SN74ALS867 used as a frequency divider: Help needed

[dufduf]

Hello,

I need to preset a frequency divider with the SN74ALS867 (http://www.datasheetcatalog.com/datasheets_pdf/S/N/7/4/SN74ALS867A.shtml)
but I don t know how to do it, since it s not very explicite in the datasheet.As it s an 8 bits, we have 8 flip flops with the inputs (A to H) and the output (Qa to Qh), then I m blocked.

Could someone give me an example?? If I want to divide by 15 for instance?
And about the clock?

Thanks a lot!!

 

[Papabravo]

As with any syncronous counter you identify the count value that activates the carry signal. In this case it is called RCO-bar. Then you subtract the divisor from that value. This is the value that you preload into the counter.

In your case the counter wil have 15 states, fourteen of which will have RCO-bar high and one of which will have it low. When RCO-bar is high you count and when RCO-bar is low you load the counter.

Is that clear?

 

[dufduf]

Ok, thanks.
So for a division by 15, should I put it like this?

S0=1
S1=0
___
ENT=0
___
ENP=0
___
RCO=0?
A=1 B=1 C=1 D=1 E=0 F=0 G=0 H=0
Qa=? Qb=? Qc=? Qd=? Qe=? Qf=? Qg=? Qh=?

(if I consider that A is the low binary)
Where do I get my frequency output?
What must be the value of the Qx?

Thanks!

 

[Papabravo]

No! You did not pay attention to what I wrote. I think that the preload value should be 0xF1 and that the state sequence should be:

0xF1..0xF2....0xFD..0xFE..0xFF

When the conter contains all ones, the OUTPUT RCO-bar goes low. When this hapens you need to change the counter from counting to loading. After the clock edge which loads the 0xF1 the RCO-bar will go away and the counter will count again from 0xF1 to 0xFF, and it will do this forever.