beaglebone-black - Flipbook - Page 55
BeagleBone Black
Fig. 6.19: User LEDs
6.3.1 Boot Con昀椀guration
The design supports two groups of boot options on the board. The user can switch between these modes via
the Boot button. The primary boot source is the onboard eMMC device. By holding the Boot button, the user
can force the board to boot from the microSD slot. This enables the eMMC to be overwritten when needed or
to just boot an alternate image. The following sections describe how the boot con昀椀guration works.
In most applications, including those that use the provided demo distributions available from beagleboard.org
the processor-external boot code is composed of two stages. After the primary boot code in the processor ROM
passes control, a secondary stage (secondary program loader – “SPL” or “MLO”) takes over. The SPL stage
initializes only the required devices to continue the boot process, and then control is transferred to the third
stage “U-boot”. Based on the settings of the boot pins, the ROM knows where to go and get the SPL and UBoot
code. In the case of the BeagleBone Black, that is either eMMC or microSD based on the position of the boot
switch.
6.3.2 Boot Con昀椀guration Design
昀椀gure-38 shows the circuitry that is involved in the boot con昀椀guration process. On power up, these pins are
read by the processor to determine the boot order. S2 is used to change the level of one bit from HI to LO which
changes the boot order.
It is possible to override these setting via the expansion headers. But be careful not to add too much load such
that it could interfere with the operation of the HDMI interface or LCD panels. If you choose to override these
settings, it is strongly recommended that you gate these signals with the SYS_RESETn signal. This ensures that
after coming out of reset these signals are removed from the expansion pins.
6.3.3 Default Boot Options
Based on the selected option found in 昀椀gure-39 below, each of the boot sequences for each of the two settings
is shown.
The 昀椀rst row in «昀椀gure-39» is the default setting. On boot, the processor will look for the eMMC on the MMC1
port 昀椀rst, followed by the microSD slot on MMC0, USB0 and UART0. In the event there is no microSD card and
the eMMC is empty, UART0 or USB0 could be used as the board source.
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Chapter 6. Detailed Hardware Design
