USB-Project/Measurement Frontend
Author: R.J. Maris
This project principally demonstrates the usage of a MSP430
controller with USB functionality. At a first glance, it seems
somewhat curious to have an USB application with an ultralow power device.
But there's at least one big argument pro USB: One can use an MSP430
application as a battery powered datalogger, while data can be
transferred from and to a PC via USB. Moreover, the USB-bus can
charge batteries without extra power sources. Of course, the USB-core
must not consume any current when the unit is operating on its own.
It is exactly this goal which is realized with this project.
Project description
The hardware is built around a 64-pin MSP430 device, typically a
MSP430F14x. The USB-functionality is realized with a chip from FTDI (see
the USB-page). This chip was chosen, because the manufacturer
has extensive software support for the host side.
Along with a photograph of top- and bottom side of the PCB, the board
and its features will be explained.
Top side of the PCB (following the keywords along the photograph)
-
A 32 kHz as well as a 6 MHz crystal are provided. This
allows for optimum flexibility in power conservation goals. The 6 MHz
crystal also provides a clock to the USB-core. An inverter in a
SOT23-5 package buffers this clock to the USB-chip. A power down of
the USB-core has no effect on the crystal operation.
-
The USB-chip provides a parallel bus to the MSP430. This costs
12 I/O lines. A serial solution was considered, too. But the max.
througput rate would be only 3 Mbps, which has an impact on software.
3 Mbps actually means approx. 3 µs/byte transfer rate. Without a
software buffer, one could not write consequtively bytes to the
USB-core. The parallel bus allows painless data transfers without a
software buffer. The buffer is in the USB-chip.
The disadvantage in terms of I/O-resources is relative: 8 bits may be
shared with other, optional resources as an LCD-display or keypad.
-
An adjustable inductor based voltage inverter converts the
3.3 V power supply to voltages in the range -3.3....-8 V.
This accomodates painless design of analog add-on circuits as well as
to serve as a PWM-driven adjustable contrast voltage for LCDs. The
add-on capability is outlined in the next paragraph. This voltage
inverter can be powered down by a digital MSP-output. Also, the positive
supply can be disabled, detail info below.
-
A Li-Ion battery (why Li-Ion?: low self-discharge rate)
is considered as a power source for stand alone operation. No special
charge control hardware is utilized here. Instead, a small powerFET
allows charge current control by the MSP. Therefore, the charging
current must be measured by the controller. This is implemented using
a micropower opamp, which connects to an ADC-input while measuring
the current in charge as well as discharge direction. This will
facilitate accurate battery capacity gauging, even when the current
consumption is low.
Bottom side of the PCB
The board has been designed for optimum context flexibility, i.e.
housing and add-on options:
-
A flexfoil connector is provided for connecting to a miniature low
cost graphic LCD.
-
The USB-port conforms to the 1.1 Full Speed standard (12 Mbps).
-
Stacking another PCB is possible via the two pin headers.
The headers are designed as through-hole mounting. This allows the
designer to use either side of the board for adding other circuitry.
The lower part of the photograph shows a typical configuration for a
small add-on where the board can be enclosed in a small housing. The
bottom side has components with height max. 4 mm, which allows
fitting in typical handheld enclosures (see the image in the left column).
Putting the pin headers at the top side - see the upper part of the
photograph - would make sense when this MSP-module serves as a slave
stacked to a bigger board.
Currently, the pin headers are 2mm gridded types. A second version of
the PCB will be created next with 1/10" gridded headers, which
allows the usage of this module with standard breadboards.
-
The JTAG-header is placed at the bottom side, because this is
the side with the USB-socket. Hence: the bottom side is normally the
top side, regardless the assembly context as outlined above. The
JTAG-header may be straight or right-angle.
Technical Details
As mentioned above, the negative power supply can be disabled. The
same applies to the positive power supply, which is softswitched by a
small powerFET.
The MSP430 itself is powered independently from this power source by
a micropower low-drop regulator, which brings down the battery
voltage range from approx. 3.3 ... 4.2 V resp. 4.5 ... 5 V
USB-bus voltage range to a safe 3.3 V level.
Several changes in the current layout must be prepared (replacement
of the DIP-housed inverter IC by an SMD type and a few small
enhancements). When these changes have been realized, a schematic
will be available.
The low power capabilities of the MSP430 are fully supported by the
surrounding chips. The opamp for the battery current measurement is a
CMOS type, TLV2241 (TI). It consumes only 1 µA (GBW = 5
kHz!). The 3.3 V voltage regulator is a XC6201 (Torex). This one
takes no more than 2 µA typically. Both components are low
cost and availability is ok.
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