# config FCLKDIV

Discussion created by Jeff Smith on May 10, 2006
Latest reply on May 11, 2006 by Dennis Shutt
I get baffled and confused every time I try to make sense of docs for calculating FCLKDIV. Using DP256B (if that ever makes a difference). I'm going to go over the example below to verify how it's done.

I suppose "FDIV" always refers to FDIV bits [5:0] (not concerned with PRDIV8).

Figure 4-1 looks incomplete, yet anywhere that I should find details it just says look at 4-1 for all my answers (yeah-right). What clk source am I starting with? I think it means OSCCLK. I guess PLL only matters if Tbus exceeds certain limits.

First, what goes on left side of the equals? There are places where the expression starts with '=' so I see no indication what the result represents. You just have to know? Then why am I reading the stupid instructions?

Questions about this wierd expression "=PRDCLK[MHz]*(5+Tbus[uS])"... what goes on left side; is it the value so far for FDIV? I'll assume the [MHz] and [uS] means I can enter 24 and 0.5 in a calculator when the value is really 24,000,000 and 0.0000005 respectively. Note, that confuses me sometimes because I already have the real value in the calc. I also wonder if there is some complex reason why I'm checking if the result is an "integer". How accurate do I need to be in decimal places? Since I'm multiplying with Tbus < 1uS it seems not likely to ever be a whole integer. But if it does come out, it's hard to visualize why I treat it differently. I see no explanation. Hopefully I don't want to know, because I don't.

I try to assume that value n in FDIV[5:0] would cause divide by n+1, but I don't see that described anywhere. It's only possibly derivable from the cryptic expressions in Fig 4-1.

Can I assume all is the same for EEPROM's ECLKDIV?

=== Example seems to work ====

oscillator = 16 MHz (xtal rating)
Fbus = 24 MHz (PLL set 3/2)
-therefore Tbus ~= 1/24 [uS] (or 0.0416666667 uS)

1. Since 16 > 12.8, I divide by 8
PRDIV8 = 1
PRDCLK = 2 MHz

2. Since 2*(5+1/24) is not rounded, I don't subtract 1
FDIV = 10

3. FCLK (between 150 kHz and 200 kHz): 2e6/(1+10)
FCLK = 181.818 kHz

4. Since (1/181,818 + 1/24) > 5 uS, FDIV is OK

Using my assumptions I end up with PRDIV8=1, FDIV=10 (0x0a), therefore FCLKDIV = (PRDIV8_MASK | 0x0a) = 0x4a.

Whew, I hope this helps people, as well as gets more explanation out of people who know what's going on.

Message Edited by imajeff on 05-10-200604:18 PM