This application report describes the boot process of the OMAP-L138 Arm® + DSP SoC. The content here also applies to the AM1808 and C6748 devices, unless otherwise specified.

There are two types of reset:

• Power-On Reset (POR) - POR occurs when both RESET and TRST are low. Internal memory is cleared during this state. The boot pins will be latched when RESET goes high.
• Warm Reset - Warm Reset occurs when RESET is low and TRST is high. Internal memory is retained during this state. The boot pins will be not be latched when RESET goes high, but instead retain the same values from the last POR.

It is important for the device to experience a POR when initially powered up. This will clear the emulation and PLL logic, as well as latch the boot pins correctly. Therefore, make sure TRST is externally pulled down on your board. Not doing so can be the cause of many boot related problems.

When the device is held in reset, all device I/Os are internally pulled down.

For the boot and configuration pins, if they are both routed out and 3-stated (not driven), it is strongly recommended that an external pullup/pulldown resistor be implemented. Although, internal pullup/pulldown resistors exist on these pins and they may match the desired configuration value, providing external connectivity can help ensure that valid logic levels are latched on these device boot and configuration pins. In addition, applying external pullup/pulldown resistors on the boot and configuration pins adds convenience to the user in debugging and flexibility in switching operating modes.

For details on how to choose values to oppose the internal pullup/pulldown resistors, see the Pullup/Pulldown Resistors section of the OMAP-L138 C6000™ DSP+ ARM® Processor Data Manual.