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July 17, 2002
93
System Design Considerations
6.5.2 Serial Flash ROM Write Protection Circuit
The serial flash ROM used with TM5500/TM5800 processors does not provide a secure mechanism for
protecting the contents from accidental erasure or accidental writes. This creates a potential risk for the serial
flash ROM to be inadvertently erased or modified in-system, which affects the Code Morphing software boot
image stored in that ROM. Therefore, to ensure a high level of system security and integrity, Transmeta
requires a special write protection circuit for the serial flash ROM. Systems using a parallel ROM to store the
Code Morphing software boot image do not have this risk and do not require this write protection circuit.
The write protection circuit uses a 22LV10 PLD to filter the chip select signals from the processor to the ROM.
The PLD intercepts write cycles to the ROM when writes are not authorized. A southbridge GPIO pin signals
the PLD when writes are authorized. The features of this circuit are:
•
It provides security.
•
It fully write protects the entire serial flash ROM.
•
It ensures that the Code Morphing software boot image is not accidentally overwritten.
•
It allows for field upgrades of Code Morphing software should they be needed.
6.5.2.1
Circuit Operation
The write protection PLD works by gating the chip select signal to the serial ROM when a write or erase
command is detected. The device has an input pin (named WP) to enable and disable this feature. When WP
is asserted, write and erase commands are not allowed to complete. When WP is negated, write and erase
commands function normally. Read commands are always unaffected by the PLD.
The polarity of the WP input is selectable with another input pin (named WPNEG). When WPNEG is pulled
up, the WP input is active-low. When WPNEG is pulled down, the WP input is active-high. WPNEG should be
pulled up if the signal driving the WP input powers up in the low state. WPNEG should be pulled down if the
signal powers up in the high state.
The PLD also has an input pin (named SEL) to select the type of serial ROM in use. The SEL pin should be
pulled down for the Atmel flash device, which is the only device currently supported.
Note
For JEDEC source code and CUPL source code, see Appendix B,
Serial Write-protection PLD Data
Note
Due to a potential race condition, do not use an ATF22LV10C part for this application.
Содержание Crusoe TM5500
Страница 1: ...TM5500 TM5800 System Design Guide July 17 2002...
Страница 6: ...July 17 2002 6 List of Tables...
Страница 8: ...July 17 2002 8 List of Figures...
Страница 50: ...July 17 2002 50 Processor Power Supplies and Power Management...
Страница 110: ...July 17 2002 110 System Design Considerations...
Страница 122: ...July 17 2002 122 System Design Checklists...
Страница 128: ...July 17 2002 128 Serial Write protection PLD Data...
Страница 130: ...July 17 2002 130 Index...