This Microcontroller with the firmware program developed in C18 C language will do the below activities.
Power Supply Section: This equipment needs a power supply
of 3.3V DC. It accepts 9 to 20V AC/DC at the DC Jack. It is rectified by a
bridge rectifier and filtered through a combination of 1000uF capacitor. The filtered voltage is fed to a Voltage regulator circuit.
Voltage regulator is the adjustable
type fixed type LM117 3.3V & LM7805 5V. Capacitor C3 & C23 (0.1uF) at the output
voltage regulator improves the filtering and avoids oscillation at regulator.
Thus the regulated 3.3V DC & 5V DC fed to all other circuits.
Voltage & current sensing and signal condition
section: This is the critical section of the
equipment. The design of this section should be such way it should not
introduce additional error in the circuit and also it should be able measure
small changes in the measured current/voltage. The circuit measures both
Current & Voltage; from complexity point of view, current measuring is
difficult when compared to voltage measuring. Below figure shows how it is
achieved.
Voltage measuring is done by way of
a voltage divider and the divided voltage is connected to A to D converter of
Microcontroller. As the Microsoft AD converter reference voltage is set as
2.5V, the Vbus voltage of 5V is divided by resistors R5 & R6.
This gives a full scale value 2.5V for 5V input. AD resolution being 12bit, the
lowest measurable voltage becomes 2.5V/2^12=0.00061V which 0.61mV. Though the
noise in the circuit may not yield this much of lowest value measurement,
definitely measurement of 1 to 2mV change should be achievable.
The general method to measure
current is to insert a known resistance to the circuit under measurement in
series and measure the voltage drop across the resistor. From the measured
voltage drop, the current can be easily found by Ohms law (I=V/R). Looking at
the lowest value of current to be measured, highest value to be measured and
the maximum drop allowed, a shunt resistance value 0.1 Ohm was chosen. To ensure
accuracy, the resistance tolerance was chosen at 1%. With this resistor, the
max USB spec current or 500mA & 900mA will yield a drop of 50mV & 90mV respectively.
After getting the voltage drop, it cannot be
directly fed to the AD converter because the drop is across the shunt resistor
and not with reference to Ground (GND). To convert this to GND referenced
voltage, various circuits we analyzed consisting of Op Amps. Elaborate checking at various semiconductor manufacturers
revealed that it may better to use dedicated IC specially built for such
purpose. The chosen IC was from Linear Technology LT®6106 which is a versatile
high side current sense amplifier.
The beauty of this IC is – by changing the 2 resistor values, suitable amplification
of the voltage drop can be achieved. The voltage at the OUT terminal is GND
referenced and it is fed to the AD converter of the Microcontroller. Inside
Microcontroller firmware, suitable division and multiplication is done so that
the current value is arrived.
DC Load Section: This is another
difficult section in the project. This section is basically to simulate
different load conditions on Vbus and to see how the voltage is maintained and
whether the voltage maintained is within the specification of USB. When
researching for this, various ides came up starting from the simple idea of
having different load resistor and connecting the same to the load through a switch
assembly (relays). After analyzing varies ideas, it was concluded to have
circuit as below in Figure.
The circuit is basically to use
Digital to Analog converter to covert a digital value given by the
Microcontroller in to an analog voltage. This is done by a dedicated DAC chip
MCP4812 from Microchip. This is a dual channel 10bit DAC with inbuilt 2.5V
reference voltage source. This connects to the Microcontroller through SPI
interface. The converted analog voltage is then fed to an Op Amp to control
MOFET. The Op Amp used is MCP617 and the MOSFET used is BUZ11 of equivalent. The current
conducted by BUZ11 MOSFET is fed back to the Op Amp. Basically the MOSFET is
operated in linear region as constant current source. The current is set by the
voltage from DAC. So by varying the digital value sent to the DAC by
Microcontroller, the current allowed by MOSFET is varied simulating different
load conditions on the Vbus of USB port. Careful selection of R18, R19 and
detailed processing Microcontroller firmware ensures the desired load range is
achieved.
Input & Output Section: The input & output section helps
to locally control the equipment. It consists of 4 push button switches and LCD
Display. The buttons are named “Menu”, “Up”, “Down” and “Enter” to manually
input and control the equipment. The switches are sensed by Microcontroller.
The Microcontroller port pins connected to the switches are in logic level 1
through pull up resister. When the switch is pressed, the corresponding port
pin is connected to GND (logic 0) this is sensed by Microcontroller firmware
and appropriate action is taken inside the controller. Refer below figure.
For local display, 2x16 Character LCD compatible to Hitachi HD44870 which is very
famous among designers is used. This LCD connects to the Microcontroller through its
Data pins (D4 to D7) and 2 control lines name RS, & E. Currently there is
difficulty in the number of free ports available in the Microcontroller to
operate this LCD so a small revision in the circuit is being carried out to
control the LCD through a Serial to Parallel shift register.
Other support components: Other support components consists of
power supply filter capacitors, crystal, various connectors, optional
Temperature / Humidity sensor DHT11, ICSP header for in-circuit programming
etc.
Development, Programming, Testing and Equipment build up: During
the project design, testing was taken up during the design itself stage by
stage. Most of the testing on hardware circuit design and firmware was done in
the bread board itself. This was done so that multiple iterations were not
required for PCB design. PCB fabrication being a costly and time consuming
activity, this approach helped to save time and money.
To simplify the testing of the hardware and firmware, it was
decided to test small small section of the hardware and firmware code as the
design was progressing. Below are list individual blocks that was identified
and tested along the project development.
- Power ON the Microcontroller with basic wiring of power, MCLR, ICSP and USB host connection.
- Program Microcontroller with slightly modified version of boot loader with the help of external programmer ICD2.
- Reaching this stage of Microcontroller operation ensured further programming of Microcontroller firmware does not need the external ICD2 programmer. The programming can be done using USB port itself with the help of boot loader. The boot loader can be activated by keeping RB2 at logic 0 at the time of power ON or reset of Microcontroller.
- Firmware developed first to communicate with PIC and to decode simple commands like, getting firmware version string, setting a port value etc. At this stage, communication with Microcontroller from PC was done through a terminal emulator program.
- SPI interface related code was written so that Microcontroller was able to communicate with DAC IC MCP4812 over SPI interface. Testing of controlling each channel, switching a channel OFF, changing output value etc were done. The output Voltage value of DAC was cross verified with a multimeter.
- LCD related code was written and sample output was checked at LCD. .canon Initialize ports RB1, RB2, RB3 & RB5 as input
- RTC module was initialized and code was written to set the time from PC interface and the same was displayed in LCD. Picture taken at this point of time is shown below for reference.
- ADC module was initialised and from PC command was sent to read a particular channel. Dummy inputs were connected to the input channels and output reading was verified for the given input.
- Code was written to read input push buttons Read the push button status in process loop.
- USB SOF packet handler interrupt was used which happens every 1ms to de-bounce push button for around 100ms
- Depending on which button s pressed, cycle through menu and sub menu routines for appropriate user control / input.
- If more than a 1 to 2 second of inactivity found at push button, leave menu/sub menu routines and come to normal
With all the testing done & hardware and firmware verified, PCB
design was finalized for sending it to PCB fabricator. It is worthwhile to
mention two points at this junction that is USB current sending & actual DC
load simulation was done at bread board level. This is because of the
complexity involved in using SMD and big through hole components involved in
these sections.
Once the PCB was received from PCB fabricator; it was checked for
any defect like any electrical continuity or short circuit problem. In fact, a
hole for LCD mounting was not drilled and the same was done using hand drilling
machine. Soldering of components were done.
Finally the board was powered up and connected to the PC for remaining
program code verification and development. The sections USB voltage &
current measurement and DC Load were programmed and finally verified. Measured
values were verified with the help of Agilent multimeter for accuracy.
Photos showing various stages of development:
Bread board
Final PCB
Assembly pictures
Complete schematic diagram: Will upload later
Firmware: Needs lot of clean-up, will give link later
PC side control software: Work under progress......
Summary: Prototype developed and basic working measurement demonstrated. Most of the project objectives were met. Below improvements needs to be taken care in Revison 2 of the design.
- Power supply section needs to be reviewed so that 5V rail has 5V whether is fed by USB or external adopter. In the current design, when powered by USB, we can not get 5V
- DC load section does not load more than 400 to 450mA even when the D/A converter gives the maximum output. This section needs to redesigned so that upto 1000mA to 1200mA loading is possible.
- PCB design review - traces close to mounting holes, trace lenght in few areas
- Firmware clean up
- PC side software completion
Further development: Though the equipment was designed to specially test the USB port
which is having the low voltage of 5V and low current of less than 1A, the same
concept can be used variety of other ways. This equipment can further be
improved by way of design & firmware modification to do the below things:
- With the help of few additional adapter cables, this can be used to
study charging characteristics of mobile phones, tablets etc which are
chargeable through USB port
- By modifying the current carrying capacity of the dummy load,
current sensing resister and high side current sensing amplifier, this equipment
can be made to study the charging and discharging characteristics of various
batteries.
- With the above it can be
even used for calibrate notebook batteries which calls for complete discharge
and charge cycles multiple times with a predefined current settings
- With suitable modification in current carrying capacity of the
dummy load, current sensing resistor, high side current sensing amplifier,
voltage divider network for voltage sensing and the firmware, this equipment
can be used for testing small solar panels etc
- With
suitable modification in current carrying capacity of the dummy load, current
sensing resistor, high side current sensing amplifier, voltage divider network
for voltage sensing and the firmware, this equipment can be used for testing
small DC power supplies.
