I changed my mind, I am going to add the High voltage supply before I go to layout.

I am going to use a switching boost regulator to give me between 6 an 12 volts to supply to Vpp on the target.  I mentioned a couple of weeks ago that I found a chip to achieve this goal.  I’ll be using the AAT1230. I created the symbol in the library and placed it on the schematic.

I need to research the highest voltages needed for Vpp, I am designing the circuit with a range from 7 to 14V for Vpp.  I am using a simple voltage divider on feedback circuit to set the regulated voltage.  With a threshold of 0.6V the setting resistors need to be calculated as follows R2/(R1+R2) = 0.6/7. From the datasheet R2 is specified at 4.99K. I simplify by inserting the value for R2 and multiplying by 7. The result is 34.93K/(4.99K+R1) = 0.6. I then multiply both sides by 4.99K+R1 to simplify again. Now the equation looks like 34.93K=2.994K+0.6 x R1. I subtract 2.994K from both sides to simplify again I get 31.936K=0.6xR1. I now divide by 0.6 on both sides of the equation and I get 53.23K = R1. To validate the number I go back to the original formula which is the desired voltage(7) times the resistor tied to ground(4.99K ohms) divided by the sum of the resistors(4.99K+53.23K = 58.22K) which has to equal my threshold value of 0.6V.  7×4.99/58.22 = 0.60 (with a little rounding)  Using the programming line I can set Vpp from 7V to 14V output in 16 steps. Each step will be just under 1/2 a volt.

In my experience Vpp is defined as a range.  For the Microchip PIC16F87X series chips the range is 13+0.5V  The output voltage can be set in that range.

I have attached this voltage divider circuit to the analog inputs of the microchip USB chip  this will allow me to check if we are getting good voltage out of the regulator.  I am also using the microchip chip to set the voltage.  If I hold the set line low for longer than 500 μS the regulator will reset down to the 7V setting. If I pulse it low for less than 500 μS it will increment up the steps.

I also tied a voltage divide to Vt to measure target voltage as well.

Sorry for all the math, but as an engineer sometimes it’s unavoidable.