Tuesday, June 26, 2007

Cold Reset in HT Technology

Cold Reset is signaled during the power-up sequence under hardware control. This section details the sources, effects, and characteristics of a HyperTransport cold reset.

Sources of Cold Reset

In addition to the hardware generation of cold reset during the powerup sequence, platform developers may also provide hooks for generating cold reset under software control. An optional method of generating a HyperTransport cold reset is defined by the specification for the secondary bus of a HT-to-HT bridge (discussed on page 278). However, software generation of cold reset for the secondary side of the Host-to-HT bridge can be implementation specific.

Resetting the Primary HT Bus

Some implementation-specific mechanism must be defined to initiate a cold reset at powerup. The HT specification does not precisely define the source of HT cold reset for the system. It may be generated by system board logic or could be incorporated into the Host to HT bridge or other HT device residing on the system board.

Further, the specification does not require a software controlled method of cold reset generation. However, a host bridge could optionally implement a mechanism similar to that provided by the bridge control register of an HT-to-HT bridge. (See next section.)

Once reset is signalled, any HT device has the option of extending it (via open drain signaling) to ensure the amount of time it needs to complete its internal initialization. In this way, reset remains asserted until the last HT device in the chain completes its initialization. All HT devices that signal cold reset must correctly sequence RESET# and PWROK.

Resetting Secondary Side of HT-to-HT Bridge

An HT Bridge is required to propagate cold reset from its primary to its secondary side, but is not allowed to propagate any form of reset from its secondary to primary side. Thus, when the HT-to-HT Bridge initiates an HT cold reset to its secondary side, it will be distributed to all devices in the downstream chain.

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