AXEL ULite SOM/AXEL ULite Hardware/Power and Reset/Boot process and bootstrap configuration

Introduction

NXP iMX6UL processor implements a very flexible boot process. This versatility comes at the price of a non trivial bootstrap configuration scheme. Typical system-on-module (SOM for short) adopter does not want/need to deal with such complexity. In other words he/she expects to manage few boot configuration issues because it is assumed they are handled by the SOM itself. Nevertheless, there are specific applications where the system integrator needs full control of all the bootstrap configuration options, even if the design is built upon a SOM.

These two needs - ease of use and configurability - clearly push in opposite directions. During the conception of the product AXELULite, specific attention has been addressed to find a viable trade off to satisfy such requirements. This effort has led to the options that are detailed in the section Options for AXELULite users. Before skipping to it, it is recommended to read this section that depicts an overview of the processor's boot process. For more details about it please refer to

Overview of NXP iMX6UL bootstrap process

When it comes to boot process, three factors play a major role:

• BOOT_MODE signals (BOOT_MODE1, BOOT_MODE0)
• eFUSE bits
• GPIO settings.

Generally speaking, three main boot modes are supported. These modes are selected by BOOT_MODE signals that are latched when processor comes out of reset.

BOOT_MODE[1:0]	Boot type
00	Boot from eFUSEs
01	Serial downloader
10	Internal boot
11	reserved

It is worth remebering that, no matter what modes is selected, when processor reset is released, internal Boot ROM is executed. Its behavior depends on selected mode as described in the following sections.

Boot from eFUSEs

The term eFUSEs refer to an one-time-programmable (OTP) memory integrated in iMX6UL processor. This memory is organized in several 32-bit words. Three of these words - named BOOT_CFG - are used to store configuration bits that affect boot process, when Boot from eFUSEs mode is selected. In this case, Boot ROM reads BOOT_CFG words and acts consequently.

BOOT_CFG includes many configuration settings. Some of them are particularly important for the boot strategy:

• BT_FUSE_SEL (shipped value = 0)
• SDMMC_MFG_DISABLE (shipped value = 0)
• FORCE_INTERNAL_BOOT.

The use of these settings will be more clear soon.

Serial downloader

Serial Downloader mode is used to download a Program Image [1] over one of the following connections:

• USB OTG1
• UART1
• UART2.

When this mode is selected, Boot ROM continuously polls for a connection over these interfaces.

It is worth remembering that, when Serial Downloader is enabled, Boot ROM needs to initialize muxing of UART1/UART2 signals as per the following table.

Ball name	Ball reference	Signal name	Mux mode
UART1_TX_DATA	TBD	uart1.TX_DATA	Alt 0
UART1_RX_DATA	TBD	uart1.RX_DATA	Alt 0
UART2_TX_DATA	TBD	uart2.TX_DATA	Alt 0
UART2_RX_DATA	TBD	uart2.RX_DATA	Alt 0

Since these pads may be used to implement different functions other than UART signals in customer's application, system integrator needs to verify that the Serial Downloader - if used - is electrically compatible with his/her own carrier board.

[1] Program Image includes - but it is not limited to - the code that is executed by the processor when Boot ROM jumps into it.

Internal boot

When this mode is selected, Boot ROM tries to retrieve the Program Image from the selected device (NOR flash, NAND flash, SD card etc.).

The selection of the device along with several further device-specific configuration settings can be done in two different ways, depending on BT_FUSE_SEL:

• 0 (shipped value): GPIOs
• 1: eFUSE.