1.3.1. Boot Monitor configuration

User switches on the motherboard or in the config.txt file control Boot Monitor configuration.


Table 1.1 shows how the setting of USERSWITCH[0] in the config.txt file determines how the Boot Monitor starts after a reset.

Table 1.1. Function of USERSWITCH[0]

OFFA prompt is displayed enabling you to enter Boot Monitor commands.
ONThe Boot Monitor executes a boot script that has been loaded into an MMC or Secure Digital (SD) card. If a boot script is not present, the Boot Monitor prompt is displayed.

The boot script can execute any Boot Monitor commands. It typically selects and runs an application image that has been stored in either NOR flash memory, or on the SD card. You can store one or more code images in flash memory and use the boot script to start an image at reset. Use the SET BOOTSCRIPT command to set the boot script file name from the Boot Monitor. See Table 1.3.


Table 1.2 describes how the setting of USERSWITCH[2:1] controls text input and output for applications STDIO calls and Boot Monitor I/O.

Table 1.2. Function of USERSWITCH[2:1]


switch 1


switch 2



or console


or console

STDIO autodetects whether to use semihosting I/O or a UART. If a debugger is connected and semihosting is enabled, STDIO is redirected to the debugger console window. Otherwise, STDIO goes to UART0.
OFFONUART0UART0STDIO is redirected to UART0. This occurs even under semihosting.
ONOFFLCDKeyboardSTDIO is redirected to the LCD and keyboard. This occurs even under semihosting.
ONONLCDUART0STDIO output is redirected to the LCD and input is redirected to the UART0. This occurs even under semihosting.

The USERSWITCH[2:1] bits do not affect file I/O operations performed under semihosting. Semihosting operation requires a debugger and a JTAG interface device.


The Boot Monitor does not use the USERSWITCH[7:3] bits and these bits are always available for user applications. If a different loader program is present at the boot location, the function of USERSWITCH and CFGSWITCH is implementation-dependent. See the configuration information in the documentation supplied with your motherboard.

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