UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 420 o f 792
NXP Semiconductors UM10237
Chapter 15: LPC24XX USB OTG controller
The dev_dma_need_clk signal is asserted on any Device controller DMA access to
memory. Once asserted, it remains active for 2 ms (2 frames), to help assure that DMA
throughput is not affected by any latency associated with re-enabling ahb_master_clk.
2 ms after the last DMA access, dev_dma_need_clk is de-asserted to help conserve
power. This signal allows AHB_CLK_EN to be cleared during normal operation.

9.1.1 Host clock request signals

The Host controller has two clock request signals, host_need_clk and
host_dma_need_clk. When asserted, these signals turn on the host’s 48 MHz clock and
ahb_master_clk respectively.
The host_need_clk signal is asserted while the Host controller functional state is not
UsbSuspend, or if the functional state is UsbSuspend and resume signaling or a
disconnect is detected on the USB bus. This signal allows HOST_CLK_EN to be cleared
during normal operation when software does not need to access the Host controller
registers – the Host will continue to function normally and automatically shut off its clock
when it goes into the UsbSuspend state.
The host_dma_need_clk signal is asserted on any Host controller DMA access to
memory. Once asserted, it remains active for 2 ms (2 frames), to help assure that DMA
throughput is not affected by any latency associated with re-enabling ahb_master_clk.
2 ms after the last DMA access, host_dma_need_clk is de-asserted to help conserve
power. This signal allows AHB_CLK_EN to be cleared during normal operation.

9.2 Power-down mode support

The LPC2400 can be configured to wake up from Power Down mode on any USB bus
activity. When the USBWAKE bit is set in the INTWAKE register, the assertion of the
USB_NEED_CLK signal causes the chip to wake up from Power Down.
Before Power Down mode can be entered when USBWAKE is set, USB_NEED_CLK
must be de-asserted. This is accomplished by clearing all of the CLK_EN bits in
OTGClkCtrl and putting the Host controller into the UsbSuspend functional state. If it is
necessary to wait for either of the dma_need_clk signals or the dev_need_clk to be
de-asserted, the status of USB_NEED_CLK can be polled in the USBIntSt register to
determine when they have all been de-asserted.
10. USB OTG controller initialization
The LPC2400 OTG device controller initialization includes the following steps:
1. Enable the device controller by setting the PCUSB bit of PCONP.
2. Configure and enable the PLL and Clock Dividers to provide 48 MHz for usbclk, and
the desired frequency for cclk. For correct operation of synchronization logic in the
device controller, the minimum cclk frequency is 18 MHz. For the procedure for
determining the PLL setting and configuration, see Section 4–3.2.12 “Procedure for
determining PLL settings”.
3. Enable the desired controller clocks by setting their respective CLK_EN bits in the
USBClkCtrl register. Poll the corresponding CLK_ON bits in the USBClkSt register
until they are set.