PIC16LF18854 Datasheet, PDF (314/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
21.0 ZERO-CROSS DETECTION
(ZCD) MODULE
The ZCD module detects when an A/C signal crosses
through the ground potential. The actual zero crossing
threshold is the zero crossing reference voltage,
VCPINV, which is typically 0.75V above ground.
The connection to the signal to be detected is through
a series current limiting resistor. The module applies a
current source or sink to the ZCD pin to maintain a
constant voltage on the pin, thereby preventing the pin
voltage from forward biasing the ESD protection
diodes. When the applied voltage is greater than the
reference voltage, the module sinks current. When the
applied voltage is less than the reference voltage, the
module sources current. The current source and sink
action keeps the pin voltage constant over the full
range of the applied voltage. The ZCD module is
shown in the simplified block diagram Figure 21-2.
The ZCD module is useful when monitoring an A/C
waveform for, but not limited to, the following purposes:
• A/C period measurement
• Accurate long term time measurement
• Dimmer phase delayed drive
• Low EMI cycle switching
21.1 External Resistor Selection
The ZCD module requires a current limiting resistor in
series with the external voltage source. The impedance
and rating of this resistor depends on the external
source peak voltage. Select a resistor value that will drop
all of the peak voltage when the current through the
resistor is nominally 300 A. Refer to Equation 21-1 and
Figure 21-1. Make sure that the ZCD I/O pin internal
weak pull-up is disabled so it does not interfere with the
current source and sink.
EQUATION 21-1: EXTERNAL RESISTOR
RSERIES
=
-^V----^P---^E---^A---^K--
310^–4
FIGURE 21-1:
VPEAK
EXTERNAL VOLTAGE
VMAXPEAK
VMINPEAK
VCPINV
DS40001824A-page 314
Preliminary
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