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OSBDM - how I did it, part two

Question asked by Jaromir Sukuba on Dec 11, 2007
Latest reply on Nov 3, 2009 by Carlos Eduardo Carvalho
Hi all,

I've been busy for a last months, mostly because of school and work (the first one thing is getting to the end, second one just starts). That's why I last appeared on this forum with something new more than half a year ago. But now things finally cooled down and while there is first snow covering lately autumn leaves, I decided to finish thing I started in March of this year.
Yes, I'm talking about my version of OSBDM :smileyhappy:

Background of the whole history about how I got to my current version is in this thread
I decided not to continue here, but this can be interpreted as it's addendum.

Well, when comparing to original OSBDM circuit, there are few major changes.

1, Old circuit uses two LVC1T45 devies. First one translates voltage levels from 5V CMOS (on JB16 side) to levels depending on supply voltage of target circuit. Second one holds RESET pin of target in appropriate level to allow entering mode after this pin released while BKGD pin is low. This works fine for all HCS08 devices with dedicated RESET pin, but don't for all other (this problematics was discussed also in other threads). That's why my circuit holds BKGD pin low while powering-up. Heart of this part is double comparator LM393 (IC1A and B on schematics). It's function was explained in my previous thread.
There is also R12(10k), allowing to bleed down capacitors in power supply under POR threshold.

2, Old circuit doesn't contain small power-supply for targets. Sometimes it's very useful - I included 3,3V regulator and mechanical switch. There is also LED indicating if it's on. Who don't need it, may omit the corresponding parts.

3, A bit more fool-proof design. I included P6KE6V8 TVS (transient voltage supressor) in circuit. It's main function is to serve as over-voltage protection, but it's also almost normal silicone diode and it can protect from reversing of target power-supply. Without this diode will be (at least) LVC driver destroyed, which is the second most expensive (and sometimes hard to find) device in OSBDM. This diode also doesn't allow to apply reversed voltage indefinitely long, but it's more robust than IC in small SOT-23 package. Note that reversing of power supply doesn't happen very often, but when developing new circuit on custom PCB's, it's a thing that just can happen.

4, New PCB. I did a "quick-and-dirty" new layout for OSBDM. I feel that this is the weakest part of my design. I'm far away of being professional in this field, but I tried to do it as good as I could. Whole desing moved through four versions. On all of them were some changes - that's why are some of components illogicaly marked (C1, C2, C3 and then C6 etc.). PCB is proposed as single sided (very important for hobbyists or students, I even did one version hand-painted) and populated mostly with SMD components. Through-hole components and wire bridges are used only where necessary. Passive components are mostly 1206 sized, very suitable for hand-soldering.
I'm sure that PCB can be done much better, if someone can do it better, I won't limit his(her) initiative and will welcome the product.

But after all, it works very well. I tested it with QG8, QG4, RD16 and GT16A devices (QG and GT in Vdd range 2.0 - 3.5V) and I can't recall any serious problems with it. I especially focused on reliable entering debug mode and it really works as I wished with my first OSBDM, when I had to push button on BKG pin of QG8 device in order to enter debug mode (very annoying).

Hope it will work for you at least as good as for me!


Zip file with schematics and PCB is here

pictures of my OSBDM version are here (each around 150kB in size)



As you can see here
I added M1 capacitor between RESET pin of JB16 and ground. Without this capacitor I had problems to get JB16 into USB ICP mode.
Interesting fact is that in my previous circuits I didn't need any capacitors. Maybe slight change between silicon revisions.


Second EDIT:

Constructive critism and/or suggestion how to improve it are very appreciated.


Message Edited by jarin on 2007-12-11 11:54 AM

Message Edited by jarin on 2007-12-11 11:58 AM