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Options to obtain frequency?

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Timmymna

New Member
Hi, wondering if anyone can give me some advice?

I am planning on using a Zilog Z85C30 serial controller to transmit data using the SDLC protocol. It will be transmitted at 1.0152MHz.

One option is to use a oscillator, 622.080MHz from Farnell which, using the equation in the datasheet, would give me:

Time Constant = [Clock Freq / (2 * Clock Mode * Baud Rate)] - 2
Where Clock Mode = 1 for synchronous transfers
Baud rate = 1015200

Time constant of 324 which would then give a baud rate of 1015460 which is the closest I've been able to get with a oscillator.

There is a 1.0152MHz crystal but the Z85C30 can only produce a max data rate of 1/4 of the clock so this is no good and there aren't any multiples of this available (that I can find)

So this is what I've got so far, I am controlling the Z85C30 using the Microchip 16bit 28pin demo board fitted with the PIC24FJ64GA002.

Can anyone offer any alternatives or advice? The oscillator is £20 so I'm just looking to see if there are any alternative possibilities.

Is there an easy, accurate, way to produce the 1.0152MHz with the PIC? TBH I've not do that much using timers and the likes so not sure if this is possible or not. Using the 1.0152MHz crystal, but would have to be bumped up at least 4x?

Would it be possible to produce it with a 555? I've just started looking into this but it’s a bit mixed what I've read about max frequency – still looking.


If anyone can point me in the right direction it would be greatly appreciated :)
Thanks, Tom.
 

blueroomelectronics

Well-Known Member
Well a 555 won't cut it. You could use the fancy PLL in a 24H series PIC to produce almost any frequency within its limits. Why not just have a custom crystal made?
 

Russ Hensel

New Member
There are programmable oscillators, one might work. How close does the timing need to be. A synchronous link usually is fairly tollerenant as you ( or your dark master ) provides the clock.
 

Diver300

Well-Known Member
Most Helpful Member
You can get oscillators made to custom frequencies. Just about all on-chip crystal oscillators have an input and an output pin. If you feed the input with the oscillation signal from the output of the oscillator, you can get rid of all other external oscillator components, layout is much less important, and you don't have to worry about adjustment and load capacitance.

I very much doubt that the baud rate generator would work with a 622.08 MHz clock. It is just going to be far too fast for it to keep up.

OnSpec Oscillators Ltd does custom oscillators and crystals, but the price for 1 off isn't much less than the £20
 

Timmymna

New Member
Thanks for the replies, I'm looking into it now :)
I've been reading up on timers, about time I learnt how to use them properly.
Is it possible to cascade two timers? Preset timer0 to x then enable timer1 preset to y then back and forth giving a overall timer of x+y? I have worked out values to give a pretty accurate signal but requiring two timers.

Thanks Diver, will have a look into OnSpec
 
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