AXEL ULite SOM/AXEL ULite Hardware/Power and Reset/Reset scheme and control signals

Info Box Applies to AXEL ULite

Introduction

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Reset scheme and control signals

The following picture shows the simplified block diagram of reset scheme and voltage monitoring.

Simplified block diagram of reset circuitry and voltage monitoring

VDD_SNVS_IN

Some signals that are related to reset circuitry are pulled-up to VDD_SNVS_IN. This voltage is generated by PMIC PF3000's VSNVS LDO/Switch and its actual value depends on:

• voltage applied to PF3000 VIN pin
  • in case of AXELULite this pin is connected to VIN_SOM power rail
• voltage applied to PF3000 LICELL pin
  • in case of AXELULite this pin is connected to pin 14 of SODIMM connector (PMIC_LICELL).

Hence it is recommended that system designer takes into account these factors in order to properly manage these signals at carrier board level.

For more details please refer to section VSNVS LDO/Switch of Freescale Semiconductor PF3000 Advance Information document.

CPU_PORn

The following devices can assert this active-low signal:

• PMIC
• multiple-voltage monitor: this device monitors critical power voltages and triggers a reset pulse in case any of these exhibits a brownout condition

Since SPI NOR flash can be used as boot device, CPU_PORn is connected to this device too. This guarantees it is in a known state when reset signal is released.

Handling CPU-initiated software reset

By default, MX6 processor does not assert any external signal when it initiates a software reset sequence. Also default software reset implementation does not guarantee that all processor registers are reset properly. For these reasons, it is strongly recommended to use a different approach that, in combination with the use of a processor's watchdog timer (WDT), provides a full hardware reset in case a software reset is issued.

This technique is implemented in XELK. At software level, U-Boot and Linux kernel software reset routines make use of processor's WDT #2 to assert the WDOG2_B reset signal. This signal in turn is routed to GPIO_1 pad (MUX mode = 1). At hardware level, this signal is AC-coupled to a 3-state output buffer (please refer to U22 chip of AxelEVB-Lite carrier board), driving PMIC_PWRON.