crystals powering multiple devices (bit-bang simulation of SPI)

[Kelvin Chu]

Hi Group,

Thank you for all the help with my question earlier about UART to SPI, this
is a follow up question...

Can the same crystal be used to power 2 different devices? Is there any
danger in using that?

The current situation is, I have a UART which support bit-bang simulation
(FTDI232BM), but the problem is that I don't know how to generate the Clock
signal, since the FTDI232BM bit-bang creates 8 GPIO lines for this, but I'm
thinking that software generated CLK signals will be highly unstable?

Since the timing signals of the FTDI232BM is not exported, I'm thinking that
if I also use the crystal to power a 555 timer, and then I will have a
signal at the correct frequency (and also a signal equivalent to the one
being generated in the FTDI232BM during sending of data?)

Thanks in advance

Kelvin

 

[Tim Shoppa]

Can the same crystal be used to power 2 different devices?

Dilithium crystal and Scotty jokes to follow...

Tim.

 

[Bob Eldred]

Hi Group,

Thank you for all the help with my question earlier about UART to SPI, this
is a follow up question...

Can the same crystal be used to power 2 different devices? Is there any
danger in using that?

The current situation is, I have a UART which support bit-bang simulation
(FTDI232BM), but the problem is that I don't know how to generate the Clock
signal, since the FTDI232BM bit-bang creates 8 GPIO lines for this, but I'm
thinking that software generated CLK signals will be highly unstable?

Since the timing signals of the FTDI232BM is not exported, I'm thinking that
if I also use the crystal to power a 555 timer, and then I will have a
signal at the correct frequency (and also a signal equivalent to the one
being generated in the FTDI232BM during sending of data?)

Thanks in advance

Kelvin

In general you CAN use a crystal to clock (not power) more than one device.
You use the xtal controlled oscillator of device one and take a clock output
from device one to drive the clock input of device two. If there is not a
clock output or input, you may with caution, take the driven terminal of the
xtal on device one as the clock output and connect it to the xtal input of
device two. It may require some series resistance to limit the drive level.
Do not connect the xtal drive pin of device two. For parallel mode crystals,
the shunt capacitors should be reduced in value to reduce crystal loading by
the added circuitry. In some cases, buffering may be required depending on
the specific circuit.

Another option is to use a ready made xtal oscillator instead of an xtal and
connect it's output to the two clock or xtal inputs on the two devices. A
555 is not a suitable device for an xtal oscillator or buffer. However, you
can make oscillators and buffers with CMOS inverters.
Bob

 

[Pooh Bear]

Hi Group,

Thank you for all the help with my question earlier about UART to SPI, this
is a follow up question...

Can the same crystal be used to power 2 different devices? Is there any
danger in using that?

The current situation is, I have a UART which support bit-bang simulation
(FTDI232BM), but the problem is that I don't know how to generate the Clock
signal, since the FTDI232BM bit-bang creates 8 GPIO lines for this, but I'm
thinking that software generated CLK signals will be highly unstable?

Since the timing signals of the FTDI232BM is not exported, I'm thinking that
if I also use the crystal to power a 555 timer, and then I will have a
signal at the correct frequency (and also a signal equivalent to the one
being generated in the FTDI232BM during sending of data?)

Yes, it's possible to use one crystal to clock multiple devices provided that
the clock signal is suitably 'strong' or adequately buffered.

555s don't use crystals though. They are RC based timers.

You may want to look at this Intel application note. It's very useful.

http://www.intel.com/design/mcs51/applnots/23065901.pdf


Graham