Fast-Bus Mode; Half-Turbo Mode - Intel PXA270 Optimization Manual

Power Optimization
Selecting the lowest possible run frequency acceptable for applications results in lower core-power
consumption.
For example, many combinations of the L, 2N
(CCCR) and B bit in the Clock Configuration register (CLKCFG) result in a core frequency of
208 MHz.
•
L=16, N
208 MHz. The system bus runs at 104 MHz and the memory bus runs at 104 MHz.
•
L=16, N
208 MHz. The system bus runs at 208 MHz and the memory bus runs at 104 MHz.
•
L=16, N
208 MHz. The system bus runs at 208 MHz and the memory bus runs at 208 MHz.
•
L=8, N
208 MHz. The system bus runs at 52 MHz and the memory bus runs at 104 MHz.
In all cases, the Intel XScale® core is running at a frequency of 208 MHz. The tradeoffs between
the four cases are the speed of the system bus and memory bus versus power consumption. The
lower the frequency of the either bus, the lower the core-power consumption. However, if an
application requires minimal memory latency or high-internal throughput, it might be preferable to
run with a higher system-bus frequency or memory bus at the expense of power.
6.2.4.1

Fast-Bus Mode

The system bus frequency can be doubled through the use of CLKCGF[B], refer to the
Bit Definitions"
is set, the system-bus speed is doubled (up to allowable maximum of to 208 MHz) and the system-
bus frequency is equal to that of the run mode frequency. Compared to when fast-bus mode is
disabled this substantially increases the maximum bandwidth of the system bus, but increases the
power usage. Refer to the
Developer's Manual for restrictions and detailed information about this mode.
Note: CCCR[L] is limited to 16 in fast-bus mode.
6.2.4.2

Half-Turbo Mode

Systems that have high bus usage requirements, but low Intel XScale® core frequency
requirements can benefit from the use of half-turbo mode. This mode causes the Intel XScale®
core to be clocked at half the selected run mode frequency. Only the core frequency is affected by
this. The bus speed configurations are unaffected in this mode. By using this to select a lower core
frequency the power used by the core is minimized. Ensure that the Intel XScale® core frequency
is sufficient for the loading on the core, otherwise system performance is degraded.
Refer to the
Manual for information on the bit settings. Refer to the
PXA27x Processor Family Developer's Manual for restrictions and detailed information about this
mode.
1. The value of N used to determine the Turbo Mode frequency is one half the value written to CCCR[2N]
6-4
1
=1.0, A=0, B=0 configures the run frequency as 208 MHz and a turbo frequency of
1
=1.0, A=0, B=1 configures the run frequency as 208 MHz and a turbo frequency of
1
=1.0, A=1, B=1 configures the run frequency as 208 MHz and a turbo frequency of
1
=2.0, A=0, B=0 configures the run frequency as 104 MHz and a turbo frequency of
table in the Intel® PXA27x Processor Family Developer's Manual. When this bit
"Fast-Bus Mode"
"CLKCFG Bit Definitions"
1
and A bits in the Core Clock Configuration register
table in the Intel® PXA27x Processor Family
table in the Intel® PXA27x Processor Family Developer's
"Half Turbo Mode"
Intel® PXA27x Processor Family Optimization Guide
"CLKCFG
table in the Intel®