Vfp Shutdown - ARM ARM1176JZF-S Technical Reference Manual

10.3

VFP shutdown

ARM DDI 0301H
ID012310
The blocks in the top level of the ARM1176JZF-S are:
• A1176RAM , that includes all the RAMs
• when you have an IEM implementation:
— the four IEM register slices
— placeholders for level shifters and clamps between all the blocks
• , that includes all the remaining logic.
ARM1176JZFSNoRAM
contains:
ARM1176JZFSNoRAM
• the VFP
• all other logic outside the VFP
• a placeholder for clamping logic between these two blocks.
With this hierarchy, you can switch off the VFP to save power, when the VFP is not in use.
There is a clamping placeholder between the VFP and the rest of the logic but this block is not
implemented in gates because it contributes to a critical path. You must add clamps to the
placeholder, either as explicit gates in the Core power domain, or as pull-down transistors that
clamp the values when the VFP is powered down.
To shutdown the VFP:
1. Save all VFP registers, if the VFP contains defined state.
2. Disable the VFP with the system control coprocessor, see c1, Coprocessor Access Control
Register on page 3-51.
Assert nVFPRESETIN LOW.
3.
Assert VFPCLAMP HIGH.
4.
5. Switch the VFP power off.
To take the VFP out of shutdown:
1. Switch the VFP power on.
When the power is stable, deassert VFPCLAMP.
2.
Deassert nVFPRESETIN.
3.
4. Enable the VFP with the system control coprocessor.
5. Restore the VFP registers, if required.
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Power Control
10-6