From: "Bertrik Sikken" <bertrik@bert...>
> Perhaps we can improve upon the existing designs
and create
> the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> Some ideas:
> * provide proper current limiting for all lines
> * add readback of the RTCK signal (return clock)
> * add readback of the targets' VCC (like the Olimex clone does)
Also:
* use proper level shifting buffers or separate buffers for the LPT port
side(5V logic) and the target side (target voltage).
* use low-pass filters (RC) + schmidttrigger receivers on the LPT port side.
* series termination resistor on each driven line, to LPT and to target.
* a (jumper selectable) nTRST drive, separate from the nRESET drive.
* optional: a 1mA LED indicator on the target side VCC.
The problems occur not just with building the thing - it's the wiggler
design itself that is a bit flakey, especially the lack of a noise filter
and schmidttrigger receiver on the CLK input at the LPT side. A 100...400
nsec RC filter should do it while still allowing up to 1 MHz clocking (above
LPT port speed levels). If this is not present, long LPT cables or improper
grounding will cause glitches that upset the JTAG state machine.
I fail to see the actual use of the RTCK signal - is this to be tested by SW
on the LPT status bits? Which CPU is so slow that this would make a
difference?
Regards,
Arie de Muijnck
Building DIY wiggler w/74VHC08
Started by ●January 16, 2006
Reply by ●January 16, 20062006-01-16
Reply by ●January 16, 20062006-01-16
----- Original Message -----
From: "donhamilton2002" <hamilton@hami...>
To: <lpc2000@lpc2...>
Sent: Monday, January 16, 2006 9:21 PM
Subject: [lpc2000] Re: Building DIY wiggler w/74VHC08 > --- In lpc2000@lpc2..., Sean <embeddedrelated@w...> wrote:
>>
>> Hamilton:
>>
>> What did you do differently between the two that you built?
>>
>> -- Sean
>>
>>
> This first one I built was from parts in my junk box.
> A scrap vector board and a db-25 connector.
>
> Through it together one late night.
> Looked like hell, but its such a simple circuit.
>
> Never worked.
>
> Second one, I built with a little more care.
> Some new parts, some scrap parts.
>
> It looked like it worked, and the slowest speed. But it was flakey.
> Failed more then it worked. ( Atmel demo board)
> So I bought one from sparkfun.com
>
> That one worked fine. So I built another just to see if I lost
> my knack in building things.
>
> The last one seemed to work OK, but still flakey.
> I just gave up and use the pre-built one.
>
> I have been building things for 20 years. This is not new.
> Whenever I see someone posting a message that they can not get
> a wiggler working, I have to ask "did you build it yourself ?".
> Most times its yes, "but its such a simple circuit !!"
If my PCB design is used, it should work every time.
Leon
From: "donhamilton2002" <hamilton@hami...>
To: <lpc2000@lpc2...>
Sent: Monday, January 16, 2006 9:21 PM
Subject: [lpc2000] Re: Building DIY wiggler w/74VHC08 > --- In lpc2000@lpc2..., Sean <embeddedrelated@w...> wrote:
>>
>> Hamilton:
>>
>> What did you do differently between the two that you built?
>>
>> -- Sean
>>
>>
> This first one I built was from parts in my junk box.
> A scrap vector board and a db-25 connector.
>
> Through it together one late night.
> Looked like hell, but its such a simple circuit.
>
> Never worked.
>
> Second one, I built with a little more care.
> Some new parts, some scrap parts.
>
> It looked like it worked, and the slowest speed. But it was flakey.
> Failed more then it worked. ( Atmel demo board)
> So I bought one from sparkfun.com
>
> That one worked fine. So I built another just to see if I lost
> my knack in building things.
>
> The last one seemed to work OK, but still flakey.
> I just gave up and use the pre-built one.
>
> I have been building things for 20 years. This is not new.
> Whenever I see someone posting a message that they can not get
> a wiggler working, I have to ask "did you build it yourself ?".
> Most times its yes, "but its such a simple circuit !!"
If my PCB design is used, it should work every time.
Leon
Reply by ●January 16, 20062006-01-16
----- Original Message -----
From: "Tom Walsh" <tom@tom@...>
To: <lpc2000@lpc2...>
Sent: Monday, January 16, 2006 9:58 PM
Subject: Re: [lpc2000] Building DIY wiggler w/74VHC08 > Ed Schlunder wrote:
>
>>I'm thinking of building the DIY wiggler so that I can program and
>>debug some LPC2103 chips on the cheap.
>>
>>I see the wiggler.txt schematic in the files area. It is using a
>>74AC244 chip to do voltage level conversion between the PC and the
>>LPC21xx chip. It only seems to use 4 line drivers of the 74AC244 chip
>>(half the chip is left unused).
>>
>>Would there be any problem with using a 74VHC08 quad 2 input AND gate
>>chip instead? I would just tie one side of each AND gate high so that
>>it can be used as 4 line drivers. This chip is cheaper and smaller
>>than the 74AC244.
>>
>>
>>
> However, it may not have the same drive levels (output current). The
> '244 series is a buffer / driver, typically producting 12ma sink and 4ma
> source. The '08 is a logic device, not a driver, you will probably
> only find that it can do +/- 4ma (if you are lucky).
>
> Output impedance would dictate how quickly signals may be driven high /
> low on the JTAG interface. Too high an impedance == lousy signal edges
> / ring / overshoot.
I think that is probably the case, contrary to what I said earlier. Ground
bounce due to poor construction techniques may be one reason why some people
have problems getting it to work reliably.
Leon
From: "Tom Walsh" <tom@tom@...>
To: <lpc2000@lpc2...>
Sent: Monday, January 16, 2006 9:58 PM
Subject: Re: [lpc2000] Building DIY wiggler w/74VHC08 > Ed Schlunder wrote:
>
>>I'm thinking of building the DIY wiggler so that I can program and
>>debug some LPC2103 chips on the cheap.
>>
>>I see the wiggler.txt schematic in the files area. It is using a
>>74AC244 chip to do voltage level conversion between the PC and the
>>LPC21xx chip. It only seems to use 4 line drivers of the 74AC244 chip
>>(half the chip is left unused).
>>
>>Would there be any problem with using a 74VHC08 quad 2 input AND gate
>>chip instead? I would just tie one side of each AND gate high so that
>>it can be used as 4 line drivers. This chip is cheaper and smaller
>>than the 74AC244.
>>
>>
>>
> However, it may not have the same drive levels (output current). The
> '244 series is a buffer / driver, typically producting 12ma sink and 4ma
> source. The '08 is a logic device, not a driver, you will probably
> only find that it can do +/- 4ma (if you are lucky).
>
> Output impedance would dictate how quickly signals may be driven high /
> low on the JTAG interface. Too high an impedance == lousy signal edges
> / ring / overshoot.
I think that is probably the case, contrary to what I said earlier. Ground
bounce due to poor construction techniques may be one reason why some people
have problems getting it to work reliably.
Leon
Reply by ●January 16, 20062006-01-16
--- In lpc2000@lpc2..., "Leon Heller" <leon.heller@b...>
wrote:
>
>
> I think that is probably the case, contrary to what I said earlier.
Ground
> bounce due to poor construction techniques may be one reason why
some people
> have problems getting it to work reliably.
>
> Leon
>
There is also another problem that can occur, and is VERY difficult to
detect sometimes. Without a Schmidt trigger input, if the input
voltage to a logic device changes too slowly through the transition
region, the output will be metastable or oscillate. It happened to me
one time with a circuit that had a clock speed of just a few hundred
kilohertz, and the logic would not work! In this case the clock input
was from an analog source that had a relatively slow transition time,
so the clock inside my logic had bursts of 50MHz oscillation on each
clock transition. When you have a scope set to look at a low frequency
signal of a few hundred kilohertz it was really hard to notice the
much higher 50MHz signal in the transistions, but the logic knew!!!!
So you need to be very careful with slew limiting the logic signals
with logic not designed to work with slow transitions.
--Dave
>
>
> I think that is probably the case, contrary to what I said earlier.
Ground
> bounce due to poor construction techniques may be one reason why
some people
> have problems getting it to work reliably.
>
> Leon
>
There is also another problem that can occur, and is VERY difficult to
detect sometimes. Without a Schmidt trigger input, if the input
voltage to a logic device changes too slowly through the transition
region, the output will be metastable or oscillate. It happened to me
one time with a circuit that had a clock speed of just a few hundred
kilohertz, and the logic would not work! In this case the clock input
was from an analog source that had a relatively slow transition time,
so the clock inside my logic had bursts of 50MHz oscillation on each
clock transition. When you have a scope set to look at a low frequency
signal of a few hundred kilohertz it was really hard to notice the
much higher 50MHz signal in the transistions, but the logic knew!!!!
So you need to be very careful with slew limiting the logic signals
with logic not designed to work with slow transitions.
--Dave
Reply by ●January 17, 20062006-01-17
Thank you for all the suggestions. I have put together an initial
schematic diagram using 74VHC14 schmitt trigger inverters and RC low
pass filters (as suggested here):
http://www.k9spud.com/jtag/
I'm not sure how to add additional signals like RTCK and nTRST. Won't
these kind of changes require software support on the PC to be useful?
My understanding was that the Macraigor software was not open source.
Where should these signals be connected to on the PC side?
Or are you just suggesting a jumper for the user to manually switch by
hand?
What is the nTRST signal useful for? I'm new to JTAG, just trying to
get my first ARM project started.
--- In lpc2000@lpc2..., "Arie de Muynck" <mygroups@x...> wrote:
>
> From: "Bertrik Sikken" <bertrik@z...>
> > Perhaps we can improve upon the existing designs and create
> > the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> > Some ideas:
> > * provide proper current limiting for all lines
> > * add readback of the RTCK signal (return clock)
> > * add readback of the targets' VCC (like the Olimex clone does)
>
> Also:
>
> * use proper level shifting buffers or separate buffers for the LPT port
> side(5V logic) and the target side (target voltage).
> * use low-pass filters (RC) + schmidttrigger receivers on the LPT
port side.
> * series termination resistor on each driven line, to LPT and to target.
> * a (jumper selectable) nTRST drive, separate from the nRESET drive.
> * optional: a 1mA LED indicator on the target side VCC.
>
> The problems occur not just with building the thing - it's the wiggler
> design itself that is a bit flakey, especially the lack of a noise
filter
> and schmidttrigger receiver on the CLK input at the LPT side. A
100...400
> nsec RC filter should do it while still allowing up to 1 MHz
clocking (above
> LPT port speed levels). If this is not present, long LPT cables or
improper
> grounding will cause glitches that upset the JTAG state machine.
>
> I fail to see the actual use of the RTCK signal - is this to be
tested by SW
> on the LPT status bits? Which CPU is so slow that this would make a
> difference?
schematic diagram using 74VHC14 schmitt trigger inverters and RC low
pass filters (as suggested here):
http://www.k9spud.com/jtag/
I'm not sure how to add additional signals like RTCK and nTRST. Won't
these kind of changes require software support on the PC to be useful?
My understanding was that the Macraigor software was not open source.
Where should these signals be connected to on the PC side?
Or are you just suggesting a jumper for the user to manually switch by
hand?
What is the nTRST signal useful for? I'm new to JTAG, just trying to
get my first ARM project started.
--- In lpc2000@lpc2..., "Arie de Muynck" <mygroups@x...> wrote:
>
> From: "Bertrik Sikken" <bertrik@z...>
> > Perhaps we can improve upon the existing designs and create
> > the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> > Some ideas:
> > * provide proper current limiting for all lines
> > * add readback of the RTCK signal (return clock)
> > * add readback of the targets' VCC (like the Olimex clone does)
>
> Also:
>
> * use proper level shifting buffers or separate buffers for the LPT port
> side(5V logic) and the target side (target voltage).
> * use low-pass filters (RC) + schmidttrigger receivers on the LPT
port side.
> * series termination resistor on each driven line, to LPT and to target.
> * a (jumper selectable) nTRST drive, separate from the nRESET drive.
> * optional: a 1mA LED indicator on the target side VCC.
>
> The problems occur not just with building the thing - it's the wiggler
> design itself that is a bit flakey, especially the lack of a noise
filter
> and schmidttrigger receiver on the CLK input at the LPT side. A
100...400
> nsec RC filter should do it while still allowing up to 1 MHz
clocking (above
> LPT port speed levels). If this is not present, long LPT cables or
improper
> grounding will cause glitches that upset the JTAG state machine.
>
> I fail to see the actual use of the RTCK signal - is this to be
tested by SW
> on the LPT status bits? Which CPU is so slow that this would make a
> difference?
Reply by ●January 17, 20062006-01-17
For 74AC08 you can have "Outputs source/sink 24 mA",
http://www.fairchildsemi.com/pf/74/74AC08.html
Adrian
--- Tom Walsh <tom@tom@...> wrote:
> Ed Schlunder wrote:
>
> >I'm thinking of building the DIY wiggler so that I
> can program and
> >debug some LPC2103 chips on the cheap.
> >
> >I see the wiggler.txt schematic in the files area.
> It is using a
> >74AC244 chip to do voltage level conversion between
> the PC and the
> >LPC21xx chip. It only seems to use 4 line drivers
> of the 74AC244 chip
> >(half the chip is left unused).
> >
> >Would there be any problem with using a 74VHC08
> quad 2 input AND gate
> >chip instead? I would just tie one side of each AND
> gate high so that
> >it can be used as 4 line drivers. This chip is
> cheaper and smaller
> >than the 74AC244.
> >
> >
> >
> However, it may not have the same drive levels
> (output current). The
> '244 series is a buffer / driver, typically
> producting 12ma sink and 4ma
> source. The '08 is a logic device, not a driver,
> you will probably
> only find that it can do +/- 4ma (if you are lucky).
>
> Output impedance would dictate how quickly signals
> may be driven high /
> low on the JTAG interface. Too high an impedance ==
> lousy signal edges
> / ring / overshoot.
>
> TomW > --
> Tom Walsh - WN3L - Embedded Systems Consultant
> http://openhardware.net, http://cyberiansoftware.com
> "Windows? No thanks, I have work to do..."
> ---------------- >
Adrian Valeanu.
SC PROSAFE SRL
J/40/11835/2002 ; CUI 15026939
Aleea Politehnicii nr. 2
Bl. 5A, sc. 1 Ap. 17 Sector 6, BUCHAREST
ROMANIA
Tel/fax +40 (21) 781.38.58
__________________________________________________
http://www.fairchildsemi.com/pf/74/74AC08.html
Adrian
--- Tom Walsh <tom@tom@...> wrote:
> Ed Schlunder wrote:
>
> >I'm thinking of building the DIY wiggler so that I
> can program and
> >debug some LPC2103 chips on the cheap.
> >
> >I see the wiggler.txt schematic in the files area.
> It is using a
> >74AC244 chip to do voltage level conversion between
> the PC and the
> >LPC21xx chip. It only seems to use 4 line drivers
> of the 74AC244 chip
> >(half the chip is left unused).
> >
> >Would there be any problem with using a 74VHC08
> quad 2 input AND gate
> >chip instead? I would just tie one side of each AND
> gate high so that
> >it can be used as 4 line drivers. This chip is
> cheaper and smaller
> >than the 74AC244.
> >
> >
> >
> However, it may not have the same drive levels
> (output current). The
> '244 series is a buffer / driver, typically
> producting 12ma sink and 4ma
> source. The '08 is a logic device, not a driver,
> you will probably
> only find that it can do +/- 4ma (if you are lucky).
>
> Output impedance would dictate how quickly signals
> may be driven high /
> low on the JTAG interface. Too high an impedance ==
> lousy signal edges
> / ring / overshoot.
>
> TomW > --
> Tom Walsh - WN3L - Embedded Systems Consultant
> http://openhardware.net, http://cyberiansoftware.com
> "Windows? No thanks, I have work to do..."
> ---------------- >
Adrian Valeanu.
SC PROSAFE SRL
J/40/11835/2002 ; CUI 15026939
Aleea Politehnicii nr. 2
Bl. 5A, sc. 1 Ap. 17 Sector 6, BUCHAREST
ROMANIA
Tel/fax +40 (21) 781.38.58
__________________________________________________
Reply by ●January 17, 20062006-01-17
Why do you need to use 5V for the LPT side? Most parts can be powered from
either 3.3v or 5v and are tolerant to either regardless of the voltage they
are running at?
-- Sean
At 05:01 PM 1/16/2006, you wrote:
>From: "Bertrik Sikken" <bertrik@bert...>
> > Perhaps we can improve upon the existing designs and create
> > the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> > Some ideas:
> > * provide proper current limiting for all lines
> > * add readback of the RTCK signal (return clock)
> > * add readback of the targets' VCC (like the Olimex clone does)
>
>Also:
>
>* use proper level shifting buffers or separate buffers for the LPT port
> side(5V logic) and the target side (target voltage).
>* use low-pass filters (RC) + schmidttrigger receivers on the LPT port side.
>* series termination resistor on each driven line, to LPT and to target.
>* a (jumper selectable) nTRST drive, separate from the nRESET drive.
>* optional: a 1mA LED indicator on the target side VCC.
>
>The problems occur not just with building the thing - it's the wiggler
>design itself that is a bit flakey, especially the lack of a noise filter
>and schmidttrigger receiver on the CLK input at the LPT side. A 100...400
>nsec RC filter should do it while still allowing up to 1 MHz clocking (above
>LPT port speed levels). If this is not present, long LPT cables or improper
>grounding will cause glitches that upset the JTAG state machine.
>
>I fail to see the actual use of the RTCK signal - is this to be tested by SW
>on the LPT status bits? Which CPU is so slow that this would make a
>difference?
>
>Regards,
>Arie de Muijnck
Reply by ●January 17, 20062006-01-17
Just throwing out some additional ideas, as I am in the same boat you are
(just starting out here):
For the +5v (is this really needed?) You can try something akin to what
you can do for a simple RS232 level driver:
http://www.piclist.com/techref/io/serial/ttl-rs232.htm
Basically using some resistors, diodes and caps you can steal power from
individual lines, gang them together if needed. You'll need diodes with
really low level drops however to make this effective.
Alternatively you could try a simple voltage doubler circuit with a pair of
normal-drop diodes. 3.3 * 2 = 6.6 - 2*0.7 = 5.2V which should work pretty
well for the current levels required.
The wall brick really isn't necessary, and will be expensive and
cumbersome. You could always use something like MAX682/3/4 charge pumps
(still cheaper than wall brick, or get a few samples for free), or
alternately something like the
LM4871 audio amp can give a simple 3.3V to 5V conversion:
http://www.edn.com/article/CA240909.html
Or possibly a NJU7261U50 switching regulator (about $1 plus only a few
external components) for 3.3V to 5V step-up at 250mA.
You may want to take a look at the 74LVC4245A part (CMOS OCTAL BUS
TRANSCEIVER AND 3.3V TO 5V SHIFTER; but without schmitt), instead of the
inverters.
As well since there are additional IO pins on the LPT side unused, is there
anything we can do with this?
Perhaps add in an LED or two to give a visual indication of what is going on?
As well instead of using a 74HC14, how about 74AC08 quad AND-gate? You can
drive higher current, and you only need 4 gates anyway. You won't get your
5V side logic or schmitt either though (again, is this necessary?)
I know of several places of getting PCBs made fairly cheap, so if we can
work out something that works well I wouldn't find footing some cash to
have a pile of PCBs made to sell to the DIY-ers out there.
-- Sean
At 11:54 PM 1/16/2006, you wrote:
>Thank you for all the suggestions. I have put together an initial
>schematic diagram using 74VHC14 schmitt trigger inverters and RC low
>pass filters (as suggested here):
>
><http://www.k9spud.com/jtag/" target="_blank" rel="nofollow">http://www.k9spud.com/jtag/>http://www.k9spud.com/jtag/
>
>I'm not sure how to add additional signals like RTCK and nTRST. Won't
>these kind of changes require software support on the PC to be useful?
>My understanding was that the Macraigor software was not open source.
>Where should these signals be connected to on the PC side?
>
>Or are you just suggesting a jumper for the user to manually switch by
>hand?
>
>What is the nTRST signal useful for? I'm new to JTAG, just trying to
>get my first ARM project started.
>
>--- In lpc2000@lpc2..., "Arie de Muynck" <mygroups@x...> wrote:
> >
> > From: "Bertrik Sikken" <bertrik@z...>
> > > Perhaps we can improve upon the existing designs and create
> > > the ultimate low-cost ARM JTAG cable, yet keeping it simple.
> > > Some ideas:
> > > * provide proper current limiting for all lines
> > > * add readback of the RTCK signal (return clock)
> > > * add readback of the targets' VCC (like the Olimex clone does)
> >
> > Also:
> >
> > * use proper level shifting buffers or separate buffers for the LPT port
> > side(5V logic) and the target side (target voltage).
> > * use low-pass filters (RC) + schmidttrigger receivers on the LPT
>port side.
> > * series termination resistor on each driven line, to LPT and to target.
> > * a (jumper selectable) nTRST drive, separate from the nRESET drive.
> > * optional: a 1mA LED indicator on the target side VCC.
> >
> > The problems occur not just with building the thing - it's the wiggler
> > design itself that is a bit flakey, especially the lack of a noise
>filter
> > and schmidttrigger receiver on the CLK input at the LPT side. A
>100...400
> > nsec RC filter should do it while still allowing up to 1 MHz
>clocking (above
> > LPT port speed levels). If this is not present, long LPT cables or
>improper
> > grounding will cause glitches that upset the JTAG state machine.
> >
> > I fail to see the actual use of the RTCK signal - is this to be
>tested by SW
> > on the LPT status bits? Which CPU is so slow that this would make a
> > difference? >
>SPONSORED LINKS
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><http://groups.yahoo.com/gads?t=ms&k=Microcontrollers&w1=Microprocessor&w2=Microcontrollers&w3=Pic+microcontrollers&w451+microprocessor&c=4&s&.sig=DvJVNqC_pqRTm8Xq01nxwg>Microcontrollers
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>microprocessor >----------
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Reply by ●January 17, 20062006-01-17
> From: "Leon Heller" <leon.heller@leon...>
> I think that is probably the case, contrary to what I said
> earlier. Ground bounce due to poor construction techniques may be
> one reason why some people have problems getting it to work
> reliably.
Some PC printer ports generate fast (<10ns) spikes of up to 1
volt amplitude on some pins. They can be very difficult to
remove, and will often trigger buffers that have TTL threshold
levels. Most printer ports now run at about 3v and if the device
is powered through the printer port, it is probably operating
way outside its datasheet specs.
Stephen
>
> Leon
>
--
Stephen Pelc, stephen@step...
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
tel: +44 23 80 631441, fax: +44 23 80 339691
web: http://www.mpeforth.com - free VFX Forth downloads
> I think that is probably the case, contrary to what I said
> earlier. Ground bounce due to poor construction techniques may be
> one reason why some people have problems getting it to work
> reliably.
Some PC printer ports generate fast (<10ns) spikes of up to 1
volt amplitude on some pins. They can be very difficult to
remove, and will often trigger buffers that have TTL threshold
levels. Most printer ports now run at about 3v and if the device
is powered through the printer port, it is probably operating
way outside its datasheet specs.
Stephen
>
> Leon
>
--
Stephen Pelc, stephen@step...
MicroProcessor Engineering Ltd - More Real, Less Time
133 Hill Lane, Southampton SO15 5AF, England
tel: +44 23 80 631441, fax: +44 23 80 339691
web: http://www.mpeforth.com - free VFX Forth downloads
Reply by ●January 17, 20062006-01-17
----- Original Message -----
From: "Stephen Pelc" <stephen@step...>
To: <lpc2000@lpc2...>
Sent: Tuesday, January 17, 2006 10:55 AM
Subject: [lpc2000] Building DIY wiggler w/74VHC08 >> From: "Leon Heller" <leon.heller@leon...>
>
>> I think that is probably the case, contrary to what I said
>> earlier. Ground bounce due to poor construction techniques may be
>> one reason why some people have problems getting it to work
>> reliably.
>
> Some PC printer ports generate fast (<10ns) spikes of up to 1
> volt amplitude on some pins. They can be very difficult to
> remove, and will often trigger buffers that have TTL threshold
> levels. Most printer ports now run at about 3v and if the device
> is powered through the printer port, it is probably operating
> way outside its datasheet specs.
Mine is powered by the target system. Leon
From: "Stephen Pelc" <stephen@step...>
To: <lpc2000@lpc2...>
Sent: Tuesday, January 17, 2006 10:55 AM
Subject: [lpc2000] Building DIY wiggler w/74VHC08 >> From: "Leon Heller" <leon.heller@leon...>
>
>> I think that is probably the case, contrary to what I said
>> earlier. Ground bounce due to poor construction techniques may be
>> one reason why some people have problems getting it to work
>> reliably.
>
> Some PC printer ports generate fast (<10ns) spikes of up to 1
> volt amplitude on some pins. They can be very difficult to
> remove, and will often trigger buffers that have TTL threshold
> levels. Most printer ports now run at about 3v and if the device
> is powered through the printer port, it is probably operating
> way outside its datasheet specs.
Mine is powered by the target system. Leon