Maxim Integrated Secure Microcontroller - Manual (Page 51)

Secure Microcontroller User’s Guide
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5. MEMORY INTERCONNECT
The secure microcontrollers are composed of microprocessors and modules. This section illustrates the
memory interconnect for the various chips and shows block diagrams of selected modules. The soft
microprocessor chips are 80-pin QFP packages that connect to a low-power CMOS SRAM. When using a
chip, the user must connect the bytewide bus to the RAM as shown in this section. In module form, the
bus is connected inside the package.
Preferred RAMs are those with the lowest possible data retention currents for the chosen memory
configuration. Note that data retention lifetime increases as RAM data retention current decreases and
battery size/capacity increases. The laws of physics decree that data retention currents can vary greatly
with temperature; be sure to select a device that meets the required data retention current over the
expected temperature range of the application. This is covered in detail in Section
6 . In general, system
designers should carefully scrutinize the SRAM data sheet to ensure the memory device meets the
specifications.
In the case of the DS5000FP, the microprocessor can connect to either one or two SRAMs. They can be
8kB or 32kB, though the case of two 8kB RAMs could be prohibitively expensive.
Figure 5-1 illustrates
the memory connection of a DS5000FP connected to one 32kB x 8.
CE1 provides the chip select, and
R/ W supplies the WE signal. A second RAM could be added by simply using
CE2 as the chip enable
with a common connection for the other signals.
In the case of DS5000-based modules including DS5000(T) and DS2250T, the SRAM is connected as
described above. Connections running between the microprocessor and RAM are not available at the pins.
The DS2250-64 has a second SRAM on
CE2 . The when present, the real-time clock is connected to CE2 .
Figure 5-2 shows the module configuration with 32kB RAM and an RTC. This is identical for DS2250 or
DS5000 modules, which differ only in form factor.
Figure 5-1. Memory Interconnect of the DS5000FP