Message Edited by peg on 2007-03-1509:18 AM
I enforce Peg's first answer.
There is absolutely no doubt in my mind that OSC2 output of the master Xtal-driven HC908QB8 may drive OSC1 input of the slave Xtal-less -QB8 preset in external clock driven mode. There is surely no violation of the drive fan-out of the first gate (the master one) and the load will be negligibly small compared to that of the oscillator circuitry itself. At such a low frequency the output swing will be very close to the power supply rails and very sharp: it will be consequently no possibility of any uncertain input drive or improper offset of the slave input oscillator gate, so there is not any reason to linearly bias the oscillator gate of the slave device as BigMac suggested (this leave you another pin available as I/O). As Peg pointed out, the two chips must be reasonably close one with the other (some centimeters, perhaps up to 10) to share the clock line.
But the main question remains WHY?
The HC908QB8 is a small, nice full featured 8bit wonder which I used in the past and impressed me at the time; it costs now 1.48$ for the cheapest version as reference price, with 12-13 available I/O pins (one or two are reserved for the oscillator, in your case). 3-4 pins for each MCU are anyway devoted to the serial SPI link and many many pins shares different functions which limits the possibility of using every function you may need to its full extent. The need of programming and possibly reprogramming the internal flash poses serious requirements on the design of the application circuitry vs. the on board programming port.
Even worse, the overheading of the firmware layer which provides the SPI link will cause operative slow down and programmer's headaches which I would avoid as much as possible.
If you need more I/O ports it would be wiser to use a bigger MCU, avoiding linking two -QB8. A natural choice in the past would be the HC908QC16, which is essentially a bigger -QB8 with up to 28 pins. But the Freescale's reference cost is a jealous secret (the 16 pin version is quoted at 1.94$) and Digi-Key quotes it at 2.228$. Availability is still a mistery. But a very more powerful MC9S08AW16CGFE is quoted 1.95$ in the 44pin version and only slightly more in the 64 pin version. This is definitely MUCH more powerful and easier to manage and program: I woudn't come back to the older -QB and -QC version after having tested it.
The only drawback I can see is the availability of the dip version for the QB8, usefull for tests and breadboards, while the AW16 is available only in small smd forms, difficult to manage without a proper pc board and some skill.
I'm a pretty old designer and tenaciously hanged to 5V logic world...
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