AN780 MICROCHIP [Microchip Technology], AN780 Datasheet - Page 6

• Manufacturer Part Number: AN780
• Description: 15-Kilogram Scale Using the TC500A and the TC520
• Manufacturer: MICROCHIP [Microchip Technology]

DS00780A-page 6
FIGURE 5: Serial interface protocol.
REFERENCE VOLTAGE CALCULATION
Now that the timing has been determined, eq3 can be used to
calculate the reference voltage:
The reference voltage does not need to be calculated very precise
since it will have to be trimmed during calibration. A 25% adjust-
ment range is enough to make up for just about any minor
calculation error.
MICROPROCESSOR PROGRAMMING
The PIC16C62A 8-Bit microcontroller was selected but any
reasonable processor/controller will suffice. The PIC16C62A is a
28-pin part that has EPROM programmability.
AN780
Read Format
Load Format
V
REF
= V
= 1V 200mS/490mS
READ
D
LOAD
D
D
D
IN
OUT
OUT
CLK
IN
CLK
(max) T
INT
OVR POL MSB
MSB
/T
DCINT
.408V
CONCLUSION
The scale works extremely well. The 8X oversampling makes it
very smooth and noise-free. The response time is within one
conversion ( 1/2 sec) for changes of 2 grams or more. Changes of
less than 2 grams are accumulated in an integrating register until
it gets to either +1 gram or –1 gram. When this happens, the current
conversion is allowed to "get through" and a new base is estab-
lished in the accumulator.
There is also a facility in the programming that allows the raw data
to be displayed. These displays show the full 17-bit conversion
results. The basic converter noise is as predicted, typically 1 to 2
counts of flicker (16-bit accuracy) with an intermittent jump of
about 3 or 4 counts (1/f noise). One count is equivalent to 1/8 gram.
The actually 60Hz power line rejection ration of the TC500A was
not measured, but judging from the 6 to 8 counts of "rolling noise"
before preloading the TC520 with 2F
The effect of the differential ratiometric reference was tested by
changing the supply voltage from +4V to +6V. Although there was
a 1 – 2 second delay due to unmatched time constants between
the reference and the strain gauge, the final readings were exactly
the same. This shows that the power supply rejection is better
than 100dB.
LSB
© 2002 Microchip Technology, Inc.
H
, it is quite adequate.
LSB