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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
239 of 571
NXP Semiconductors
UM10316
Chapter 15: LPC29xx USB OTG interface
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.
8.2 Clocking and power management
The OTG controller clocking is shown in
.
A clock switch controls each clock with the exception of ahb_slave_clk. When the enable
of the clock switch is asserted, its clock output is turned on and its CLK_ON output is
asserted. The CLK_ON signals are observable in the OTGClkSt register.
To conserve power, the clocks to the Device, Host, OTG, and I2C controllers can be
disabled when not in use by clearing the respective CLK_EN bit in the OTGClkCtrl
register. When the entire USB block is not in use, all of its clocks can be disabled by
clearing the PCUSB bit in the PCONP register.
When software wishes to access registers in one of the controllers, it should first ensure
that the respective controller’s 48 MHz clock is enabled by setting its CLK_EN bit in the
OTGClkCtrl register and then poll the corresponding CLK_ON bit in OTGClkSt until set.
Once set, the controller’s clock will remain enabled until CLK_EN is cleared by software.
Accessing the register of a controller when its 48 MHz clock is not enabled will result in a
data abort exception.