On Wed, Nov 23, 2016 at 02:34:03PM +0100, Andrei Pistirica wrote:
> From what I understand, your suggestion is:
> (ns | frac) * ppb = (total_ns | total_frac)
> (total_ns | total_frac) / 10^9 = (adj_ns | adj_frac)
> This is correct iff total_ns/10^9 >= 1, but the problem is that there are
> missed fractions due to the following approximation:
> frac*ppb =~ (ns*ppb+frac*ppb*2^16)*2^16-10^9*2^16*flor(ns*ppb+frac*ppb*2^16,
> 10^9).
-ENOPARSE;
> An example which uses values from a real test:
> let ppb=4891, ns=12 and frac=3158
That is a very strange example for nominal frequency. The clock
period is 12.048187255859375 nanoseconds, and so the frequency is
83000037.99 Hz.
But hey, let's go with it...
> - using suggested algorithm, yields: adj_ns = 0 and adj_frac = 0
> - using in-place algorithm, yields: adj_ns = 0, adj_frac = 4
> You can check the calculus.
The test program, below, shows you what I meant. (Of course, you
should adjust this to fit the adjfine() method.)
Unfortunately, this device has a very coarse frequency resolution.
Using a nominal period of ns=12 as an example, the resolution is
2^-16 / 12 or 1.27 ppm. The 24 bit device is much better in this
repect.
The output using your example numbers is:
$ ./a.out 12 3158 4891
ns=12 frac=3158
ns=12 frac=3162
$ ./a.out 12 3158 -4891
ns=12 frac=3158
ns=12 frac=3154
See how you get a result of +/- 4 with just one division?
Thanks,
Richard
---
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
static void adjfreq(uint32_t ns, uint32_t frac, int32_t ppb)
{
uint64_t adj;
uint32_t diff, word;
int neg_adj = 0;
printf("ns=%u frac=%u\n", ns, frac);
if (ppb < 0) {
neg_adj = 1;
ppb = -ppb;
}
word = (ns << 16) + frac;
adj = word;
adj *= ppb;
adj += 500000000UL;
diff = adj / 1000000000UL;
word = neg_adj ? word - diff : word + diff;
printf("ns=%u frac=%u\n", word >> 16, word & 0xffff);
}
int main(int argc, char *argv[])
{
uint32_t ns, frac;
int32_t ppb;
if (argc != 4) {
puts("need ns, frac, and ppb");
return -1;
}
ns = atoi(argv[1]);
frac = atoi(argv[2]);
ppb = atoi(argv[3]);
adjfreq(ns, frac, ppb);
return 0;
}
