibits the use of program memory on the expanded bus. In this illegal event, special
circuits will automatically force the partition to a location of 7FFFh. This means all 32kB memory on the
bytewide bus is designated program memory. The second contradictory case is to select a range of 8kB,
and to choose a partition of greater than 8kB. This results in the range as the limiting factor. Addresses
above the range are automatically deflected to the expanded bus. No data memory is allocated in NV
RAM for this configuration.