Stopping the clock on a 68000 system

[uriah]

I have a piece of test equipment that requires the clock to be stopped
before it can be used on the board your working on. It tests IC in
circuit and I am using it on a 68000 cpu controlled system. One book
said to ground each leg of the crystal and to remove any timer chips
from the circuit to stop the clock. Is this correct or is there some
other way to go about this. Can you ground a leg of one of the pins on
the 68000? will grounding both legs of the crystal cause any damage?

Thanks
Russ

 

[Don Lancaster]

I have a piece of test equipment that requires the clock to be stopped
before it can be used on the board your working on. It tests IC in
circuit and I am using it on a 68000 cpu controlled system. One book
said to ground each leg of the crystal and to remove any timer chips
from the circuit to stop the clock. Is this correct or is there some
other way to go about this. Can you ground a leg of one of the pins on
the 68000? will grounding both legs of the crystal cause any damage?

Thanks
Russ

Seems to me you would want to GATE the clock, not stop it.

Restarting a crystal takes many hundreds to many thousands of
unpredictable cycles.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[[email protected]]

I have a piece of test equipment that requires the clock to be stopped
before it can be used on the board your working on. ..using it on a 68000 cpu controlled system.

The 68000 is not stable with its clock stopped (it needs the clock
active for dynamic
RAM-like registers, and because there are charge pump biasing networks
that
use the clock). There IS a pin for HALT on this CPU, which brings all
operations to
a stop. Probably you want to HALT the processor, instead of not
clocking it.

A few CMOS processors can be halted by gating the clock off, but it's
not generally
good practice.

 

[mkaras]

I have a piece of test equipment that requires the clock to be stopped
before it can be used on the board your working on. It tests IC in
circuit and I am using it on a 68000 cpu controlled system. One book
said to ground each leg of the crystal and to remove any timer chips
from the circuit to stop the clock. Is this correct or is there some
other way to go about this. Can you ground a leg of one of the pins on
the 68000? will grounding both legs of the crystal cause any damage?

Thanks
Russ

If you are using a crystal just connected to an X1 and X2 pin pair then
it will be sufficient to ground the X1 pin (using the typical naming
practice where X1 is the input to the onboard oscillator inverter). On
the other hand if you are using an external crystal oscillator device
these often have an output enable pin which is typically held in the
"enabled" state on the board. Overcome this enabled state and you can
get the oscillator to stop outputting its clock signal.

- mkaras

 

[Joerg]

The 68000 is not stable with its clock stopped (it needs the clock
active for dynamic
RAM-like registers, and because there are charge pump biasing networks
that
use the clock). There IS a pin for HALT on this CPU, which brings all
operations to
a stop. Probably you want to HALT the processor, instead of not
clocking it.

A few CMOS processors can be halted by gating the clock off, but it's
not generally
good practice.

A bit OT here but before a reader thinks "oh, mine is all static so
it'll be fine to short the crystal": Even modern uCs can become
unpredictable or do unexpected stunts. A clock fault often is muffled by
an internal "pinch hitter" clock kicking in and that can throw a curve
during such ICT procedures. Also, timers could run into a dead end and
all that stuff.