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Isolated Power Configuration

If USB must be galvanically isolated from the curcuit, digital isolators, such as ISO7240M, MAX14934, Si8640 may be used between NSDSP and PIC as shown below, In this case NSDSP and USB side of the isolators is powered by USB, while the PIC side of the isolators is powered by the board.

Schematics

For the locations of the NSDSP pins, refer to the pinout diagram. Only PIC pins used for connecting NSDSP are shown. For the list of connections necessary to run the PIC, refer to the datasheet for your PIC device.

This schematics does not show any ESD protection devices which may or may not be needed for the application.

To enable programming and debugging, MCLR, PGC, and PGD pins must be connected. Values of R1, R2, and R3 depend on the sourcing abilities of the isolators they're connected to. Typical values are 1kΩ, but if isolators cannot source enough current to drive the resistors, the values of R1, R2, and R3 must be increased. Resistors up to 10kΩ are Ok, but higher resistance may interfere with high speed programming rates. R2 and the corresponding isolator is necessary only if PGC line is bidirectional, which is the case only when debugging dsPIC33E/PIC24E and some of PIC32 devices. The propagation delay on PGC and PGD lines cannot exceed 10ns at the highest programmming speed. Longer propagation delays are Ok if programming speed is decreased. Traces connecting PGC and PGD pins on both sides of the isolation barrier must be kept as short as possible. If longer traces are needed, it is best not to run them parallel to each other. Do not connect any diodes or capacitors to PGC or PGD pins.

 

PGC and PGD pins

If your PIC has several pairs of PGD/PGC pins, you can use any pair, but you must use it as a pair. Do not combine pins from different pairs.

PGC or PGD pins are busy during programming and debugging. However, you can use them during run time.

Using PGC and PGD as inputs is impossible because the output from the isolators will drive both pins. NSDSP pins can be configured to be driven high or low, but putting them into high impedance state will not work because of the isolators.

To use PGC and PGD as outputs, connect them directly or through protective resistors. The combined impendance of the connected devices must be at least 10kΩ. If it is higher than 10kΩ, you may consider increasing values of resistors R1 and R3 to decrease power consumption. Make sure that the devices connected to these pins can tolerate high frequency signaling during programming or debugging.

To use PGC and PGD to detect USB state, configure NSDSP to alter the state of the pins depending on USB state. Once configured, you can read the state of these pins to detect when USB power is connected, when USB connection is established, and when NSDSP enters UART mode. However, when NSDSP is not powered the state of PGC and PGDO pins depend on the construction of the isolator - some isolator models are high by default, others are low. There may be glitches on PGC and PGDO pins when NSDSP is powering up and down. To prevent the glitches, a 100kΩ pull-down resistors may be connected to NSDSP's PGC and PGDO pins.

If not using PGC and PGD configure them as inputs.

 

MCLR pin

Some PIC devices require an MCLR circuit which includes capacitor to the ground and pull-up resistor (see datasheet for your PIC). If such circuit is used, isolate the circuit from the PIC's MCLR pin with 470Ω (or more) resistor, and make sure that the output of the isolator connects directly to the PIC's MCLR pin without any resistors in-between.

If your PIC device allows using MCLR as a GP input, you must enable MCLRE and LVP. Isolator for MCLR requires special care. Its default output state must be high. Otherwise, your PIC will reset.

 

UART pins

To communicate with the host computer through USB, your PIC can use UART. RX and TX pins must be connected as shown. Isolators for these pins may have longer propagation delays, but must support the intended baud rate. When not in use, NSDSP drives its TX pin high at all times. To keep the TX pin high while USB is disconnected, select the isolator with high default state. There may be glitches on TX line when NSDSP is powering up and down. To prevent the glitches, a 100kΩ pull-up resistor may be connected to NSDSP's TX pin.

If UART is not needed, do not install the isolatos for TX/RX, leave TX pin unconnected, and connect RX to VDD.

If UART flow control is required, RTS and CTS pins must be connected too. While NSDSP is not in the UART mode with flow control, the state of RTS is undefined. If flow control is not needed, do not install isolators for RTS/CTS, leave RTS unconnected and connect CTS to ground.

For more details on UART communicatons, click here

 

There may be special considerations for your specific PIC device. Please visit the Supported Devices section, find your device and read the documentation.

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