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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
30 of 571
NXP Semiconductors
UM10316
Chapter 3: LPC29xx Clock Generation Unit (CGU)
3.7 Clock detection
All of the clock sources have a clock detector, the status of which can be read in a CGU
register. This register indicates which sources have been detected.
If this is enabled, the absence of any clock source can trigger a hardware interrupt.
3.8 Bus disable
This safety feature is provided to avoid accidental changing of the clock settings. If it is
enabled, access to all registers except the RBUS register (so that it can be disabled) is
disabled and the clock settings cannot be modified.
3.9 Clock-path programming
The following flowchart shows the sequence for programing a complete clock path:
4.
CGU1 functional description
The CGU1 block is functionally identical to the CGU0 block and generates two clocks for
the USB interface and a dedicated output clock. The CGU1 block uses its own PLL and
fractional dividers. The PLLs used in CGU0 and CGU1 are identical.
The clock input to the CGU1 PLL is provided by one of two base clocks generated in the
CGU0: BASE_ICLK0_CLK or BASE_INT1CLK. The base clock not used for the PLL can
be configured to drive the output clock directly.
The CGU1 provides the following three base clocks (
):
Fig 13. Programming the clock path
Configure XO50MOSC
in normal mode with
HF pin enabled
Configure PLL to use
XO50MOSC as input
and generate 80MHz
(Fin = 10 MHz
and Fcco = 160 MHz)
with 3-phase output
enabled
Wait for PLL to lock
Configure FR clock
to 40 MHz
Configure FDIV5 to use
120° PLL output and
generate ~3.6866 MHz
Configure UART_CLK
to use FDIV5 and
divide by 2
Table 12.
CGU1 base clocks
Base clock
Parts of the device clocked by this branch clock
BASE_OUT_CLK
clock out pin
BASE_USB_CLK
USB clock
BASE_USB_I2C_CLK
USB OTG I2C clock