User guide
Neuron C Programmer’s Guide 39
treats as 820 microsecond ticks. This means that a timer duration is actually
0.999 times the specified duration.
For example, for a Series 3100 device with a 2.5 MHz clock, a specified timeout of
99 ms would result in an expected duration of 96.67 ms.
The complete formulas for calculating the low and high durations are:
L
=
E
- (11*
S
+ 1)
H = E
+ (11*
S
+ 1)
The high duration with a 2.5 MHz clock and a specified timeout of 99 ms would
thus equal 141.67 ms; the low duration is 51.67.
Note: The number "11" in the formulas above is based on a typical worst case
scenario. In the absolute worst case, that is, the maximum number of timers,
network variables, addresses, and so on, this number can be as high as 32.
In addition, the high duration may be increased by
network management delay
(NMD), an additional skew introduced by network management message
processing. Normally, this term is 0. But, if a device were to process a network
management message, the upper range for any given timeout could be
significantly increased. For example, adding a domain to a device would result in
an NMD of anywhere from 300 ms to (300 + 838*
S
) ms. In general, network
management operations of this type occur infrequently. It is always good
practice to take a device offline, if possible, before sending further network
management messages.
To measure an event’s duration, a timer can be polled before and after the event,
and the difference can be calculated. To measure the duration of events less than
50 milliseconds, use the get_tick_count( ) function instead of the software timers
(see the
Neuron C Reference Guide
).
Repeating Timers
For repeating timers, there is no cumulative drift other than that produced by
the difference in
D
and
E
. The
N
th timeout for repeating timers occurs in the
range of
LR
to
HR
, where:
ER = E * N
and
LR
=
ER
- (11*
S
+ 1)
HR
=
ER
+ (11*
S
+ 1)
For repeating timers, intermediate timeout events are lost if the following is true:
abs
(AR
-
ER
) ≥
E
ER
-
AR
>
E
where
AR
is the actual duration of the repeating timer.