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→Note on NVCC_AXEL_I/O_3.3V/1.8V usage
Implementing correct power-up sequence for i.MX6 processors is not a trivial task because several power rails are involved. Axel SOM simplifies this task and embeds all the needed circuitry. The following picture shows a simplified block diagram of PSU/voltage monitoring circuitry:
[[File:AxelUltra-power-sequence.png | 741px800px]]
The PSU is composed of two main blocks:
# PMIC transitions from OFF to ON state
# PMIC initiates power-up sequence needed by MX6 processor
# NVCC_AXEL_I/O_3.3V/1.8V signal is raised; this active-high signal indicates that carrier board circuitry interfacing Axel SoM's I/O has is powered. This signal can be used to be powered manage carrier board power up sequence in order to prevent back powering (from SoM to carrier board or vice versa). For additional information, please refer to the [[Power_%28AxelUltra%29#Note_on_NVCC_AXEL_I.2FO_3.3V.2F1.8V_usage | Note]] below.
# configurable I/O power rails (NVCC_CSI_EXT, NVCC_EIM_EXT, NVCC_SD3_EXT, NVCC_LCD_EXT) are powered by carrier board
# PMIC VGEN6 LDO is turned on (this is the last regulator turned on automatically by PMIC)
# CPU_PORn is released.
==== Note on NVCC_AXEL_I/O_3.3V/1.8V usage ====
NVCC_AXEL_I/O_3.3V/1.8V is generally used on carrier board to drive loads such as DC/DC enable inputs or switch on/off control signals. Depending on the kind of such loads, NVCC_AXEL_I/O_3.3V/1.8V might not be able to drive them properly. In these cases a simple 2-input AND port can be used to address this issue. The following picture depicts a principle schematic showing this solution. VDD_SOM denotes the power rail used to power AXEL ULTRA SoM.
[[File:Axel-power-good.png]]
==== Note on NVCC_EIM_EXT ====
If the SPI NOR flash is mounted on the AXEL ULTRA SoM, the NVCC_EIM_EXT input signal can't be configured as an extended range voltage, but it must be connected to a +3.3V rail (because the SPI bus used internally for the NOR flash shares some pins of the EIM bank).
=== Power rails and related signals ===
* NVCC_SD3_EXT: this rail powers MX6's NVCC_SD3 domain
* NVCC_LCD_EXT: this rail powers MX6's NVCC_LCD domain
* NVCC_AXEL_I/O_3.3V/1.8V: this output signal is used to indicate when carrier board's circuitry interfacing Axel's I/Os has to be powered up.
* VGEN1: PMIC's VGEN1 regulator output; this regulator is not used by any Axel's internal load. Output voltage can be selected by user.
* VGEN2: PMIC's VGEN2 regulator output; this regulator is not used by any Axel's internal load. Output voltage can be selected by user.
