voltage switching for Vpp

[elfcurry]

I'm looking for a simple way to switch between two voltages for EPROM programming.

The chips I'm programming at present are S87C652, which are OTP 8051 derivatives and need Vpp to be switched between logic '1' and 12.75V, at 10 or 20mA (from memory).

I started making an 8751 programmer many years ago but stalled on this aspect and hand drew one of my very few PCBs which I still have. Then I used a relay because I couldn't think of anything else but I though it would cause problems. I don't think I ever used it on a real chip because I wanted to burn a windowed 8751 and didn't have an eraser so I shelved it.

Later I made a programmer for PIC 16C84 using an analogue switch for Vpp control which worked but felt like the wrong way to use them. Maybe it's OK though, I don't know.

There may be a simple solution using transistors but I'm still vague about using them. I don't really want a solution which is tied to a fixed value for Vpp as this varies from chip to chip. Maybe using two FETs would work as I believe their 'on' resistance is low.

I'm assuming I'll use an external variable PSU for Vpp and have a software controlled switchover between 5V and Vpp but willing to consider other solutions if simple and cheap.

 

[ericgibbs]

I'm looking for a simple way to switch between two voltages for EPROM programming.

The chips I'm programming at present are S87C652, which are OTP 8051 derivatives and need Vpp to be switched between logic '1' and 12.75V, at 10 or 20mA (from memory).

I started making an 8751 programmer many years ago but stalled on this aspect and hand drew one of my very few PCBs which I still have. Then I used a relay because I couldn't think of anything else but I though it would cause problems. I don't think I ever used it on a real chip because I wanted to burn a windowed 8751 and didn't have an eraser so I shelved it.

Later I made a programmer for PIC 16C84 using an analogue switch for Vpp control which worked but felt like the wrong way to use them. Maybe it's OK though, I don't know.

There may be a simple solution using transistors but I'm still vague about using them. I don't really want a solution which is tied to a fixed value for Vpp as this varies from chip to chip. Maybe using two FETs would work as I believe their 'on' resistance is low.

I'm assuming I'll use an external variable PSU for Vpp and have a software controlled switchover between 5V and Vpp but willing to consider other solutions if simple and cheap.

hi,
The method I used was a LM317 [ the TO92 version].

By switching, with logic, the lower Vset resistor its possible to switch +5V or +12.75V.
I used a couple of simple pots to get the two voltages correct.

Do you follow OK.?

EDIT: this is a circuit you could consider.

Change the Vset to suit the EEPROM that you need to program

 

[3v0]

**broken link removed** describes an opamp solution he used in ICD2 clones.

The opamp solution was never as popular as transistors but it takes less parts. I used it in a programmer cluster where I distributed a regulated voltage sufficent to run the opamp. Each programmer board had a 5V regulator and VPP is set by the 47K pot. The intention was to make several of the programmer boards but I only had two made when I ran into the Bill's Junebug PICkit2 USB clone.

Note: Luthar's page used to be one of the top hits when you googled for ICD2 clones. It has fallen so far on the list that I included the image and link here in an effort to help people find this old but good gem.

 

[Pommie]

Whilst Luthar was the originator of the ICD2 clones, I would point you to our own Bill. If you download his designs for the inchworm and junebug they both demonstrate how to switch the 12V line with minimal components.

Mike.

 

[3v0]

I added a link to my previous post.

Luthar's info in dated but to the point regarding the VPP question. If you want to build a programmer Bill has schematics on his site for the Inchworm an ICD2 clone and the PICkit2 clone the Junebug. He also has kits.

The Junebug is the way to go because its USB interface is fast. The RS232 based ICD2 programmers are too slow to work with the large memories used in many PICs.

3v0

Whilst Luthar was the originator of the ICD2 clones, I would point you to our own Bill. If you download his designs for the inchworm and junebug they both demonstrate how to switch the 12V line with minimal components.

Mike.

 

[elfcurry]

Thanks Eric. I was not too sure about your solution to start with as I didn't understand the resistor switching and would have to get a LM317. Now I've looked at the gif you included and I realise I have a LM317T (bigger package, I assume that's irrelevant) it looks good and I'll probably give it a try.

3v0, thanks too. I like the op-amp solution as well. I've even got an LM358 too so I might try that.

I'm spoilt for choice. What kind helpful people.