Can I use one shared crystal?

[SBS]

Anthony Fremont ([email protected]) ha scritto:

::: BTW, I need to use a crystal with nominal frequency
::: of 16 MHz. So the best soultion should be to use a crystal
::: for each device, shouldn't it?

:: Possibly the easiest and least expensive anyway.

Very clear, thank you!

Thanks to everyone who has shared his knowledge on this
thread.

 

[colin]

As told before: A crystal oscillator has an input and an (inverting)
output. A crystal connected in be input and output will cause the
whole thing to oscillate at the crystal's resonant frequency.

If you want to use one crystal for 2 devices, you should place the
devices close together. Connect the crystal between the crystal
oscillator pins of device A, connect the crystal oscillator output
from device A to the oscillator input from device B with a small
capacitor (say 1nf).

The datasheets should tell you more about such a setup and which pins
are input and output.

Like this ??


+------+
| | |
| | |
__|__ --- 0
\ / [ ] / \
\ / --- / \
0 | -----
| | |
| | |
+------+----()---+
1nf
CPU A CPU B

yes thats it thanks saved me drawing it.

I don't think this will work.

I think the drive of CPU A output will be loaded too much to drive the
crystal and the input of CPU B.

output CPU A should be capable of driving a cmos input,
as long as CPU B is a modern type, and CPU A isnt a realy weedy type.
dont forget you usualy need capacitors to ground on both pins,
and ofc the capacitance of the CPU B input means you might need slightly
less on that node.

Let me know how it works.

yes id like to know too.

Colin =^.^=

 

[Donald]

colin ([email protected]) ha scritto:

:: I dont see why one cant drive the other,

Thank you Colin, can you draw a picture which explains
this situation, please?

I can't understand how to connect both devices to the single
crystal... :-(


As told before: A crystal oscillator has an input and an (inverting)
output. A crystal connected in be input and output will cause the
whole thing to oscillate at the crystal's resonant frequency.

If you want to use one crystal for 2 devices, you should place the
devices close together. Connect the crystal between the crystal
oscillator pins of device A, connect the crystal oscillator output
from device A to the oscillator input from device B with a small
capacitor (say 1nf).

The datasheets should tell you more about such a setup and which pins
are input and output.

Like this ??


+------+
| | |
| | |
__|__ --- 0
\ / [ ] / \
\ / --- / \
0 | -----
| | |
| | |
+------+----()---+
1nf
CPU A CPU B

yes thats it thanks saved me drawing it.

I don't think this will work.

I think the drive of CPU A output will be loaded too much to drive the
crystal and the input of CPU B.

output CPU A should be capable of driving a cmos input,
as long as CPU B is a modern type, and CPU A isnt a realy weedy type.
dont forget you usualy need capacitors to ground on both pins,
and ofc the capacitance of the CPU B input means you might need slightly
less on that node.

Let me know how it works.

yes id like to know too.

Colin =^.^=

Yes, the caps to ground were a exercise for the user.

donald

PS: I wouldn't use this in production. But it might work on the bench.
If the OP has a board were he can try several chip of both types,
CPU A and CPU B to see if they are consistant.

 

[jasen]

colin ([email protected]) ha scritto:

:: I dont see why one cant drive the other,

Thank you Colin, can you draw a picture which explains
this situation, please?

I can't understand how to connect both devices to the single
crystal... :-(

.--------------. .--------------.
| | | |
| device 1 | | device 2 |
| | | |
| | | |
| Xin Xout | | Xin Xout |
`---O-----O----' `---O-----O----'
| | |
| +--------------'
+-|#|-+
| |
=== ===
| |
| |
--+-- --+--
///// /////

Bye.
Jasen

 

[colin]

.--------------. .--------------.
| | | |
| device 1 | | device 2 |
| | | |
| | | |
| Xin Xout | | Xin Xout |
`---O-----O----' `---O-----O----'
| | |
| +--------------'
+-|#|-+
| |
=== ===
| |
| |
--+-- --+--
///// /////

Bye.
Jasen

oo thats quite a good drawing too, you guys have more patience than I do.

Colin =^.^=