Share the same crystal on tqo chips with internal oscillators. How to?

[neazoi]

Hello,
These two chips use the same frequency crystal for their internal oscillators (3.5795MHz)

1. Will there be a difference if I use a 3.8MHz crystal instead, with the same results, or is the frequency that critical?

2. Can the TP5088 share the same crystal with the 74hc943, since they operate at the same crystal frequency?
How can this be achieved?
I can see that both use a gate (although one is inverted) for the oscillator. So maybe I can somehow get the output of the internal oscillator in one chip and drive the input gate of the second?

Thank you

 

[KlausST]

Hi,

1) Yes, there will be a difference.
3.8MHz is about 6% OFF, which most probably violates standard frequency accuracy.
But if Tx and Rx frequency are the same magnitude OFF then you just run the MODEM on your own frequency.
As so often: it depends on your requirements.

2) Theoretically yes. Usually the MASTER´s oscillator OUT is connected to the SLAVE´s oscillator IN.
In detail it depends on the specified input and ouput levels.
And the TP5088 has internal OSCILLATOR_ENABLE which may cause problems.

Klaus

 

[neazoi]

Hi,

1) Yes, there will be a difference.
3.8MHz is about 6% OFF, which most probably violates standard frequency accuracy.
But if Tx and Rx frequency are the same magnitude OFF then you just run the MODEM on your own frequency.
As so often: it depends on your requirements.

2) Theoretically yes. Usually the MASTER´s oscillator OUT is connected to the SLAVE´s oscillator IN.
In detail it depends on the specified input and ouput levels.
And the TP5088 has internal OSCILLATOR_ENABLE which may cause problems.

Klaus

KlausST, regarding the 1 point, I am terribly sorry, I meant to run it on 3.58MHz instead of 3.5795. This was a typo. Does it make a real difference in the bell103?

Regarding the 2 point, then I could use the tp5088 oscillator (connect a crystal there) and drive the modem IC with this?
The 74hc943 data states: "If a suitable clock is already available in the system. XTALD can be driven."
So maybe just generate the clock with the tp5088 and drive the modem chip STALD pin with that and leave the other pin unconnected?

 

[KlausST]

Hi,

1) I still have no idea whether you want to meet some standards or just run your own system.

2) the "typical" operation is:
* HC943: continous clock
* TP5088: gated clock

With your solution the HC943 gets a gated clock. Not continous anymore.

Thus I´d rather do it the other way round. (Without detailed reading).

Klaus

 

[neazoi]

Hi,

1) I still have no idea whether you want to meet some standards or just run your own system.

2) the "typical" operation is:
* HC943: continous clock
* TP5088: gated clock

With your solution the HC943 gets a gated clock. Not continous anymore.

Thus I´d rather do it the other way round. (Without detailed reading).

Klaus

Thanks. Regarding the point 1, I would like to meet the bell103 system that the modem supports. On it's page 7 it states a 3.58MHz crystal. However, on page 3 it states a 3.5795MHz crystal. This is why I am confused if there is an actual difference.

Regarding point 2, I see now, a gated clock is enabled only for the time interval that the dtmf chip wants, so I will try to do it the other way around and see if it works.

By the day, the D inputs to the data latch, do they need external pull up resistors or just feed 5v to them for logic 1 and left these unconnected for logic 0?

 

[KlausST]

Hi,

1) are you sure you get a 3.58MHz XTAL? Please check.
2) you need to read the datasheet on your own. It´s rather clear it has internal pullup.

Klaus