This document describes a microcontroller-based control and monitoring system for a DC-HVDC power supply module used in the India-based Neutrino Observatory (INO) project. The system uses microcontrollers to control and monitor the voltage applied to Resistive Plate Chambers (RPCs) via DC-HVDC converters. It allows setting the voltage at a specified rate and monitoring voltage and current. The system has been prototyped and future work involves integrating it into the RPC detector readout module and mass producing it for use in the INO experiment.

1. A microcontroller based control and monitoring system on an
SPI interface for a DC-HVDC power supply module
Srinidhi Bheesette*
Electronics and Telecommunication Engineering
Terna Engineering College
Nerul, Navi Mumbai, INDIA, 400706
Email: srinidhi.bheesette@gmail.com
Upendra Yadav
Electronics and Telecommunication Engineering
Terna Engineering College
Nerul, Navi Mumbai, INDIA, 400706
Email:upenndra137@gmail.com
Abstract
India-based Neutrino Observatory (INO) collaboration is planning to
build a massive 50,000 ton Iron Calorimeter (ICAL) detector. Particle
detectors called Resistive Plate Chambers (RPCs) – about 30,000 in
number, will be used as active detector elements. RPCs require a high
voltage of about 10KV to be applied across its parallel glass
electrodes for producing the required uniform field needed for their
operation. A differential voltage (±5KV) solution using two low-
ripple DC-HVDC units is proposed. This solution is superior from the
points of view of cost besides ease of integration, which is a crucial
for ICAL detector.
Aim of this work is to design and build a microcontroller based
control and monitoring system on an SPI interface for the twin DC-
HVDC controllers based power supply module. Main functions of the
control and monitoring system are setting the required voltage at the
specified ramp up or ramp down rate and monitoring the voltage and
load current of the module.
General Terms
INO: Rs. 1350 crore mega science project funded by the Government
of India; ICAL: State-of-the-art, world's largest magnetised iron
calorimeter detector for neutrino physics research; RPCs: Modern,
gaseous, ultra-fast, particle detectors made of a pair of glass
electrodes.
Keywords
DC-HVDC converters, microcontroller, SPI interface, ADC, DAC,
control and monitoring.
Introduction
India-based Neutrino Observatory (INO) [1]
collaboration is currently engaged in building a
large magnetized iron tracking calorimeter (ICAL)
of 50Kiloton in total weight, using atmospheric
neutrinos as source. Main goal of this basic particle
physics experiment is to accurately measure
parameters which govern oscillations between three
flavours of these absolutely tiny particles. The basic
criteria for selecting this type of detector are large
target mass, good energy and angular resolution and
identification of the electric charge of muons. The
experiment will be situated in an underground
cavern of about 132m×26m in area, which will be
built at the end of a 2km tunnel inside Bodi hills
near Madurai in Tamil Nadu. This experiment is
declared by the Government of India as one of the
country's mega science experiments under XII-plan
period.
1. Magnetised Iron Calorimeter (ICAL)
The ICAL detector will have a modular structure of
total lateral size 48m×16m and will consist of a
stack of 151 horizontal layers of 56mm thick low
carbon steel plates interleaved with 40mm gaps to
house the active detector layers called Resistive
Plate Chambers (RPCs). The height of the detector
will be 14.5m.
The detector will be built layer by layer using
4000mm×2000mm×56mm low carbon steel plates,
locked in position using spacers. The detector is
magnetised to a field of about 1.3Tesla, which helps
charge identification of the detecting particles.
The ICAL detector will be subdivided into three
modules of size 16m×16m (Fig. 1). Salient features
of the ICAL detector and its active detector
elements are summarised in the Fig. 2. A total of
28,800 RPCs of dimension 2m×2m will be needed
for his experiment. A dedicated R&D programme is
undertaken by the collaboration to develop and
mass produce these active detector elements
indigenously with help of local industry.
*Corresponding author. Mobile: 09892341794

2. Figure 1: ICAL detector
2. Resistive Plate Chambers
Resistive Plate Chambers [2] are rugged and low-
cost gas detectors and are extensively used in
ongoing and planned high energy and astroparticle
physics experiments for the detection of charged
particles. They have excellent spatial and temporal
resolution leading to applications for time of flight
measurements, tracking detectors and digital
calorimetry due to their large signal amplitudes.
Figure 2: ICAL features and specifications
RPC is composed of two parallel electrodes usually
made of commercial float glass or bakelite. The
electrodes are separated by suitable spacers glued to
both electrodes at regular intervals in such a way
that they channel the gas flow through the chamber
uniformly. T-shaped spacers are glued at the edges
of the electrodes to make the whole module gas
tight. To distribute high voltage on the electrodes
uniformly, their outer surfaces are coated with a thin
layer of graphite paint.
Figure 3: Schematic of RPC detector
An electric field is applied between two electrodes.
When a high voltage of about 10KV is applied
across the glass electrodes and a suitable gas mixer
is flown in the detector, the RPC will work as a
particle detector. The detector draws very little
current (about 200nA) from the power supply. High
voltage power supply is one of the crucial
components of the RPC detector system.
Readout of the RPC is performed by external
orthogonal metal pickup strips. Localized charge
produced by the avalanche or the streamer induces
charge on the appropriate strips. Typical signal
amplitudes in avalanche and streamer mode are 5-
10mV and 100-200mV (across 50Ω load
respectively. These tiny detector signals are
processed and recorded, in order to acquire all the
required information that goes into verifying the
physics principles.
3. Electronics and DAQ scheme for the ICAL
ICAL front-end electronics essentially comprises of
a preamplifier followed by a leading-edge
discriminator stage. This is implemented by an
indigenously developed 8-channel ASIC. Each of
the channels comprises of regulated Cascode trans-
impedance amplifier, differential amplifier,
common threshold comparator and LVDS output
driver. Amplifier’s output of a selected channel is
also available across a 50Ω output buffer through a
built-in analog multiplexer.

3. Figure 4: RPCDAQ module
The ICAL front-end electronics will be located near
and along the X-plane and Y-plane edges of the
RPC gas gaps. The pickup signals will be brought to
these front-end boards using appropriate flexi
cables/structures. The boards will be designed with
a form-factor of about 240mm (length) × 25mm
(width) and mounted vertically on long dimension.
Figure 5: Overall scheme of ICAL electronics
The RPCDAQ module (Fig. 4), which is positioned
at a corner of the RPC is the heart of the electronics
readout system of the RPC. It controls and acquires
data from the entire RPC using a high-end FPGA
(Xilinx XC6SLX100FGG676 or Altera
EP4CE115F29 chips), a multi-hit 200ps resolution
Time-to-Digital Converter (TDC) chip, 5Gs/s
waveform sampler, ambient parameter sensor
module, Wiznet W5300 network controller and
other high density components. A soft-core
processor embedded in the FPGA takes charge of
all the supervisory functions of the RPCDAQ
module.
The RPCDAQ module is also designed to
communicate with the high voltage supply
controller (the topic of our project) using an SPI bus
so that it can control and monitor the high voltage
and measure the RPC current. Overall scheme of the
electronics, trigger, data acquisition and network
sub-systems is given in Fig. 5.
4. High voltage options for RPC
RPCs require a high voltage of about 10KV to be
applied across its parallel glass electrodes for
producing the required uniform field for their
operation. A differential voltage of ±5KV is usually
preferred from other considerations.
Conventional solution for the high voltage supply
comprises of a centralised power supply, along with
associated cabling to each detector. In this we have
two options: a channel at 10kV or two channels at
±5kV. Additionally it also possible to power 4
RPCs with a single HV channel. But here the cable
diameter is an integration issue and the connectors
are also expensive. Therefore a distributed or local
solution using DC-HVDC units is being proposed
by the collaboration. This solution is superior from
the points of view of cost besides ease of
integration, which is often turned out to be a crucial
aspect in case large scale detectors.
Two linear, low ripple DC-HVDC converters [3-5]
are chosen due to their small size, as these units are
planned to be mounted inside the RPC itself. The
converter produces the output voltage proportional
to the input control voltage and also provides
monitoring voltages proportional to the output
voltage and load current. ICAL will use 60,000 DC-
HVDC converter units.
The system comprises of two HVDC convertors
which produce proportional output in the range of
0-6000V to the applied input 0-5V. We have used
EMCO [3] made Q60 (positive) and Q60N
(negative) converters. The EMCO Q-Series is a
broad line of ultra-miniature, high reliability DC
HVDC converters supplying up to 6,000 volts in
only 0.125 cubic inches and up to 10,000 volts in
only 0.614 cubic inches. These component-sized

4. converters are ideal for applications requiring
minimal size and weight. The output is directly
proportional to the input voltage and is linear from
<0.7V input to maximum input voltage, allowing
for an adjustable output voltage with a peak to peak
ripple of <1%. Shown in Fig. 6 are the linearly
plots (input versus output voltages) for Q60(+6kV)
and Q60N(-6kV) that we obtained.
Figure 6: Calibration of the DC-HVDC converters
5. Overall scheme of our project
In our project we wish to control the input/control
voltage of the DC-HVDC converters using
embedded DAC channels in the microcontroller
ATxmega32A4U [6]. The set voltage and the load
currents are readout by the ADC channels
embedded in the microcontroller. Thus, main
hardware functions of the microcontroller are to
control its ADC and DAC channels. Besides, it
must also communicate with the RPCDAQ module
using the SPI communication bus so that this
functional interface is extended to the detector's
central data acquisition scheme.
Figure 7: Calibration plot of the ADC (Channel 0)
The ATxmega32A4U microcontroller provides 12
ADC channels of 12-bit resolution and is capable of
converting up to 2MSPS. The input selection is
flexible, and both single-ended and differential
measurements can be implemented. We have tested
the ADC channels by reading the input voltages
through ADC Channel 0 and 1. Shown in Fig. 7 is
the calibration plot for Channel 0.
Figure 8: Calibration of a DAC channel
The ATxmega32A4U microcontroller also provides
two-channel, 12-bit, one MSPS Digital to Analog
Converters (DAC). We have calibrated these DAC
channels. Data for one of them is shown in Fig. 8.
Figure 9: Overall scheme of the project
A schematic of the overall scheme of the project is
given in given in Fig. 9. It may be noted that ADC
and DAC connections to and from the DC-HVDC
units represent only logical connections. In practice,
the DAC output (range 0-3.3V) is processed
through a difference amplifier and a variable power
supply regulator before reaching the DC-HVDC
control input. This is needed to condition the DAC
output for offset and dynamic range which is
needed for our application. Similarly, the voltage
monitoring output is processed through an
attenuator and an inverter before fed to the ADC

5. (range 0-2.07V) input. These circuits are detailed in
Fig. 10.
The Serial Peripheral Interface (SPI) is a high-speed
synchronous data transfer interface using three or
four pins. It supports full-duplex communication
and allows fast communication among peripheral
devices and controllers. We used ATmega32U4
device as master to simulate the RPCDAQ module,
which is going to be used in the ICAL detector and
HV controller is built around an ATxmegaA4U
device (slave). SPI is used as communication
interface between master and slave.
Figure 10: Signal conditioning circuits for DAC and ADC
6. Software sequence of the project
As mentioned in Section 5, RPCDAQ module is
simulated in our project using an ATmega32U4
module. An LCD module is connected to the
ATmega32U4 for providing the user interface. The
ATmega32U4 is also equipped with a couple of
push-button switches to key in commands to ramp-
up or ramp-down the high voltage by 100 volts.
Firmware is developed [7] and loaded into both
ATmega32U4 (Master) and ATxmega32A4U
(Slave) boards to implement the above mentioned
functionality. Individual software sequences are
listed below:
a. Master
Power up: Vset = 0
Initialise LCD display
(A) Poll Push buttons; If no input Goto (A)
If 'Up' key pressed:
Vset = Vset + 100; If Vset > 3000 then Vset = 3000
Goto (A)
If 'Down' key pressed:
Vset = Vset - 100; If Vset < 0 then Vset = 0
(B) Calculate DAC data
Transmit DAC data to the Slave on SPI
Wait for the DC-HVDC voltage to stabilise
Read ADC data from the Slave
Calculate Vmon
Display Vmon on the LCD
Goto (A)
Commands:
[00][DAC data] = Set Vset (DAC data)
[01][ADC data] = Send Vmon (ADC data)
b. Slave
Power up: DAC data = 0
(A) Set voltage to DC-HVDC converters
Wait for the DC-HVDC voltage to stabilise
Read ADC data
Store ADC data
(B) Listen to SPI port: Goto (B)
If Set Command: Read DAC data; Goto (B)
If Mon Command: Send ADC data; Goto (B)
Command:
[02][ADC data] = Sending Vmon (ADC data)
7. Status of current work
Extensive market survey was done to select a
suitable DC-HVDC module, which will meet all the
specifications of the ICAL project. To begin with
we have procured the EMCO units and completed
their characterisation and calibration. We then did a
similar survey for selecting suitable
microcontrollers for the slave (DC-HVDC
controller) as well as the RPCDAQ simulator
(master). We then procured these chips and built the
boards. We designed and built rest of the circuitry
on 'mother boards' so that the circuits could be
expanded easily if needed. We have followed this
approach because we are engaged in developing a
new system.
We integrated a couple of push-buttons and a 20x4
character commercial LCD display module for the
master. Similarly we built quad operational
amplifier, variable power supply regulator based

6. signal conditioning circuits for the ADC and DAC.
We have also wired all necessary circuits for the
DC-HVDC modules and other ancillary circuits.
We setup and used the integrated software platform
called Atmel Studio, on which we developed the
entire software for handling the ADC [8], DAC,
digital I/O, SPI ports and LCD [9]. As we
developed module by module, we have tested these
modules independently before integrating them in
all one. Picture of a 'crude' but actual working setup
of this project is shown in Fig. 11.
Figure 11: Actual working setup of the project
8. Future outlook
As mentioned already, the functionality of the
master is going to be implemented in the FPGA,
which constitutes major part of the RPCDAQ
module. We will move all the hardware and
software design features of the master into this
FPGA. We will also simultaneously spruce up the
slave design and design a high density, small form-
factor board, taking into account the specific
integration issues of the RPC detector as well as the
power supply, cable and other resources which are
planned to be available at that time.
We will then integrate and test the completed
system on an actual RPC detector before submitting
our design to the INO collaboration for their
scrutiny, validation and feedback, if any. If accepted
by the collaboration - after needed refinements and
modifications, this module will hopefully be mass
produced and integrated into about 30,000 RPC
detector units.
This scheme was developed based on the EMCO Q-
series DC-HVDC converters. We have also found
that HVM technologies produces low-ripple, small
volume converters, which are much easier to control
and monitor. Similarly EMCO has also come up
with new AG series of converters, which are also
very tiny and simple to operate. The collaboration
might want to opt for these devices in future, even
though both of these are very expensive at this
moment.
Acknowledgements
We would like to thank the INO collaboration
(Department of High Energy Physics, Tata Institute
of Fundamental Research, Mumbai) for giving us
this exciting opportunity to develop this system for
a world class experiment. We are grateful to Mr.
S.S.Upadhya, Mr. Sandeep Duhan and Dr.
Satyanarayana Bheesette, for their able guidance
and constant encouragement. Without them, this
work would not have been possible. We are
thankful to Mr. M.Saraf, Mr. S.R.Joshi, Mr.
R.R.Shinde, Ms. Sonal Dhuldhaj and Mr.
A.D'Souza for their help throughout this project.
References
1. INO Collaboration, INO Project Report
INO/2006/01, (2006); http://www.ino.tifr.res.in
2. Satyanarayana Bheesette, Design and
Characterisation Studies of Resistive Plate
Chambers Ph.D. Thesis, (2009)
3. EMCO High Voltage Corporation:
http://www.emcohighvoltage.com
4. HVM Technology: http://www.hvmtech.com
5. iseg Spezialelektronik GmbH: http://www.iseg-
hv.com
6. ATMEL Corporation: http://www.atmel.com
7. Muhammad Ali Mazidi and Janice Gillispie
Mazidi, The 8051 Microcontroller and
Embedded Systems, Prentice Hall, (1999)
8. http://www.avrfreaks.com
9. http://extremeelectronics.co.in