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STPC4HEBI Datasheet(PDF) 64 Page - STMicroelectronics

Part No. STPC4HEBI
Description  X86 Core PC Compatible Information Appliance System-on-Chip
Download  93 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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STPC4HEBI Datasheet(HTML) 64 Page - STMicroelectronics

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DESIGN GUIDELINES
64/93
Release 1.5 - January 29, 2002
6.2. STPC CONFIGURATION
The STPC is a very flexible product thanks to
decoupled clock domains and to strap options
enabling a user-optimized configuration.
As some trade off are often necessary, it is
important to do an analysis of the application
needs prior to design a system based on this
product. The applicative constraints are usually
the following:
- CPU performance
- graphics / video performances
- power consumption
- PCI bandwidth
- booting time
- EMC
Some other elements can help to tune the choice:
- Code size of CPU Consuming tasks
- Data size and location
On the STPC side, the configurable parameters
are the following:
- synchronous / asynchronous mode
- HCLK speed
- MCLK speed
- CPU clock ratio (x1, x2)
- Local Bus / ISA bus
6.2.1. LOCAL BUS / ISA BUS
The selection between the ISA bus and the Local
Bus is relatively simple. The first one is a standard
bus but slow. The Local Bus is fast and
programmable but doesn't support any DMA nor
external master mechanisms. The Table 6-1
below summarize the selection:
Before implementing a function requiring DMA
capability on the ISA bus, it is recommended to
check if it exists on PCI, or if it can be
implemented differently, in order to use the local
bus mode.
6.2.2. CLOCK CONFIGURATION
The CPU clock and the memory clock are
independent unless the "synchronous mode"
strap option is set (see the STRAP OPTIONS
chapter). The potential clock configurations are
then relatively limited as listed in Table 6-2.
The
advantage
of
the
synchronous
mode
compared to the asynchronous mode is a lower
latency when accessing SDRAM from the CPU or
the PCI (saves 4 MCLK cycles for the first access
of the burst). For the same CPU to Memory
transfer performance, MCLK as to be roughly
higher by 20MHz between SYNC and ASYNC
modes
(example:
66MHz
SYNC
=
96MHz
ASYNC).
In all cases, use SDRAM with CAS Latency
equals to 2 (CL2) for the best performances.
The advantage of the asynchronous mode is the
capability to reprogram the MCLK speed on the
fly. This could help for applications were power
consumption must be optimized.
Regarding PCI bandwidth, the best is to have
HCLK at 100MHz as it gives twice the bandwidth
compared to HCLK at 66MHz.
The last, and more complex, information to
consider is the behaviour of the software. In case
high CPU or FPU computation is needed, it is
sometime better to be in DX2-133/MCLK=66
synchronous mode than DX2-133/MCLK=100
asynchronous mode. This depends on the locality
of the number crunching code and the amount of
data manipulated.
The Table 6-3 below gives some examples. The
right column correspond to the configuration
number as described in Table 6-2:
Obviously, the values for HCLK or MCLK can be
reduced compared to Table 6-2 in case there is no
need to push the device at its limits, or when
avoiding to use specific frequency ranges (FM
radio band for example).
Table 6-1. Bus mode selection
Need
Selection
Legacy I/O device (Floppy, ...), Super I/O
ISA Bus
DMA capability (Soundblaster)
ISA Bus
Flash, SRAM, basic I/O device
Local Bus
Fast boot
Local Bus
Boot flash of 4MB or more
Local Bus
Programmable Chip Select
Local Bus
Table 6-2. Main STPC modes
CMode
HCLK
MHz
CPU clock
clock ratio
MCLK
MHz
1
Synchronous
66
133 (x2)
66
2
Asynchronous
66
133 (x2)
100
3
Synchronous
100
100 (x1)
100
Table 6-3. Clock mode selection
Constraints
C
Need CPU power
Critical code fits into L1 cache
1
Need CPU power
Code or data does not fit into L1 cache
3
Need high PCI bandwitdh
3
Need flexible SDRAM speed
2


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