ICD2 Variable VPP?

[Mike - K8LH]

In the original ICD2 schematic MC uses an MCP41010 digital pot connected to the '877 RB1, RB6, and RB7 pins for the SPI interface and driving an MC34063 switching regulator.

Does the ICD2 really control the VPP voltage? If so, is there any reason you can think of that would prevent me from designing a little 8-pin 12F683 boost regulator with an SPI interface that would emulate the 256-steps of the digital pot to provide the same "variable VPP" function in a single circuit for my ICD2-USB clone design?

TIA, Mike

 

[Jay.slovak]

Hi Mike,

I am designing new ICD2 clone as well and I have been thinking about this too. Although I do have the MCP part on the shelf, I will replace it with a simple trimmer set to 12.5V (this value works the best for me). I used to program 18F with 13V without problems, so fixed VPP shouldn't be a problem.

EDIT: 12F683 emulating MCP + DC-DC booster is just not worth the effort IMHO

 

[Mike - K8LH]

Hi Jay,

So, what are you using to generate VPP?

Mike

 

[Jay.slovak]

Currently MC34063 but will switch to (simplier) MAX632.

 

[Mike - K8LH]

Ok, so you've got an IC and a trimmer (pot). That's cool.

I agree that it may be too much work/effort. If it could be done though, I think I'd prefer a single IC solution with ICD2 controlled variable VPP. Less board space, more control, great "cool" factor (grin).



Mike

 

[William At MyBlueRoom]

In the original ICD2 schematic MC uses an MCP41010 digital pot connected to the '877 RB1, RB6, and RB7 pins for the SPI interface and driving an MC34063 switching regulator.

Does the ICD2 really control the VPP voltage? If so, is there any reason you can think of that would prevent me from designing a little 8-pin 12F683 boost regulator with an SPI interface that would emulate the 256-steps of the digital pot to provide the same "variable VPP" function in a single circuit for my ICD2-USB clone design?

TIA, Mike

Here's the MCP41010 in a ICD2 schematic from CZ...

 

[Mike - K8LH]

Yes, I've seen that schematic Bill. Thanks.

I was thinking about something like the drawing below. A single chip that emulates the 256-step MCP41010 digi' pot and provides the variable boost VPP output voltage.

Regards, Mike

 

[Nigel Goodwin]

Yes, I've seen that schematic Bill. Thanks.

I was thinking about something like the drawing below. A single chip that emulates the 256-step MCP41010 digi' pot and provides the variable boost VPP output voltage.

Doesn't the PICKIT1 already do that to generate the 13V Vpp from the 5V USB voltage?.

 

[Mike - K8LH]

Hi Nigel,

That PWM drive is right out of the PICKit2 schematic.

This circuit, if it can be done, would basically combine the function of the MCP41010 digital pot' and the MC34063 switching regulator from the original ICD2 design, all in a single IC. And, the ICD2 firmware is already set up to control it. Cool, huh?

Regards, Mike

 

[Nigel Goodwin]

Hi Nigel,

That PWM drive is right out of the PICKit2 schematic.

This circuit, if it can be done, would basically combine the function of the MCP41010 digital pot' and the MC34063 switching regulator from the original ICD2 design, all in a single IC. And, the ICD2 firmware is already set up to control it. Cool, huh?

Yep!, it certainly is!

 

[William At MyBlueRoom]

Should'nt there be feedback? I think since the ICD2 uses a linear PS and a LM317 once the voltage is set that's that.
But with a boost converter you'll have to monitor the output somehow dynamically...

 

[Mike - K8LH]

Please check out the original ICD2 schematic with MCP41010 and MC34063. The '877/A monitors the voltage via ADC and then adjusts the digi' pot. This should work, shouldn't it? Or should I consider 12F683 A2D PID loop to make it all that much more "bullet proof"?

Mike

 

[Jay.slovak]

I would certainly add some sort of Feed-back to the 12F683 (and a suitable control algoritm). Without it, the generated voltage could change as the current draw changes.

 

[Mike - K8LH]

Hi Jay,

Do you think it's worth putting the effort into the project? I'm still not sure how the ICD2 firmware utilizes that digital pot control function. Does it actually vary VPP voltage depending on Target device?

Regards, Mike

 

[Jay.slovak]

Hi Mike,

As I already said "12F683 emulating MCP + DC-DC booster is just not worth the effort IMHO".

It is very little known about ICD2s control over Vpp. My guess is, each firmware sets it's own Vpp (12.5V for 18F, 12F for 12F683, whatever) and keeps it that way. I think the value is Hard-coded in the Firmware.

PS: Don't take me wrong. I think that 12F683 solution is elegant. I just don't think developing it for saving one IC and build only one or two ICDs is a waste of your (precious) development time. But, if you were to make a thousand ICDs, price savings could be significant.

 

[williB]

Hi mike ,
a quick question,
what frequency would the pwm for the boost converter be running ?

 

[Jay.slovak]

I have computed the Vpp range ICD2 is capable of achieving.

 

[William At MyBlueRoom]

Mike I like your idea for an SPI PIC boost converter, heres one with some code. I'd like to put it in the next project. I've built a LC meter using a 16F628A (project on the net) excellent for testing those pesky inductors.

**broken link removed**

The fellow wrote it in BASIC... I have an ICE for a PIC12CE674 so I can test the code real time in circuit. We can port it to a more modern PIC.

 

[Mike - K8LH]

Mike I like your idea for an SPI PIC boost converter, heres one with some code. I'd like to put it in the next project. I've built a LC meter using a 16F628A (project on the net) excellent for testing those pesky inductors.

**broken link removed**

The fellow wrote it in BASIC... I have an ICE for a PIC12CE674 so I can test the code real time in circuit. We can port it to a more modern PIC.

I ran across that very same article recently and it pointed me to the MC application note with the boost formulas.

After reading the article I realise you're correct and I/we need to close the loop right at the 12F683. Good news is the 12F683 will support the A/D function simply by changing the hardware slightly.

Your MAX232 doubler is really quite elegant but I'm interested in this PIC 1-chip solution for my ICD2-USB clone which will not have a MAX232 serial port on it.

-----

Which LC Tester did you build? How does it work?

I came across an old design using an 'F84 and I ported it to a '628A and eliminated the LM311 comparator by using the built-in '628A comparator (and measuring the frequency using TMR1). Unfortunately my prototype board version wasn't very stable and I never quite got back to modify it for an LM311.

Mike

 

[Mike - K8LH]

Hi mike ,
a quick question,
what frequency would the pwm for the boost converter be running ?

Hi Willi,

I haven't worked through the formulas yet, nor have I had time to throw the circuit together on a prototype board. But I'd be happy to keep you up to date as research continues.

Take care. Regards, Mike