DSP 104-plus, Справочное руководство

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158004.B00 37
the A/D chip can be driven from an external voltage source, or from the on-board
VCC supply voltage. A solder link allows this selection to be made. The accuracy of
the measurement of course will be limited by the accuracy of the VREF voltage.
Measurements can be made as “single-ended” or “differential” measurements, as
programmable options. In single-ended mode each of the four inputs are measured
with respect to the AGND (0V) pin. (AGND is connected to the digital GND at a single
point on the TP400). In differential mode the difference between two inputs is
measured. The difference between ADC0 and ADC1 can be measured, as can the
difference between ADC2 and ADC3.
Measurements can also be made as unipolar or bipolar, as programmable options. In
Unipolar mode an input voltage of AGND will give a digital output of 0000h, and an
input voltage of VREF will give a digital output of 0FFFh. In bipolar mode an input
voltage of VREF/2 will give a digital output of 0000h, an input voltage of VREF will
give a digital output of 07FFh, and an input voltage of 0V will give a digital output of
800h. Voltages between 0V and VREF/2 are treated as negative voltages, and
converted into twos complement negative numbers. It is probably only sensible to use
bipolar mode in conjunction with differential mode, where the difference between two
inputs can be negative as well as positive.
Sample software for the A/D converter is provided on the TP400 Utilities Disk. This
software makes measurements using the A/D converter. The software may be used
as a guide to users who wish to write their own A/D code.
A temperature sensor is provided on the TP400. This is mounted underneath the
Geode GX1 processor chip, and thus measures a temperature that will be related to
that of the Geode GX1 package. Thus the temperature of the Geode GX1 can be
estimated. This feature was added to allow the power management software to slow
the processor down if it got too hot. At the time of writing this power management
feature has not been implemented.
The temperature sensor is connected via a solder link to ADC3. If ADC3 is required
to measure an off-board voltage then the solder link can be removed.
The temperature sensor is the National Semiconductor LM60C. It has a voltage
output given by:
V = (T x 0.00625) + 0.424
where T is temperature in degrees C.
Expressed differently, the output is 6.25mV per degree C, with an offset such that 0
degrees C gives 424mV.
Alternatively:
T = (V - 0.424) x 160
The data sheets for the MAX1247 and the LM60C are on the TP400 Utility Disks.
Due to a lack of connectors the analog to digital convertor chip shares pins on
connector J6 with the SoundBlaster audio circuitry. Appendix B describes how pins on
this connector can be assigned to one function or the other.