A local bus is described which is designed for use in a multi-chipcomposed
microinstrumentation system. The bus is able to transmit a digital
code, bitstream, analog voltage, frequency, duty-cycle and also provides
calibration facilities, service request and interrupt request for the smart
sensors. Corresponding sensor bus interface was implemented in a 1.6 m
CMOS process and successfully tested in a local sensor network.Junta Nacional de Investigação Científica e Tecnológica - Praxis XXI-BD/5181/95.
STW  - Project DEL 55.3733.
TUDelft

Bartek, M.

Correia, J. H.

Cretu, E.

Wolffenbuttel, R. F.

Universidade do Minho: RepositoriUM

A Local Bus for MCM-BasedMicroinstrumentation SystemsJ. H. Correia, E. Cretu, M. Bartek and R.F. WolffenbuttelDelft University of TechnologyDept. of Electrical EngineeringLab. for Electronic InstrumentationMekelweg 4, 2628 CD Delft, The NetherlandsTel.: +31 15 2781602, Fax: +31 15 2785755e-mail: higino@ei.et.tudelft.nlAbstractA local bus is described which is designed for use in a multi-chip-composed microinstrumentation system. The bus is able to transmit a digitalcode, bitstream, analog voltage, frequency, duty-cycle and also providescalibration facilities, service request and interrupt request for the smartsensors. Corresponding sensor bus interface was implemented in a 1.6 mCMOS process and successfully tested in a local sensor network.IntroductionA microinstrumentation system, as shown schematically in Fig.1, includes all features of acomplex measurement system on the smallest possible material carrier; a silicon chip. Thesystem is composed of an universal platform which is to be populated with the required sensors,micro-actuators and a microcontroller using MCM (Multi Chip Module) techniques, to solve aparticular measurement problem.The platform provides all infrastructural functions, such as power/thermal management of thesystem, test facilities and an on-system sensor bus [1]. The sensor bus should be versatileenough to ensure efficient communication between all sensors and systems on the platform,however, should be simple enough to be on-chip merged within platform [2]. As an additionalfeature the sensor bus should be able to handle both digital and semi-digital signals, such aspulse width and frequency modulated pulse series. Moreover, self-test should be implementedover the bus by using analog excitation signals and simultaneous semi-digital or digital readout.The basic Integrated Smart Sensor bus (ISS-bus [3]) has been upgraded for this purpose,while maintaining downward compatibility. The basic ISS serial bus includes twocommunication wires and allows digital data transfer using the Manchester encoding schemeand semi-digital data transfer. The enhanced version of the ISS bus is based on a singlecontroller to coordinate the activity on the bus and includes a maskable interrupt mechanism,which makes it very suitable for implementation on the microinstrumentation platform.µFig.1: A microinstrumentation system.Design and implementationBesides the simplicity, this bus interface has two convenient features which makes it verysuitable for an integrated instrumentation system, composed from several heterogeneoussubsystems. Firstly, analog data can be transferred over the bus. Data generated by a sensor withlimited signal processing capability usually comes in this form, so this requirement is necessary,but not present in the usual standard interfaces, which are designated to interconnect only digitalsubsystems (their degree of abstraction is too high).Secondly, the use of the Manchester encoding scheme for transmission of the data at thelogical level adds further flexibility. In such a scheme, the clock is embedded into the dataallowing four logical level instead of having two (see Fig.2). The physical structure (physicallayer) of the classic ISS bus consists of two wires, a data line and a clock line, both open draindriven. Beside these two communication lines, the subsystems have two other common powersupply wires, one for the positive supply and the other for the ground. The data line allows ahalf-duplex communication between the modules connected to the bus [4].In order to increase the flexibility, in the enhanced ISS bus a second data line was added, tobe used for duplex transmission (e. g. in the case of an on-line sensor calibration or testingprocedure). This supplementary line will be used only in the case of a duplex communication,all the rest of the protocol makes use of only the clock and primary data line.In the case of the embedded systems, particularly for instrumentation systems, one would liketo also exist some facilities for the sensor modules to announce the controller when they haveavailable data [5], or more generally, when some particular event happened. In the developedinterface this flexibility is obtained by adding an interrupt request and a service request protocol.An interrupt request message, if it is not disabled by the configuration used for that particularmodule, could be sent over the bus in any moment, even if the controller was in the middle ofanother conversation. A service request instead is allowed only if the bus is in idle state, that is,no other communication takes place in that moment. Nevertheless, the treatments of bothinterrupt and service requests by the controller are unified in a single mechanism, since bothrepresent the same abstraction: a request message from a slave module. Except for the requestmessages, all the other messages are initiated by the controller and are frame oriented.The length of the frame is variable, depending on its significance. The structure of the businterface is shown schematically in Fig.3. These functions have been realised in a 1.6 mCMOS process. A photograph of the chip (1.5x0.7 mm2) is shown in Fig.4.Fig.2: Manchester encoding scheme.Fig.3: The block diagram of the bus interface.Fig.4: A microphotograph of the bus interface.Experimental resultsA typical addressing sequence transmitted serially over the data bus has been recorded and isshown in Fig.5. The frame transmitted was composed by start bit, 4 bits for the sensor addressmore 4 bits related with sensor internal configuration. The chip has a power consumption of500 W (5V@100kHz) and 2mW (5V@4MHz). A network with three bus interfaces wasimplemented and the request block was successfully tested.µClock Data“1” “0” “Error” “Idle” “Bus Free”Bus linesline driverline receiverClkclock bufferManchesterDecoderAddress RegisterConfiguration registerAddress+ConfigurationControlSensororSmart DataCal_lineManchesterEncodingDigital InputBitstream, Analog, PWM, Frequency (Input)Programmable IntegratorService and InterruptRequest ControlInterrupt RequestService RequestSensorSensor  Bus InterfaceµFig.5:A plot of clock and data lines from the oscilloscope.ConclusionsThe features of the microinstrumentation bus interface are:- Simplicity of structure; only two communication wires are used in the minimumconfiguration.- Reliable data transfer by using the Manchester encoding with error detection schemes.- Flexibility of signal type, as synchronous and asynchronous transmission of digital datais possible in combination with semi-digital signals, such as bitstreams, or even analogsignals.- Flexibility of signal handling based on a maskable interrupt mechanism.- Sensor self-test capability over the bus using separate directional data lines.- To be used as a separate die in microsystems and also suitable for on-chip integration withsensors.The features of the bus claimed above have been demonstrated, which makes it suitable forimplementation in a practical microinstrumentation system. An update version of the chip in a0.7 m CMOS process will decrease significantly the total chip area and the powerconsumption.AcknowledgementsThis work is supported in part by STW (project DEL 55.3733), TUDelft and JNICT- Portugal(Program Praxis XXI-BD/5181/95).References[1] K. D. Wise, Integrated Microsystems:Device and Technologies challenges, Proc. ESSDERC’95, TheHague, 1995, pp.15-24.[2] R.F.Wolffenbuttel, Silicon sensors and circuits: on-chip compatibility, Chapman-Hall-1996.[3] J.H.Huijsing, F. Riedijk, G. v. Horn, Developments in integrated smart sensors, Proc. Transducers ‘93,pp.320-326.[4] A. Mason, N. Yazdi, K. Najafi, K. D. Wise, A low-power wireless microinstrumentation system forenvironmental monitoring, Proc. Transducers ‘95, pp. 107-110.[5] J. Bryzek, Introduction to IEEE-P1451 the emerging hardware independent communication standard forsmart transducers, Proc. Eurosensors X - 1996 Belgium, pp.853-876.Ch1Ch2µ

A local bus for MCM-based microinstrumentation systems

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A local bus for MCM-based microinstrumentation systems

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