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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
157 of 571
NXP Semiconductors
UM10316
Chapter 13: LPC29xx USB device
8.
Clocking and power management
This section describes the clocking and power management features of the USB Device
Controller.
8.1 Power requirements
The USB protocol insists on power management by the device. This becomes very critical
if the device draws power from the bus (bus-powered device). The following constraints
should be met by a bus-powered device:
1. A device in the non-configured state should draw a maximum of 100 mA from the bus.
2. A configured device can draw only up to what is specified in the Max Power field of
the configuration descriptor. The maximum value is 500 mA.
3. A suspended device can draw a maximum of 500
μ
A.
8.2 Clocks
The USB device controller clocks are shown in
8.3 Power management support
To help conserve power, the USB device controller automatically disables the AHB master
clock and usbclk when not in use.
When the USB Device Controller goes into the suspend state (bus is idle for 3 ms), the
usbclk input to the device controller is automatically disabled, helping to conserve power.
However, if software wishes to access the device controller registers, usbclk must be
active. To allow access to the device controller registers while in the suspend state, the
USBClkCtrl and USBClkSt registers are provided.
When software wishes to access the device controller registers, it should first ensure
usbclk is enabled by setting DEV_CLK_EN in the USBClkCtrl register, and then poll the
corresponding DEV_CLK_ON bit in USBClkSt until set. Once set, usbclk will remain
enabled until DEV_CLK_EN is cleared by software.
When a DMA transfer occurs, the device controller automatically turns on the AHB master
clock. Once asserted, it remains active for a minimum of 2 ms (2 frames), to help ensure
that DMA throughput is not affected by turning off the AHB master clock. 2 ms after the
last DMA access, the AHB master clock is automatically disabled to help conserve power.
If desired, software also has the capability of forcing this clock to remain enabled using the
USBClkCtrl register.
Table 116. USB device controller clock sources
Clock source
Description
AHB master clock
Clock for the AHB master bus interface and DMA
AHB slave clock
Clock for the AHB slave interface
usbclk
Dedicated 48 MHz clock from CGU1 (BASE_USB_CLK)