5.1 Boot mode selection
The internal ROM memory is used to store the boot code. After a reset, the Arm processor starts its code execution from this
memory. The bootloader code is executed every time the part is powered-ON, is reset, or wakes up from a deep power-down while
in a low power mode.
Depending on the values of the CMPA bits, ISP pin, and the image header type definition, the bootloader decides whether to boot
from internal flash or run into ISP mode. See Section 6.5 FFR region definitions in
LPC55S6x/LPC55S2x/LPC552x User manual
(document UM11126).
The state of port pin, PIO0_5, at Reset determines the boot source of the part or if the handler is invoked. The LPC55S6x/
LPC55S2x/LPC552x/55S1x will read status of the ISP pins to determine boot source, as shown in
Table 8 .
Table 8. Boot mode and ISP download modes based on ISP pins
Boot mode ISP0 (PIO0_5 pin) Description
Passive boot HIGH
The LPC55S6x/LPC55S2x/LPC552x will look for valid image in the internal flash,
if no valid image is found, the LPC55S6x/LPC55S2x/LPC552x will enter ISP boot
mode based on DEFAULT_ISP_MODE bits defined in Table 9.
ISP boot LOW
One of the serial interfaces (UART0, I2C1, SPI3, HS_SPI, USB0, USB1) is used
to download image from host into internal flash. The first valid probe message on
USART, I2C, SPI or USB locks in that interface.
Table 9 shows the ISP pin assignments and is the default pin assignment used by the ROM code that cannot be changed.
Table 9. ISP pin assignment
ISP pin Port pin assignment
ISP0 PIO0_5
USART_ISP mode
FC0_TXD PIO0_30
FC0_RXD PIO0_29
I2C_ISP mode
FC1_SDA PIO0_13
FC1_SCL PIO0_14
SPI ISP mode
FC3_SCK PIO0_6
FC3_SSEL0 PIO0_4
FC3_MISO PIO0_2
FC3_MOSI PIO0_3
HS_SPI_SCK PIO1_2
Table continues on the next page...
NXP Semiconductors
Boot mode configurations
Hardware Design Guidelines for LPC55(S)xx Microcontrollers, Rev. 0, 30 October 2020
Application Note 11 / 24
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