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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
18 of 808
NXP Semiconductors
UM10360
Chapter 3: LPC17xx System control
On the assertion of a reset source external to the Cortex-M3 CPU (POR, BOD reset,
External reset, and Watchdog reset), the IRC starts up. After the IRC-start-up time
(maximum of 60
μ
s on power-up) and after the IRC provides a stable clock output, the
reset signal is latched and synchronized on the IRC clock. Then the following two
sequences start simultaneously:
1. The 2-bit IRC wake-up timer starts counting when the synchronized reset is
de-asserted. The boot code in the ROM starts when the 2-bit IRC wake-up timer times
out. The boot code performs the boot tasks and may jump to the flash. If the flash is
not ready to access, the Flash Accelerator will insert wait cycles until the flash is
ready.
2. The flash wake-up timer (9-bit) starts counting when the synchronized reset is
de-asserted. The flash wakeup-timer generates the 100
μ
s flash start-up time. Once it
times out, the flash initialization sequence is started, which takes about 250 cycles.
When it’s done, the Flash Accelerator will be granted access to the flash.
When the internal Reset is removed, the processor begins executing at address 0, which
is initially the Reset vector mapped from the Boot Block. At that point, all of the processor
and peripheral registers have been initialized to predetermined values.
shows an example of the relationship between the RESET, the IRC, and the
processor status when the LPC17xx starts up after reset. See
for start-up of the main oscillator if selected by the user code.
Fig 5.
Example of start-up after reset
valid threshold
processor status
V
DD(REG)(3V3)
IRC status
RESET
GND
60
μ
s
1
μ
s; IRC stability count
boot time
boot code executing
user code
boot code
execution
finishes;
user code starts
IRC
starts
IRC
stable
supply ramp-up
time