MA28139 Datasheet, PDF (11/34 Pages) Dynex Semiconductor – OBDH Bus Terminal

MA28139
The requirement to respect set-up and hold times for the
capture of the Edge Detector outputs by the LOSC high-to-low
transition means that LOSC signals which are harmonically-
related to the Litton modulation clock but whose phase can not
be controlled can never be guaranteed to provide reliable
operation.
For asynchronous local oscillator signals, there will be no
concern if we are simply able to place two or more Litton DSn
high-to-low edges into each LOSC cycle, so that:
τ_MOD ≤ τ_LOSC - t_SU - t_HOLD
and the time-out period of 16 τ_LOSC is hence approximately
8 bit periods or more.
In summary, slow local oscillator clocks which cause
relatively long timeout periods ≥ 8 bit periods are not
considered a problem; very long time-outs can be reliably
implemented. For shorter time-out periods, however, it is
necessary to avoid harmonic relationships between the Litton
modulation clock and the local oscillator. The simplest
practical method for avoiding such relationships would be to
arrange for the ratio
n = τ_MOD / τ_LOSC
to have a half-integer value such that n = 0.5, 1.5, 2.5,
...using an independent crystal oscillator if necessary.
However, suppose that the periods of the modulation clock
and the local oscillator clock are such that the relationship
between them is:
τ_MOD = m τ_LOSC
where m is a positive integer.
In order to respect the setup and hold times, t_SU + t_HOLD
respectively, between the DSn ↓, and LOSC ↓ edges, it is
necessary to avoid such harmonic relationships; it can be
shown that around these spot frequencies it is necessary to
ensure that either:
w τ_MOD ≥ x τ_LOSC + t_SU + t_HOLD
or
y τ_MOD ≤ z τ_LOSC - t_SU - t_HOLD
where the integer constants w, x, y and z are given in the
table below.
Since two modulation clock cycles occur per bit, the time-
out period at these harmonics will then be:
16 τ_LOSC ≈ 16 τ_MOD / m ≈ 8 / m bit periods.
mw x
y
z Approx. time-out
period (bit periods)
1 15 15 17 17 8
2
7 15 8 15 4
3
5 15 5 15 2.67
4
3 13 4 15 2
5
3 15 3 15 1.6
6
2 13 2 11 1.33
7
1
7
1
7 1.14
8
1
9
2 15 1
9
1
9
1
9 0.88
10 1 11 1
9 0.8
11 1 11 1 11 0.73
12 1 13 1 11 0 67
13 1 13 1 13 0.62
14 1 15 1 13 0.57
15 1 15 1 15 0.53
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