Digilent Max32 Справочное руководство - Страница 6

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Max32™ Board Reference Manual

When the MPIDE opens the serial communications connection on the PC, the DTR pin on the FT232R chip is driven

low. This pin is coupled through a capacitor to the MCLR pin on the PIC32 microcontroller. Driving the MCLR line

low resets the microcontroller, restarting execution with the boot loader.

This automatic reset action (when the serial communications connection is opened) can be disabled. To disable

this operation, there is a cuttable trace on the bottom of the board between the pins on JP5. JP5 is normally not

loaded. If the trace between the pins on JP5 has been cut, the automatic reset operation can be restored by

loading JP5 and inserting a shorting block across it.

Two red LEDs (LD1 and LD2) will blink when data is being sent or received between the Max32 and the PC over the

serial connection.

The header connector J12 provides access to the other serial handshaking signals provided by the FT232R.

Connector J12 is not loaded at the factory and can be loaded by the user to access these signals.

3.2

Power Supply

The Max32 is designed to be powered either via USB or from an external power supply. There is an automatic

switchover circuit that causes the external supply to be used if both supplies are present.

The power supply section in the Max32 uses two voltage regulators. The first regulates the external voltage to 5V

to power the VCC5V0 bus. The second regulates the VCC5V0 bus to 3.3V to provide power to the VCC3V3 bus that

powers the PIC32 microcontroller.

The 5V voltage regulator is normally an NCP1117. The board is designed to be able to also use an LM1117, but the

NCP1117 is the part normally used. The NCP1117 is rated for an output current of 1A (the LM1117 is rated for

800mA). The dropout voltage of the NCP1117 is a maximum of 1.2V at the rated output current (1.3V for the

LM1117).

There is a reverse-polarity protection diode in the external power supply circuit. Considering the diode drop plus

the forward drop across the regulator, the minimum input voltage to the regulator should be 7V to produce a

reliable 5V output. The absolute maximum input voltage of both the NCP1117 and the LM1117 is 20V. The

recommended maximum operating voltage is 15V.

For input voltages above 9V, the regulator will get extremely hot when drawing high currents. Both the NCP1117

and the LM1117 have output short circuit protection and internal thermal protection and will shut down

automatically to prevent damage.

The 3.3V regulator is a Microchip MCP1725. This regulator is rated for a maximum output current of 500mA. The

absolute maximum input voltage for the MCP1725 is 6V. This regulator has internal short circuit protection and

thermal protection. It will get noticeably warm when the current consumed by the VCC3V3 bus is close to the

500mA maximum.

The 5V power bus VCC5V0 can be powered from one of three sources:



The USB5V0 bus when the board is operating under USB power



The output of the on-board 5V regulator when operating from an external 7V – 15V supply



Directly from the external supply when operating from a regulated 5V external supply with jumper JP1 in the

BYP position.

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