{"id":6728,"date":"2022-01-24T04:41:49","date_gmt":"2022-01-24T04:41:49","guid":{"rendered":"http:\/\/fastbitlab.com\/?p=6728"},"modified":"2023-09-09T15:46:44","modified_gmt":"2023-09-09T10:16:44","slug":"linux-device-driver-programming-lecture-6-booting-sequence-beaglebone-black-hardware","status":"publish","type":"post","link":"https:\/\/fastbitlab.com\/blog\/linux-device-driver-programming-lecture-6-booting-sequence-beaglebone-black-hardware\/","title":{"rendered":"Linux Device Driver Programming Lecture 6- Booting sequence of Beaglebone Black hardware"},"content":{"rendered":"
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Booting sequence of Beaglebone Black hardware<\/b><\/span><\/h1>\n
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In this article, let’s explore the Boot sequence of BBB. There are different Boot sequences possible, as shown in Figure 1, and you can also alter the boot sequence.<\/span><\/p>\n

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Figure 1.AM335X Boot Sequence<\/span><\/figcaption><\/figure>\n

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To alter the boot sequence, there is a boot button on the board, and the boot button is S2, as shown in Figure 2.<\/span><\/p>\n

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Figure 2. Boot button S2<\/span><\/figcaption><\/figure>\n

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If you don’t press the boot button and if you give the power to the board, then this boot sequence will be followed, as shown in Figure 3.<\/span><\/p>\n

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Figure 3. Boot sequence if Boot button (S2) not pressed<\/span><\/figcaption><\/figure>\n

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That means the hardware will first try to boot from the onboard eMMC, that is, embedded MMC.<\/span><\/p>\n

If it doesn’t find any valid partition, then it would check the next boot source, that is, the SD <\/span>card. If it doesn’t find any valid boot images over there, then it would go to UART0, and then it would <\/span>try to boot from USB0.<\/span><\/span><\/p>\n

That’s a default boot sequence if you don’t press the boot button. As I said already, the board already comes with a pre-installed Debian OS  on the eMMC. That’s the reason whenever you give power to the board, the board boots from the eMMC. You need not press anything. Just give the power to the board it boots. <\/span><\/p>\n

But, in our case, we don’t want to boot from the eMMC. Remember that. We want to boot from an SD card. That’s why we have to go above the boot sequence(Figure 3).<\/span><\/p>\n

You have to press the boot button during power-up. Whenever you give power to the board,  the boot button must be pressed. In that case, the first boot source becomes SPI, which would be invalid in our case that’s why the board tries to boot from MMC0, which is nothing but a MicroSD slot.<\/span><\/p>\n

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Figure 4. Boot sequences when S2 is pressed<\/span><\/figcaption><\/figure>\n

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We have to keep valid partition and valid boot images on the SD card so that the board can boot from <\/span>the micro SD card. In our case, this would be the boot sequence( refer to Figure 4). <\/span><\/p>\n

But you need not to keep pressing the boot button S2 again and again during power-up; there is one hack, which I will cover in later articles. That we can make our board boots from the microSD card whenever we give power to the board, we will explore that later.<\/span><\/p>\n

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Figure 5. SYSBOOT[4:0]<\/span><\/figcaption><\/figure>\n

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Actually, the boot button S2 affects a register called SYSBOOT(Figure 5).<\/span><\/p>\n

SYSBOOT is a register, or it is a bit field of a control register of the SOC. A pressing or not pressing of the S2 button affects the contents of the SYSBOOT. That’s what makes the board to select between different boot sequences. Here is an S2 button(in Figure 6); this is a boot button.<\/span><\/p>\n

Just check your board near the microSD card slot, you see one button, that is the boot button named as S2.<\/span><\/p>\n

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Figure 6. Boot Power and Reset button<\/span><\/figcaption><\/figure>\n

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And you also see two more buttons. One is the power button, and another one is the reset button(Refer Figure 6.)<\/span><\/p>\n

Let’s understand the use case of these power button, reset button, and boot button.<\/span><\/p>\n