couple of questions (oscillator and voltage levels)

The atmel data sheet I found,

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, says, in paragraph 7.4, "Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an On-chip Oscillator, as shown in Figure 7-2. Either a quartz crystal or a ceramic resonator may be used.

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving other clock inputs and in noisy environments. ..."

So, yeah, run it in full-swing mode, and it's a TTL clock.

Before I do the rest of your homework for you, Why don't you go ahead and take a look at the other data sheets involved? I'm tempted to guess that the answer to your next question is, "Pull-up", but I wouldn't want to give you a bum steer, and looking up that Atmel clock spec for you just wore me out. Especially that 3 volt stuff - that's like interstellar travel to a well-digger like me. :-)

Good Luck! Rich

Reply to
Rich Grise
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Hi, pre-thanks for any tips.

I am working on a little homebrew design that requires interfacing a xilinx XC9536XL to an atmel atmega88.

Things are going ok (my first real cpld design...they are VERY neat!), but I have a couple of questions.

1) Can I use the 20mhz crystal that is connected to the X1 and X2 crystal driver inputs on the atmega88 to also clock the CPLD by running a line from X2 to one of the global clock inputs on the cpld? The atmel data sheet sort of mentions you can drive another device from X2 by setting a fuse, but it is not very clear what type of clock drive they are using. I also do not know what the gclk input on the xilinx expects in the way of clock input. The data sheet seems to lack that information. I realize I can use a clock oscillator chip, but at double the cost of a plain crystal. I am trying to minimize cost. Should I just bite the bullet and go with a clock oscillator to drive both devices?

2) The atmega88 specs input high level of 0.6VCC for guaranteed logic 1. To run it at 20mhz it needs a VCC of 4.5 to 5V.

If I use VCC = 5V, then input high level would be 3V. The XC9536XL has 5 volt tollerant I/O but of course it is a 3.3V part. Is it going to be *ok* to rely on a 3.3V high signal from the XC9536XL to register as a logic 1 on the atmega88? The atmel data sheet does say that typical threshhold for logic 1 is about 2.5V, so there is some headroom. I realize I could use an interface chip between the cpld and ucontroller, but again, I am trying to minimize cost and size. If things are ok, I would like to leave the design as is (cpld outputs hooked directly to the cpu).

Thanks again!


Reply to

Thanks. I had been looking (accidentally) at an older data sheet for a mega16 part, which has a different paragraph about the clock..not *quite* as clear as the mega44 data sheet (I just decided the 44 would work in my design today). You are right, it is pretty clear it will work as the clock drive for the cpld gclk pin. Saves a buck..


Reply to

Kewl! ;-)

Let us know how that 3.3v==>5V thing works out, OK?

Thanks! Rich

Reply to
Rich Grise, but drunk

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