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UM10375
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 3 — 14 June 2011
141 of 368
NXP Semiconductors
UM10375
Chapter 10: LPC13xx USB device controller
The usb_clk clock can be either provided by the main clock or a dedicated USB PLL (see
). The USB PLL can be powered down if it is not used for the usb_clk in the
PDRUNCFG register (
) to conserve power.
10.9.3 Power management support
To help conserve power, the USB device controller automatically disables PCLK and
USB_MainClk when not in use.
The assertion of USB_Suspend(_N) signal indicates that there was no activity on the USB
bus for the last 3 ms. At this time an interrupt is sent to the processor on which the
software can start preparing the device for suspend.
If there is no activity again for the next 2 ms, the USB_NeedClk signal will go low. This
shuts off the USB_MainClk automatically. Once the USB_MainClk is switched off, internal
registers in the USB clock domain will not be visible to the software.
When the activity is detected on the bus, USB_Suspend(_N) is deactivated and
USB_NeedClk signal is activated. This process is fully combinatorial and hence no
USB_MainClk is required to activate the USB_NeedClk signal.
The usb_clk_enable signal is provided by the SYSAHBCLK bit 14 (see
) which
enables the clock to the USB register block.
In addition, the on-chip device PHY can be powered down in the PDRUNCFG register
(
) if the USB device function is not needed.
Table 161. USB device controller clock sources
Source
Clock name
Description
ahb_sys_clk
PCLK
This is the system clock. Minimum frequency of this clock is
16 MHz.
usb_clk (see
USB_MainClk
USB_MainClk is the 48 MHz
500 ppm input clock. This
clock does not need to be synchronized with the system clock
(PCLK). Gating of this clock is possible by an external control
block using the USB_NeedClk signal. This clock will be used
to recover the 12 MHz clock from the USB bus