How To Modify a Core’s Voltage By Writing The Scc

How to Modify a Core’s Voltage by Writing the SCC RPC Register revision 1.1
Revision History for Document
1.0 1.1 Initial release. Correct typo; 0x07 shsould have been 0x70

1 Introduction
You do not have to use the RCCE power management calls to change the voltage in an SCC power domain. With RCCE, you can only change the frequency, and the voltage may then change along with it. This ensures that you do not enter an unacceptable electrical state. You do still have the capability of changing the voltage on a core directly, but not with the RCCE API. Note that when you change the voltage on a core, you actually change the voltage for all cores in that core’s power domain. You do not modify the frequency. The best way to see how to change the voltage is to look at the way RCCE does it. RCCE is more than a message-passing library you can use for your applications. The entire RCCE source code is available and reading RCCE code is an excellent way to understand how the SCC works.

2 The RPC Register
Change the voltage by writing a value to the RPC register. RPC stands for Rock Creek Power Controller. This is sometimes called the VRC register for Voltage Regulator Controller. The RPC register is at physical address 0xfb000000. You cannot read this register. You won’t get an error if you try; you’ll just get a returned value that doesn’t mean anything. Bit 16 must be 1. Bits 10-8 specify the SCC power domain. A power domain is a set of 2x2 tiles or 8 cores. Note that this is the SCC power domain number, not the RCCE power domain number. The SCC domain number reads 4, 5, and 7 on the bottom row left to right and 0, 1, and 3 on the upper row, left to right.
RCCE: 3 SCC: 0 RCCE: 4 SCC: 1 RCCE: 5 SCC: 3

RCCE: 0 SCC: 4

RCCE: 10 SCC: 5

RCCE: 2 SCC: 7

The bit pattern for the RPC register looks as follows.
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

October 2, 2011

Page 1 of 4

Intel Labs

1

SCC pwr dmn

voltage value

Bits 7 through 0 specify...

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