Using Multiple Propellers?

mhamen3

I'm planning on designing a board that will have 2 props on it, pretty much right next to eachother, and I'm wondering if they can share a crystal. Are there any possible disadvantages to this?

Thanks

Clock Loop

It CAN be done, but it is extremely unconventional, and should only be done by experienced EE's that know the complications to sharing crystals.

If you look at my attached schematics in this thread, you will see how I accomplished using a single crystal on two props.
http://forums.parallax.com/showthread.php/124520-Anyone-that-wants-more-data-up-the-USB-serial-pipe-Dual-FTDI-Parallel-Props-(hanno!)

http://forums.parallax.com/attachment.php?attachmentid=72244&d=1281101659


I wouldn't recommend getting the clock from a single crystal like this, many would agree its unpredictable due to differences in boot time.
(if you mass produce enough of these, or reboot enough times, eventually, one of the props will accidentally not boot(so just reboot pcb if you can), its rare, but can happen)

The better way would be to hook the crystal up to a single prop, and use that prop to send out a 5mhz signal out of one of its pins, that goes to the XI of the second prop

Thread: http://forums.parallax.com/showthread.php/124487-EtPhoneHome-v1.0-A-Nokia-6150(like)-pcb-redone-with-two-propellers.-PCD8544-l?highlight=etphonehome

Schematic showing proper dual prop crystal config. (prop that has 5mhz crystal, outputs a 5mhz signal on p27 to the XI of secondary prop.)
(take note that the props need specific crystal settings in the CON section to differentiate between XI and crystal.
http://forums.parallax.com/showthread.php/124487-EtPhoneHome-v1.0-A-Nokia-6150(like)-pcb-redone-with-two-propellers.-PCD8544-l?highlight=etphonehome

Ariba

If both Props have its own EEPROM, booting should not be a problem. You need a way to program them separatly (two PropPlug headers).

The main problem with sharing the clock between two Props is that the oscillator stops when the "Master-Prop" is in reset state.
So you can not let run the second Prop while the first is reseted. Depending on the application this may be a proplem or not.
This also happens if the first Prop provides a clock to the second over an I/O pin.

Andy

mhamen3

Thanks for the insight guys. I had planned on giving each prop its own eeprom and programming headers. It would really be one main board with a prop and all the accessories I need and a smaller board with just the prop, eeprom and a programming header that I could sandwich on top if I need it. It really wouldn't kill me to put crystals on both, I was just curious.

WBA Consulting

mhamen3, Not sure where you are going with your design, but would stacking two of my M44D40+ modules work for you? http://forums.parallax.com/showthread.php/131867 If you have all the parts, I would gladly mail you two PCBs to try it out.

Duane Degn

I think the M44D40+ modules look really nice. There are several other options for small Propeller boards.

Tubular has the Und3rb3lly but I don't think he sells them. At least I couldn't get him to sell me one (but he gave me two).

Cluso99 also has small Propeller boards.

I've cut one of Jazzed TetraProp boards into small single or double Propeller boards. These are so handy I've thought about designing a MonoProp board with the same pin layout.

I used to use this PCB from SparkFun to make my own small Propeller boards. By cutting a couple of the smt pads on the back of the board, a SOIC EEPROM can be used with it.

I haven't used the SparkFun board for a while since there are so many good small Prop board options these days.

mhamen3

Thanks for the links guys, I like the look those. WBA, the form factor of your M44D40+ is pretty much what I was going for but a bit wider. I would rather just use socketed DIP40's on both boards; cheaper to replace. And I'm capable of isolation milling my own boards so building it all on my own would be relatively cheap.

WBA Consulting

mhamen3, glad you came up with a solution. Just as an FYI, the M44D40+ module is the same width as socketed DIP40's. Post #8 on the other thread shows pics of a version of the M44D40+ made with DIP40 chips instead of the QFN.