The TI parallel port JTAG tool: MSP-FET430PIF gets its operating power
from two signal lines of the parallel port of a PC. As a result, the
operating voltage is often less than 2.9V and the available current is
very limited. Like many other JTAG tools from TI, there are two
alternative ways to power the target board. For target board without
its own power source (or with its own power source disconnected),
MSP-FET430PIF can supply the power through JTAG pin #2. But the
voltage and current of this tool may not be enough for the target. For
target board with its own power source, pin #2 should not be connected
and pin #4 should be connected instead. But if the voltages of the two
power sources are very different or if the two are not turned on at
the same time, there are problems too.
The circuit board inside MSP-FET430PIF is actually designed with
optional external power supply to eliminate the above problems. The
schematic and component layout diagram can be found on pages 62 and 63
of the TI document SLAU138K.PDF.
As shown in the schematic, J3: Ext_PwR and D3: LL103 can be used to
bring external power to the input of the voltage regulator U3:
TPS77001. Also, the optional R29 and R30 can be used to adjust the
operating voltage. Thus it is fairly simple to enable this
unimplemented feature. (Do the following at your own risk!)
Before you start to change anything, you may want to check the
operating voltage (for example, at pin #2 of JTAG connector) and the
amount of current it is capable of delivering. But you do not have to
do this.
The two halves of the plastic shell of the tool are hooked together at
two points on each side. There are no screws, you can insert a small
flat screw drive at those four points and pull the two halves apart.
The circuit board is held to one of the two halves by two screws. You
may not even need to take the board out.
Some of the components are not loaded on the board. In particular, J3,
D3, R29 and R30 are probably missing. You need to bring an external
power between 4V and 5.5VDC to where J3 should be and you may need to
add a diode to where D3 should be. After that, the available current
should be much larger, but the voltage may still be too low due to the
voltage regulator.
After you added the external source, you can now increase the
operating voltage by adding R30 in parallel with the existing R25:
2k7. In principle, you can calculate the desired value of the resistor
to use. In practice, you may just try and err. The smaller the value
of R30 (in parallel to R25: 2k7), the higher the voltage. R30=10k will
increase the voltage by about 9%, R30=5k will increase the voltage by
about 15%. When R30 is too low, the resulting voltage may be too high
and damage the device!
In the unlikely event that the operating voltage is already to high
without R30, you can add R29 (in parallel with the existing R24: 3k6)
to decrease the regulated voltage.
