101-1006 Rabbit Semiconductor, 101-1006 Datasheet - Page 49

• Manufacturer Part Number: 101-1006
• Description: MODULE POWERCORE FLEX 3800
• Manufacturer: Rabbit Semiconductor

Specifications of 101-1006

• Module/board Type: MPU Core Module
• For Use With/related Products: RCM3800
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

The input capture software runs as an interrupt routine that is triggered by the end-of-ramp
interrupt. The interrupt routine will read the time delay from the start of the ramp to the
signal measured and store it in a memory table. The software will then set up the next
measurement, generally by switching an n to 1 multiplexer to direct the next comparator
output to the pulse capture stop input. The reference voltage is measured to calibrate the
slope of the ramp. The pulse capture feature of the Rabbit 3000 microprocessor is used to
measure the time between the start of the ramp and when the ramp reaches the analog
input voltage that is being measured.
W
Figure 14. Using Multiplexer for A/D Converter Channels
In order to filter out noise picked up in the
ramp signal that results from coupling of the
PCB traces to nearby fast-switching digital
signals, the ramp is filtered by an RC circuit at
each comparator package by an RC circuit
consisting of a 3.3 kΩ resistor and a 2.2 nF
capacitor. This RC filter delays the ramp by
about 7 µs, which is the same as 10 mV of
ramp movement. The 3 db point of the filter is
Figure 15. Ramp Signal Offset
approximately 25 kHz. This is adequate to
from RC Filter
eliminate high-frequency spikes coupled to the
PCB trace. The same filter is used for the start-
of-ramp pulse and the reference pulse, so the
delay of the ramp is canceled out. When the ramp switches direction from downward to
upward, the ramp has sufficient negative swing to allow the transient to die down before
the start-of-ramp pulse is generated.
User’s Manual
41