Sensoray 118 - Manual (Page 35)

Sensoray Model 118 Smart A/D™ Instruction Manual 33 Timing
8 Timing
Three timing parameters are important from the
application developer’s viewpoint: Channel Update
Rate, Data Age and Communication Latency. This
section discusses these parameters along with some
important related issues.
8.1 Slot Time
Channel Slot Time is the length of time required for the
Smart A/D to completely process one sensor channel.
Each channel slot time encompasses three functional
phases as shown in the following diagram:
In the first phase, a sensor channel is excited and its sense
signals are routed through to the Smart A/D measurement
section. The embedded microcomputer configures the
signal path as appropriate for the sensor type, then the
analog front end is allotted time to stabilize.
The digitizer acquires the sensor data value in the second
phase. The Update Rate, which is described in the next
section, can be increased by reducing the duration of the
time slot Digitize phase. See Section 6.1.3 for details.
A Recover phase occurs at the end of the time slot. The
function of this phase is to reset the analog front end in
preparation for the next conversion.
Computationally intensive processes, such as
linearization, alarm processing and software filtering,
don’t affect the slot time because they execute
concurrently during the subsequent channel slot time.
8.2 Update Rate
Update Rate is defined as the number of times each
second that a sensor channel acquires new sensor data.
8.2.1 Primary Influences
The two primary influences on update rate are the
number of active channels and the channel slot time.
Due to timing uncertainties, the update rate is expressed
as a range of values bounded by minimum and maximum
times. The minimum and maximum update rate for any
active channel is given by these functions:
The time difference between the minimum and maximum
update rate is due to the automatic, interleaved
measurement of internal Smart A/D reference standards.
Other than their timing impact, these measurements—
which are scheduled and executed autonomously by the
embedded processor—are transparent to the application
program. At most, one internal standard will be
measured per every sixteen sensor channel
measurements.
Clearly, the update rate increases as the number of active
channels decrease. Channels can be removed from the
scan loop by disabling them with the SetSensorType
command with the Disabled sensor definition code.
For example, a Smart A/D that has ten active channels
and is running at the default channel slot time (22
milliseconds) would have a worst-case channel update
rate ranging from 4.1 to 4.5 samples per second:
8.2.2 Secondary Influences
Another influence on the update rate is the frequency of
communication between host and Smart A/D.
The Smart A/D firmware is designed to minimize the
impact of communications on the update rate. Even so,
communication activity will sometimes interrupt
scanning, which in turn will stretch the current channel’s
slot time in a non-deterministic way.
Settle Digitize Recover
time
One Slot Time
UpdateRate
min
1
NumActiveChans 1 +
( )
SlotTime
×
------------------------------------------------------------------------------------------ =
UpdateRate
max
1
NumActiveChans SlotTime
×
---------------------------------------------------------------------------- =
UpdateRate
min
1
10 1 +
( )
0.022
×
---------------------------------------- 4.1Hz
≈
=
UpdateRate
max
1
10 0.022
×
------------------------- 4.5Hz
≈
=