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CPU NorthBridge Performance Tuning
The CPU NorthBridge (CPU NB) clock speed determines the efficiency and bandwidth capacity of the Memory
controller. L3 cache runs at this frequency as well. CPU NB performance tuning gives a measurable boost for
overall system performance. In particular it can reduce the Memory latency and improve L3 cache bandwidth
and latency. In some scenarios the CPU NB should be tuned in order to take the full advantage of overclocked
Memory frequency and bandwidth. In general the CPU NB clock should be at least two times the Memory clock
(example in case of DDR3-1600: 2 x 800MHz = 1600MHz NB clock).
In terms of power delivery and voltage regulation the latest AM2+ and AM3/AM3+ motherboards have a
dedicated power plane for the CPU NB Voltage rail. The voltage level of CPU NB power plane (CPU NB VID)
can be set independently from the CPU Core Voltage rail (CPU VID).
The frequency of CPU NB is determined by the CPU NB Multiplier (CPU NB FID) and by the HT reference clock
(HT ref. clock, 200MHz by default). CPU NB Multiplier can not be changed on the fly (as various memory
parameters and memory training is based on the NB clock value). A reboot is required in order to adjust the
CPU NB Multiplier. Therefore the performance tuning of CPU NB is mostly done thru the BIOS menu. Note that
CPU NB Multipliers are integer values and therefore the adjustment will be in steps of 200MHz or more
(depending on HT ref clock value). The combination of “HT ref clock” and the “CPU NB Multiplier” adjustment in
the BIOS menu will allow finer steps.
HT reference clock adjustments can be done on the fly via the AMD OverDrive™ utility or via the motherboard
vendors utility and this allows finer granularity for CPU NB Performance tuning and may reduce the time
required for tuning (as there is no need to reboot the system between different HT ref. clock values).
PLEASE NOTE THAT THE AMD PRODUCT WARRANTY DOES NOT COVER DAMAGES CAUSED BY
RUNNING COMPONENTS OUTSIDE OF SPECIFICATION INCLUDING DAMAGES CAUSED BY
OVERCLOCKING (EVEN WHEN OVERCLOCKING IS ENABLED WITH THE AMD OVERDRIVE UTILITY).
CPU NB performance tuning with AMD OverDrive™ utility (HT ref. clock):
1) Set CPU NB and HT Link multiplier to known good values (example: 2000MHz or x10)
2) Set CPU Core Multiplier(s) / Voltage to a known stable setting before proceeding with CPU NB
performance tuning (to ensure that CPU core doesn’t cause stability issues / crashes)
3) Set Memory clock and timings to known stable settings (example: DDR3-1333 CL9-9-9) and note that
the Memory interface will also be overclocked when HT ref. clock is adjusted. Memory must be able to
run 10% above the default clock during CPU NB tests. Set Memory to DDR3-1333 (or DDR3-1600
depending on your memory specification) and boost Memory voltage to 1.600V to ensure stability
4) Adjust “HT ref. clock” slider in AOD (2MHz steps for accuracy, 5MHz steps for less accurate but faster
results)
a. NOTE: If you motherboard doesn’t have BIOS support for AOD HT Ref. clock adjustment you
could use the motherboard vendors software utility for this step