System Address Mapping
Élan™SC520 Microcontroller User’s Manual
4-15
4.3.5.2.1
Noncacheable, Write-Protected, or Nonexecutable SDRAM Regions
In the default condition, the entire SDRAM region is cacheable and executable by the CPU,
and read/writable by the CPU, PCI bus master, and GP bus DMA controller cycles. There
may be some system configurations in which specific portions of SDRAM require restricted
access which can be accomplished by enabling specific attributes. A few common examples
follow:
■
An SDRAM region that contains only code can be marked as write-protected with an
attribute in the PAR register. This prevents the CPU and any bus master from illegally
writing over the code in SDRAM due to faulty programming. In addition, an interrupt can
be generated to the CPU when a violation occurs to assist in debug of the illegal write
condition.
■
An SDRAM region that contains only data can be marked as nonexecutable with an
attribute in the PAR register. If a software task attempts to branch to that location and
resume execution due to a software bug, the CPU will read an illegal opcode, forcing
an exception. The exception handler will then facilitate debugging the program that
caused the illegal condition.
4.3.5.3
Configuring GP Bus Peripheral Space
Configuring space for GP bus peripherals is accomplished via PAR register programming.
This section describes a few system configuration examples beyond the normal
programming of chip select regions.
4.3.5.3.1
Configuring a Chip Select for Noncontiguous Memory or I/O Space
Some peripheral subsystems may require a
single chip select that must be asserted in
noncontiguous address locations. For example, an I/O device can contain multiple
integrated functions that are each addressed at separate, noncontiguous I/O addresses
(such as a custom ASIC). In this case multiple PAR registers can be used to define each
individual address region, but all can be mapped to the same chip select by programming
the TARGET field to GP bus and the ATTR field to the same chip select. This is most useful
when working with a highly fragmented I/O map such as defined in PC/AT systems, where
there is little unused I/O address space.
This can also be accomplished by programming a
single PAR register to cover the entire
range of addresses, which results in some wasted address space.
4.3.5.3.2
Positive Decoding Example
Some peripherals connected to the GP bus may perform their own address decoding from
the GP bus addresses and do not require a chip select. In this case, the same steps are
followed for programming the configuration registers, but the pin multiplexing registers do
not need to be programmed to allow the actual chip select to be driven on a pin, thus
allowing the pin to be used for other functions.
If multiple positive decoding regions are required in an application, the PAR registers for
each reason can be programmed to map to the same unused chip select, to conserve pin
functions.
4.3.5.3.3
Configuring the Élan™SC520 Microcontroller to Use an External Super I/O Chip
It may be desirable to connect a commercially available Super I/O chip on the GP bus in
an ÉlanSC520 microcontroller system (for example, systems requiring a keyboard or IDE
drive can implement this device).
In this case, since the Super I/O implements two UARTs programmed at the same address
as the ÉlanSC520 microcontroller’s integrated UARTs, the internal UARTs can be disabled
to support the COM1 and COM2 ports in the Super I/O chip, if desired. In this case, when
Содержание Elan SC520
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Страница 28: ...Introduction xxviii lan SC520 Microcontroller User s Manual...
Страница 42: ...Architectural Overview 1 14 lan SC520 Microcontroller User s Manual...
Страница 78: ...System Initialization 3 22 lan SC520 Microcontroller User s Manual...
Страница 108: ...Clock Generation and Control 5 10 lan SC520 Microcontroller User s Manual...
Страница 118: ...Reset Generation 6 10 lan SC520 Microcontroller User s Manual...
Страница 148: ...System Arbitration 8 24 lan SC520 Microcontroller User s Manual...
Страница 214: ...SDRAM Controller 10 36 lan SC520 Microcontroller User s Manual...
Страница 230: ...Write Buffer and Read Buffer 11 16 lan SC520 Microcontroller User s Manual...
Страница 288: ...GP Bus DMA Controller 14 22 lan SC520 Microcontroller User s Manual...
Страница 316: ...Programmable Interval Timer 16 8 lan SC520 Microcontroller User s Manual...
Страница 328: ...Software Timer 18 4 lan SC520 Microcontroller User s Manual...
Страница 346: ...Real Time Clock 20 12 lan SC520 Microcontroller User s Manual...
Страница 360: ...UART Serial Ports 21 14 lan SC520 Microcontroller User s Manual...
Страница 414: ...AMDebug Technology 26 8 lan SC520 Microcontroller User s Manual...