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5V signalling support on the PCI and Local Bus interface | Cypress Semiconductor

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5V signalling support on the PCI and Local Bus interface

Last Updated: August 28, 2009


- Can the PCI-DP support 5V signaling on the PCI side and Local Bus side?

- Can I drive 5V inputs on the PCI-DP?

Response: The PCI-DP is a 3.3V part which is 5V tolerant (on both the PCI and local bus sides).  This means that it is 3.3V PCI compliant and also meets all PCI requirements in the 5V PCI signaling environment, except for the I/O clamp to Vdd (not possible when 3.3V part is 5V tolerant). The PCI-DP meets PCI timing requirements in both 3.3V and 5V signaling environments. The main issue is a grey area in the PCI specification due to lack of an explicit definition of "5V tolerance".  Different vendors selling 3.3V parts interpret "5V tolerance" in different ways. Since the motivation behind maintaining 5V compatible devices stems from a 5V chip being able to drive directly into a 3.3V chip. Taking a TTL environment, the output spec for TTL is > 2.4V, so it could swing all the way up to 5V for a 5V chip. The 3V part on the receiving end (also TTL) must be able to stomach this and also not clamp the 5V driver to the 3.3V rail, hence no input clamp to VDD. This isn't directly related to PCI per-se. Use of either 5V or 3.3V is possible on the Local Bus side as well, due to the 5V tolerance and compatibility of signaling levels.

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