Beginners question

Dear Group,

I have a xilinx xc9536 , a 22 bit counter, and a 5 bit counter.
The 22 bit counter is used as a 48mhz clock divider for the
other 5 bit counter, hence bit 22 is used as humanly visible
clock. The counter stops when it reaches 11111.
The problem I am having is that when I apply my digital
multi meter to the pins of the counter to check if the count
has finished, then the counter seems to suddenly start
counting again. I also noticed that if before applying the
multimeter I disconnect the oscillator from the chip this will
not happen, so it seems the oscillator is making the design
unstable. Any ideas?
Also, as a beginner, which book do I need to understand
the tricks and quirks of xc9536 high speed design. Should
I be designing on a behavioural level, or should I use low
level?

My clock code is as follows

reg [21:0] nn;
assign cdiv= nn[21];
always @(posedge c0)
  nn=nn+1;

my other clock goes like this

always @(posedge clock or posedge reset1)
  begin
    if (reset1)
        aa <= 0;
    else aa <= aa + 1;
  end

The clocks are physically connected from external pins with a wire.
For the oscillator I use gck1.

Many thanks.

> The problem I am having is that when I apply my digital
> multi meter to the pins of the counter to check if the count
> has finished, then the counter seems to suddenly start
> counting again. I also noticed that if before applying the

	48 MHz ? With a multimeter ?

	Just solder a LED instead... or put a resistor at the end of your  
probe... resistor scope probe is THE low-tech low-cost "but it works!" way  
for high speed probing ;)

	A few possible causes :

	- you're wearing wool or something and generate enough static electricity  
to zap the circuits ; ground yourself properly before it costs you a  
replacement board
	- the parasitic cap of the probe is enough to cause mayhem, ringing, etc

> multimeter I disconnect the oscillator from the chip this will
> not happen, so it seems the oscillator is making the design
> unstable. Any ideas?

	Well, without clock, all synchronous logic is on blind-deaf mode, and you  
used synchronous logic for your counter, so...

Dear User,

> 48 MHz ? With a multimeter ?
>
> Just solder a LED instead... or put a resistor at the end of your
> probe... resistor scope probe is THE low-tech low-cost "but it works!" way
> for high speed probing ;)
>

I do use a led, and this is why I divide the clock with 22 bits, so I can
actually see the led flashing with different speeds on different counter
pins.


> A few possible causes :
>
> - you're wearing wool or something and generate enough static electricity
> to zap the circuits ; ground yourself properly before it costs you a
> replacement board
> - the parasitic cap of the probe is enough to cause mayhem, ringing, etc
>
> > multimeter I disconnect the oscillator from the chip this will
> > not happen, so it seems the oscillator is making the design
> > unstable. Any ideas?
>
> Well, without clock, all synchronous logic is on blind-deaf mode, and you
> used synchronous logic for your counter, so...

Yes, this is silly of me, of course without a clock it cannot move.

I was just wondering if there were any special checks I should make
because I am using 50mhz. I think I can use a PIC to simulate a slower
clock, I'll try that.

And I forgot to mention as well that I have a "done" pin, which goes high
when counter reaches "11111". Again, if I touch that pin with a led it will
start counting again. This register actually controlls if the clock is
multiplexed into the divider counter, when done=1 it stops the oscillator
from
reaching the divider. Clearing it would cause the counter to continue.

Maybe when I touch the "done" pin with a led it acts as a momentary
short ground. I can't think why that would be, my led is very low current,
it is limitted with a relatively large 10K.

I also have some other inputs, which once the clock is on 11111 should
not make any difference, but again if I touch them with a led, or even
just leave them floating, the counter starts to count again.

> I was just wondering if there were any special checks I should make
> because I am using 50mhz. I think I can use a PIC to simulate a slower
> clock, I'll try that.

I hate to ask the obvious - but have you simulated the design at this
clock rate? I haven't used the xc95xx stuff but certainly when I use a
Spartan, simulation is the key to making a deisgn that does what I
want it - and if it works, it narrows any problems to the hardware.

Tonico wrote:
> Dear Group,
> 
> I have a xilinx xc9536 , a 22 bit counter, and a 5 bit counter.
> The 22 bit counter is used as a 48mhz clock divider for the
> other 5 bit counter, hence bit 22 is used as humanly visible
> clock. The counter stops when it reaches 11111.
> The problem I am having is that when I apply my digital
> multi meter to the pins of the counter to check if the count
> has finished, then the counter seems to suddenly start
> counting again. I also noticed that if before applying the
> multimeter I disconnect the oscillator from the chip this will
> not happen, so it seems the oscillator is making the design
> unstable. Any ideas?
<snip>

Do you use bypass capacitors from power to ground?

On Jul 25, 5:30 am, <Tonico> wrote:
> Dear Group,
>
> I have a xilinx xc9536 , a 22 bit counter, and a 5 bit counter.
> The 22 bit counter is used as a 48mhz clock divider for the
> other 5 bit counter, hence bit 22 is used as humanly visible
> clock. The counter stops when it reaches 11111.
> The problem I am having is that when I apply my digital
> multi meter to the pins of the counter to check if the count
> has finished, then the counter seems to suddenly start
> counting again. I also noticed that if before applying the
> multimeter I disconnect the oscillator from the chip this will
> not happen, so it seems the oscillator is making the design
> unstable. Any ideas?
> Also, as a beginner, which book do I need to understand
> the tricks and quirks of xc9536 high speed design. Should
> I be designing on a behavioural level, or should I use low
> level?
>
> My clock code is as follows
>
> reg [21:0] nn;
> assign cdiv= nn[21];
> always @(posedge c0)
>   nn=nn+1;
>
> my other clock goes like this
>
> always @(posedge clock or posedge reset1)
>   begin
>     if (reset1)
>         aa <= 0;
>     else aa <= aa + 1;
>   end
>
> The clocks are physically connected from external pins with a wire.
> For the oscillator I use gck1.
>
> Many thanks.

I don't quite understand Verilog, but I don't see that the output of
your 22-bit counter has any affect on your 5-bit counter.  Shouldn't
you have something like:
  if cdiv = 1
    aa <= aa + 1;

-Dave Pollum

> <snip>
>
> Do you use bypass capacitors from power to ground?

I do have one 0.1 uF multi layered ceramic (dipped) on the power/gnd
pins on the right facet.

>
> I hate to ask the obvious - but have you simulated the design at this
> clock rate? I haven't used the xc95xx stuff but certainly when I use a
> Spartan, simulation is the key to making a deisgn that does what I
> want it - and if it works, it narrows any problems to the hardware.
>

Is this modelsim?

I've done "Simulate Behavioural Model" from the ISE, it worked.

BTW
I had to reduce the divider counter to 1 bit, otherwise  there are
4 million simulation steps per second timer clock.