8 Board Controller Functions

8.1 Voltage Monitor

The Nano Debugger uses an ADC channel to monitor the voltage on the target device. Reading the target voltage is controlled by the TVR (Target Voltage Readout) field in the board configuration - if this feature bit is cleared, then voltage monitoring is disabled.

The voltage monitor is a background task running in the Nano Debugger firmware that samples the ADC at regular intervals.

When used as an on-board debugger, the board designer sets the VMIN field in the board configuration, which specifies the minimum operating voltage value. Below this voltage, the Voltage Monitor registers that the target voltage is off. Handling of attempted programming and debugging operations in the off-state depends on the target device type.

The current operating voltage can be read out using a Microchip IDE or pymcuprog:

pymcuprog getvoltage

To read the set-point for the supply voltage, use:

pymcuprog getsupplyvoltage

USB Voltage

The Nano Debugger can also read the voltage on the USB VBUS line, which is its power source. This is primarily a convenience feature.

The USB voltage can be read using a Microchip IDE or pymcuprog:

pymcuprog getusbvoltage

8.2 Voltage Control

The Nano Debugger can supply the target device with its own voltage source. This is optional for both on-board and standalone debugger configurations.

To change the operating voltage, use one of the Microchip IDEs or pymcuprog:

pymcuprog --setsupplyvoltage

Tip: There is an easy option to adjust the target voltage by copying a drag-and-drop command text file to the board, which supports a set of commonly used target voltages. See section Special Commands for further details.

The mechanism used by the Nano Debugger for controlling target voltage depends upon the board configuration in place.

Table 8-1. Board configuration for voltage control
Field	Description	Notes
ANALOG_FEATURES: TVS	Target voltage set	Indicates that the target voltage can be controlled
VMIN	Minimum operating voltage	Setting the voltage below this value will fail
VMAX	Maximum operating voltage	Setting the voltage above this value will fail
VTG	Default operating voltage	Voltage used when no voltage is set
VREG	Voltage regulator configuration	Indicates which regulator is mounted on the board

Voltage Regulator Configuration 0 - None

No voltage control is available when this configuration is used.

Voltage Regulator Configuration 1 - Curiosity Nano

Note: This configuration is used on Curiosity Nano boards (adjustable voltage boards).

When the VREG board configuration is set to 0x01, the Nano Debugger uses a DAC to regulate the voltage on the target device by manipulating the voltage on the feedback pin of a low drop-out regulator. The Nano Debugger monitors the output of this regulator as described in Voltage Monitor, and regulates the voltage accordingly.

The target voltage regulator is a MIC5353 variable output LDO. The Nano Debugger can adjust the voltage output supplied to the board’s target section by manipulating the MIC5353’s feedback voltage. The hardware implementation is limited to an approximate voltage range of 1.7V to 5.1V.

MIC5353 supports a maximum current load of 500 mA. It is an LDO regulator in a small package, placed on a small printed circuit board (PCB), and can reach thermal shutdown at loads lower than 500 mA. The maximum current load depends on the input voltage, the selected output voltage, and the ambient temperature. The figure below shows the safe operating area for the regulator, with an input voltage of 5.1V and an ambient temperature of 23°C.

Figure 8-1. Target Regulator Safe Operation Area

: The profile shown here was measured on a Curiosity Nano board and may vary depending on the PCB.

The voltage output of the target regulator is continuously monitored (measured) by the Nano Debugger. An error condition is flagged if a deviation of 100 mV above or below the expected voltage is detected. In this case, the voltage regulator will be switched off.

: The error detection will also handle deviations in an externally applied voltage. For more information, see Voltage Off Pin (VOFF).

8.3 Voltage Off Pin (VOFF)

: The VOFF pin is only relevant when an adjustable regulator is used.

Instead of using the on-board target regulator, an external voltage can be used to power the target device. When the Voltage Off (VOFF) pin is shorted to the ground (GND) pin, the Nano Debugger firmware disables the target regulator, making it safe to apply an external voltage to the VTG pin.

The VOFF pin is used to allow a Curiosity Nano base board to provide power to a mounted Curiosity Nano kit. Voltage is applied via the VTG pin; otherwise, the two power supplies would be in contention. The base board pulls the VOFF pin low when it is connected.

The VOFF pin can be tied low or released at any time, and this change will be detected by a pin-change interrupt to the Nano Debugger, which will then control the target voltage regulator accordingly.

Warning: Applying an external voltage to the VTG pin without shorting VOFF to GND may cause permanent damage to the board.

Warning: Do not apply any voltage to the VOFF pin. Let the pin float to enable the power supply.

Warning: Applying a higher voltage than the maximum operating voltage of the target device may cause permanent damage to the board.

Info: If the Nano Debugger shuts off the target regulator, it will begin blinking the status LED rapidly, to indicate an error condition. Once the error is resolved the Nano Debugger will switch on the target regulator and stop blinking the status LED.

Programming, debugging, and data streaming are still possible with an external power supply. The USB cable powers the debugger and signal level shifters. Both regulators, the debugger, and the level shifters are powered down when the USB cable is removed.

Info: In addition to the power consumed by the target device, approximately 100 µA will be drawn from any external power source to power the on-board level shifters and voltage monitor circuitry when a USB cable is plugged into the DEBUG connector on the board. When the USB cable is unplugged, some current is used to supply the level shifter’s voltage pins, with a worst-case current consumption of approximately 5 µA. Typical values may be as low as 100 nA.

8.4 Voltage Control Errors

This section summarizes the most common issues that can arise with the power supply.

Target Voltage Shuts Down

Not reaching the set target voltage can occur if the target section draws too much current at a given voltage, causing the thermal shutdown safety feature of the MIC5353 regulator to activate. To avoid this, reduce the current load of the target section.

Target Voltage Setting is Not Reached

The USB input voltage (specified to be 4.4-5.25V) limits the maximum output voltage of the MIC5353 regulator at a given voltage setting and current consumption. If a higher output voltage is needed, use a USB power source with a higher input voltage or an external voltage supply on the VTG pin.

Target Voltage is Different From Setting

An externally applied voltage to the VTG pin without setting the VOFF pin low can cause this. If the target voltage fluctuates by more than 100 mV above or below the voltage setting, the Nano Debugger will detect it, and the internal voltage regulator will shut down. To fix this issue, remove the applied voltage from the VTG pin, and the Nano Debugger will enable the voltage regulator when the new condition is detected. Note that the PS LED will blink rapidly if the target voltage is below 100 mV of the setting but will remain on if it is more than 100 mV above it.

No, or Very Low Target Voltage and PS LED is Blinking Rapidly

A full or partial short circuit can cause this and is a specific instance of the issue described above. Remove the short circuit, and the Nano Debugger will re-enable the target voltage regulator.

No Target Voltage and PS LED is Lit 1

This situation occurs if the target voltage is set to 0.0V. To fix this, set the target voltage to a value within the specified voltage range for the target device.

No Target Voltage and PS LED is Lit 2

This situation can occur when power jumper(s) are cut and the target voltage regulator is set to a value within the specified voltage range for the target device. To fix this, solder a wire or bridge between the pads or, if a pin header is mounted, add a jumper.

VBUS Output Voltage is Low or Not Present

If the VBUS output voltage is low or missing, the most likely reason is a high current drain on VBUS, which has caused the current limiter to trip and cut off VBUS completely. To fix this issue, reduce the current consumption on the VBUS pin.

8.5 ID System

The ID line on the Nano Debugger supports a mechanism used to identify base boards and extensions that are connected to the Xplained Pro (XPRO) extension header on those base boards.

Note: Only Microchip Xplained Pro extensions and Curiosity Nano base boards feature the identification system.

During power-up, the Nano Debugger scans the ID line for both base boards and extensions and presents this information to the IDE upon request.

Note: An extension or base board connected after powering up the Nano Debugger will not be detected.

Tip: For more information on the ID system, see the Xplained Pro Hardware Development Kit (HDK).