NSHVX - HVP ExtensionConnectors Bare NSHVX Alternative HVP Curcuit
NSHVX - HVP (High Voltage Programming) Extension
While most PIC microcontrollers support LVP (Low Voltage Programming, where MCLR pin is controlled by logic level voages), some of the PIC devices can only be programmed through HVP (High Voltage Programming which requires higher voltages on MCLR - 8 to 13V). Many PIC devices support both LVP and HVP. It is preferrable to program such devices with LVP, however there may be circumstances which demand HVP programming.
NSDSP programmers cannot generate voltages above VDD levels, and therefore only support LVP programming. If HVP is necessary, NSHVX - an HVP extension for NSDSP - must be used.
Using NSHVX is very easy - it simply attaches to any of the NSDSP-1 programmers (see the picture on the left) and the combination can be used as an HVP programmer.
Most of the signals, with the exception of MCLR and LVP, pass through NSHVX unchanged (see more info about pinout). When NSDSP drives MCLR high, NSHVX connects the output MCLR pin to VDD. When NSDSP drives the MCLR pin low, NSHVX raises voltage on the MCLR pin to high voltage necessary for HVP programming. For most situations, this behaviour is sufficient for successful HVP programming. However, in some rare cases, it may be necessary to drive the MCLR pin low to properly program the target PIC. In such cases, NSDSP drives LVP pin high. NSHVX monitors LVP pin. When the pin is driven high, NSHVX does not produce high voltage on MCLR, but passes the MCLR signal from NSDSP to the PIC unchanged.
The LVP and RTS pins are shorted together inside NSDSP. Therefore, the output RTS pin must be left unconnected or connected to high impedance load. If the output RTS pin is actively driven by the external circuit, NSHVX will not work correctly.
NSHVX has a red LED, which turns on when NSHVX outputs high voltage on the MCLR pin.
NSHVX function is summarized in the following table:
|Input||MCLR Output Voltage||LED|
Some of the PIC microcontrollers require minimum voltage of 10 or 11V on MCLR. However, in our tests all the supported PIC devices could be programmed successfully with 9V, except PIC16(L)F627(A), PIC16(L)F628(A), and PIC16(L)F648A. NSHVX always outputs 9V. Therefore, NSHVX will not work with PIC16(L)F627(A), PIC16(L)F628(A), or PIC16(L)F648A.
NSHVX requres up to 30 mA of power, which must be supplied by either NSDSP, or externally through the VDD pin. Power consumption may be higher if the MCLR has any external loads.
NSHVX always operates on the same VDD voltage as NSDSP. NSHVX itself will work in wide VDD range from 1.8 to 5.5V. However, both NSDSP and PIC may impose restrictions on the operating voltage.
When using NSHVX, the programming software must be configured to use HVP. If you use NSDS Programmer or NSDS Gang Programmer, click on the "Advanced" button and then check the "High Voltage Circuit installed" box. If you use nsprog add the -h option.
9V voltage is dangerous for components designed to work at 3.3 or 5V. It can kill the target PIC. If the MCLR pin is accidentally shorted to ICSPCLK or ICSPDAT pins, NSDSP chip may be killed too. Therefore, it is safer to avoid using NSHVX unless necessary. If LVP programming can be performing, it is better to use LVP.
NSHVX keeps the MCLR pin at VDD voltage when NSDSP is not connected. Connecting NSDSP whith USB cable attached will also keep MCLR voltage at VDD. However, when the USB cable is not connected, some of the NSDSP models (especially NSDSP-1-I) may cause NSHVX to output 9V on MCLR. To avoid such situations, always connect NSDSP to USB before powering your circuit.
NSHVX has a red LED which lights up when 9V is generated on VDD. The LED is powered by VDD and the brihtness may be low when VDD is below 2V.
Alternatives to NSHVX
You can build your own HVP circuit, which requires an external source of high voltage. Such circuit can replace NSHVX in most scenarios. However, NSHVX also uses signaling on the PGM/LVP pin, which may be necessary to program some of the PIC devices. Navigate to device support pages, find your PIC, and read the HVP section to see if NSHVX is necessary for your PIC.
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