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PDF AN214 Data sheet ( Hoja de datos )
| Número de pieza | AN214 | |
| Descripción | Smart Transducer Interface Module | |
| Fabricantes | Microchip Technology | |
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AN214
The PICmicro® MCU as an IEEE 1451.2 Compatible
Smart Transducer Interface Module (STIM)
Authors: Richard L. Fischer
Microchip Technology Inc.
Jeff Burch
Agilent Technologies
INTRODUCTION
This application note discusses the organization of the
PICmicro MCU and related firmware for developing
IEEE 1451.2 Smart Transducer Interface Modules
(STIM). Although a detailed understanding of the
1451.2 is not required, a working knowledge of 1451.2
concepts like Transducer Electronic Data Sheet
(TEDS), STIMs, data models, etc., is assumed. The
purpose of this application note is to provide the devel-
opers of a PICmicro MCU based STIM with some key
hardware and firmware building blocks, that may be uti-
lized in the development of a STIM.
IEEE-1451.2 OVERVIEW
The IEEE 1451 standard is composed of four parts,
1451.1, 1451.2, 1451.3 and 1451.4. The combination
of the four parts (some parts are approved and others
are pending) define the signal chain from the analog
sensor to the digital network (See Figure 1). The main
objectives of the IEEE 1451 standard are to:
• Enable plug and play at the transducer level
(sensor or actuator) by providing a common com-
munication interface for transducers.
• Enable and simplify the creation of networked
smart transducers.
• Facilitate the support of multiple networks.
The focus of this application note deals with the 1451.2
part of the standard.
The IEEE 1451.2 standard provides the means to con-
nect sensors and actuators to a digital system, typically
a network, via a Network Capable Application Proces-
sor (NCAP). Figure 2 depicts a general distributed
measurement and control application and Figure 3
depicts the functional boundaries relating to the trans-
ducer interface standard, namely IEEE 1451.2.
In simplest terms, the IEEE-1451.2 standard does the
following:
• Defines a digital interface for connecting trans-
ducers to microprocessors.
• Describes a TEDS, “electronic data sheet”, and its
data formats.
• Defines an electrical interface, read and write
logic functions to access the TEDS, and a wide
variety of transducers.
This standard does not specify signal conditioning, sig-
nal conversion, or how the TEDS data is used in appli-
cations.
http://www.DataSheet4U.net/ There is currently no defined independent digital com-
munication interface between transducers and micro-
processors. Each vendor builds its own interface.
Without an independent, openly defined interface,
transducer interfacing and integration are time con-
suming and duplicated efforts by vendors are econom-
ically unproductive. This interface provides a minimum
implementation subset that allows self-identification
and configuration of sensors and actuators, and also
allows extensibility by vendors to provide growth and
product differentiation.
2000 Microchip Technology Inc.
Preliminary
DS00214A-page 1
datasheet pdf - http://www.DataSheet4U.net/
1 page  
AN214
WHAT IS A STIM?
The IEEE 1451.2 standard introduces the concept of
the STIM. A STIM can range in complexity from a sin-
gle sensor or actuator, to many channels (up to 255
channels) of transducers (sensors or actuators). Exam-
ples of STIM’s are shown in Figure 5.
A transducer channel is denoted "smart" in this context
because of the following three features:
• It is described by a machine-readable Transducer
Electronic Data Sheet (TEDS).
• The control and data associated with the channel
are digital.
• Triggering, status, and control are provided to
support the proper functioning of the channel.
A STIM contains the TEDS, logic to implement the
transducer interface, the transducer(s) and any signal
conversion or signal conditioning. A STIM is controlled
by a NCAP module by means of a dedicated digital
interface. This interface is not a network. The NCAP
mediates between the STIM and a digital network, and
may provide local intelligence. It is desirable that the
STIM and NCAP add little size or cost to the trans-
ducer(s) they describe and interface. TEDS memory
requirements are typically less than two kilobytes.
These size and cost considerations also restrict the
computing power available in the NCAP.
FIGURE 5: EXAMPLES OF 1451.2 SMART TRANSDUCER INTERFACE MODULES
5A) Temperature Sensor STIM
Temperature
Sensor
Microcontroller
ADC
TEDS
1451.2
STIM
5B) Eight Channel Digital I/O STIM
In Microcontroller 1451.2
TEDS
Out
http://www.DataSheet4U.net/
STIM
5C) Four Channel Sensor STIM
Temperature Sensor
Pressure Sensor
Flow Sensor
pH Sensor
Microcontroller
TEDS
ADC
1451.2
STIM
5D) Sensor and Actuator STIM
Temperature Sensor
Pressure Sensor
Proportional Valve
Relay
Microcontroller
ADC TEDS
DAC
Digital
1451.2
STIM
2000 Microchip Technology Inc.
Preliminary
DS00214A-page 5
datasheet pdf - http://www.DataSheet4U.net/
5 Page 
FIGURE 9: TRIGGERED CHANNEL ZERO READ
AN214
12
345
67
Figure 9 represents a 1451.2 bus data transport during
a triggered channel 0 read. Note that six bytes are
transferred across the 1451.2 bus for this transaction
(i.e., function address of 128, channel of 0, MSB of
channel 1, LSB of channel 1, MSB of channel 2 and
LSB of channel 2). In this measurement, the logic ana-
lyzer trigger signal condition was the STIM acknowl-
edgment of the NCAP trigger (first falling edge of the
NACK signal). Various points of interest are identified
on the figure and discussed below.
1. Channel Update Time.
2. STIM Handshake Time.
3. Channel Read Setup Time. The NCAP will wait
this long before dropping NIOE to begin the read
data transaction. In the stimtemp.c implementa-
tion, the NCAP delay gives the STIM time to per-
form any status updates and interrupt
processing. In a STIM with measurement hard-
ware, this time would most likely be dominated
by ADC conversion delays.
4.
http://www.DataSheet4U.net/
5.
6.
7.
The increased delay between the end of the
channel selector byte and the transmission of
the first byte of the data frame. This delay hap-
pens when the PICmicro device is processing
the function code and dispatching to the
transducer_read() function.
This is the 8 DCLK edges that clock the transfer
of a single byte across the 1451.2 interface. In
the STIM, the DCLK rate is 5Mbps.
This is the time for the PICmicro device to per-
form the byte-handshake.
The channel 0 read operation is longer than the
Channel Aggregate Hold-off Time.
2000 Microchip Technology Inc.
Preliminary
DS00214A-page 11
datasheet pdf - http://www.DataSheet4U.net/
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet AN214.PDF ] | |
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