
UM10800
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© NXP Semiconductors N.V. 2016. All rights reserved.
User manual
Rev. 1.2 — 5 October 2016
10 of 487
NXP Semiconductors
UM10800
Chapter 3: LPC82x Boot ROM
The bootloader code is executed every time the part is powered on or reset. The
bootloader can execute the ISP command handler or the user application code. A LOW
level after reset at the ISP entry pin is considered as an external hardware request to start
the ISP command handler via USART.
For details on the boot process, see
.
Remark:
SRAM location 0x1000 0000 to 0x1000 0050 is not used by the bootloader and
the memory content in this area is retained during reset. SRAM memory is not retained
when the part powers down or enters Deep power-down mode.
Assuming that power supply pins are at their nominal levels when the rising edge on
RESET pin is generated, it may take up to 3 ms before the ISP entry pin is sampled and
the decision whether to continue with user code or ISP handler is made. The bootloader
performs the following steps (see
1. If the watchdog overflow flag is set, the bootloader checks whether a valid user code
is present. If the watchdog overflow flag is not set, the ISP entry pin is checked.
2. If there is no request for the ISP command handler execution (ISP entry pin is
sampled HIGH after reset), a search is made for a valid user program.
3. If a valid user program is found then the execution control is transferred to it. If a valid
user program is not found, the bootloader attempts to load a valid user program via
the USART interface.
Remark:
The sampling of pin the ISP entry pin can be disabled through programming
flash location 0x0000 02FC (see
Section 25.5.3 “Code Read Protection (CRP)”
).
3.5.2 ROM-based APIs
Once the part has booted, the user can access several APIs located in the boot ROM to
access the flash memory, optimize power consumption, and operate the USART and I2C
peripherals.
The structure of the boot ROM APIs is shown in