Booting - Other Kinds of Boot Sequences

Other Kinds of Boot Sequences

Some other processors have other kinds of boot modes.

There are alternative techniques for booting CPUs and microcontrollers:

• Some modern CPUs and microcontrollers (for example, TI OMAP) or sometimes even DSPs may have boot ROM with boot code integrated directly into their silicon, so such a processor could perform quite a sophisticated boot sequence on its own and load boot programs from various sources like NAND flash, SD or MMC card and so on. It is hard to hardwire all the required logic for handling such devices, so an integrated boot ROM is used instead in such scenarios. Boot ROM usage enables more flexible boot sequences than hardwired logic could provide. For example, the boot ROM could try to perform boot from multiple boot sources. Also, a boot ROM is often able to load a boot loader or diagnostic program via serial interfaces like UART, SPI, USB and so on. This feature is often used for system recovery purposes when for some reasons usual boot software in non-volatile memory got erased. This technique could also be used for initial non-volatile memory programming when there is clean non-volatile memory installed and hence no software available in the system yet.
• It is also possible to take control of a system by using a hardware debug interface such as JTAG. Such an interface may be used to write the boot loader program into bootable non-volatile memory (e.g. flash) by instructing the processor core to perform the necessary actions to program non-volatile memory. Alternatively, the debug interface may be used to upload some diagnostic or boot code into RAM, and then to start the processor core and instruct it to execute the uploaded code. This allows, for example, the recovery of embedded systems where no software remains on any supported boot device, and where the processor does not have any integrated boot ROM. JTAG is a standard and popular interface; many CPUs, microcontrollers and other devices are manufactured with JTAG interfaces (as of 2009).
• Some microcontrollers provide special hardware interfaces which can't be used to take arbitrary control of a system or directly run code, but instead they allow the insertion of boot code into bootable non-volatile memory (like flash memory) via simple protocols. Then at the manufacturing phase, such interfaces are used to inject boot code (and possibly other code) into non-volatile memory. After system reset, the microcontroller begins to execute code programmed into its non-volatile memory, just like usual processors are using ROMs for booting. Most notably this technique is used by Atmel AVR microcontrollers, and by others as well. In many cases such interfaces are implemented by hardwired logic. In other cases such interfaces could be created by software running in integrated on-chip boot ROM from GPIO pins.

Most digital signal processors have the following boot modes:

• Serial mode boot
• Parallel mode boot, such as the host port interface (HPI boot)

In case of DSPs there is often a second microprocessor or microcontroller present in the system design, and this is responsible for overall system behavior, interrupt handling, dealing with external events, user interface, etc. while the DSP is dedicated to signal processing tasks only. In such systems the DSP could be booted by another processor which is sometimes referred as the host processor (giving name to a Host Port). Such a processor is also sometimes referred as the master, since it usually boots first from its own memories and then controls overall system behavior, including booting of the DSP, and then further controlling the DSP's behavior. The DSP often lacks its own boot memories and relies on the host processor to supply the required code instead. The most notable systems with such a design are cell phones, modems, audio and video players and so on, where a DSP and a CPU/microcontroller are co-existing.

Many FPGA chips load their configuration from an external serial EEPROM ("configuration ROM") on power-up.