PIC16LF18854 Datasheet, PDF (316/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
21.2 ZCD Logic Output
The ZCD module includes a Status bit, which can be
read to determine whether the current source or sink is
active. The OUT bit of the ZCDxCON register is set
when the current sink is active, and cleared when the
current source is active. The OUT bit is affected by the
polarity bit.
21.3 ZCD Logic Polarity
The POL bit of the ZCDxCON register inverts the
ZCDxOUT bit relative to the current source and sink
output. When the POL bit is set, a OUT high indicates
that the current source is active, and a low output
indicates that the current sink is active.
The POL bit affects the ZCD interrupts. See Section
21.4 “ZCD Interrupts”.
21.4 ZCD Interrupts
An interrupt will be generated upon a change in the
ZCD logic output when the appropriate interrupt
enables are set. A rising edge detector and a falling
edge detector are present in the ZCD for this purpose.
The ZCDIF bit of the PIR2 register will be set when
either edge detector is triggered and its associated
enable bit is set. The INTP enables rising edge inter-
rupts and the INTN bit enables falling edge interrupts.
Both are located in the ZCDxCON register.
To fully enable the interrupt, the following bits must be set:
• ZCDIE bit of the PIE2 register
• INTP bit of the ZCDxCON register
(for a rising edge detection)
• INTN bit of the ZCDxCON register
(for a falling edge detection)
• PEIE and GIE bits of the INTCON register
Changing the POL bit will cause an interrupt, regard-
less of the level of the EN bit.
The ZCDIF bit of the PIR2 register must be cleared in
software as part of the interrupt service. If another edge
is detected while this flag is being cleared, the flag will
still be set at the end of the sequence.
21.5 Correcting for VCPINV offset
The actual voltage at which the ZCD switches is the
reference voltage at the non-inverting input of the ZCD
op amp. For external voltage source waveforms other
than square waves, this voltage offset from zero
causes the zero-cross event to occur either too early or
too late. When the waveform is varying relative to VSS,
then the zero cross is detected too early as the
waveform falls and too late as the waveform rises.
When the waveform is varying relative to VDD, then the
zero cross is detected too late as the waveform rises
and too early as the waveform falls. The actual offset
time can be determined for sinusoidal waveforms with
the corresponding equations shown in Equation 21-2.
EQUATION 21-2: ZCD EVENT OFFSET
When External Voltage Source is relative to Vss:
asin


V_--V---c_-P--p_-E--i_-A-n_--K-v_{-}
TOFFSET = -----2----_----_-----_F----_r--_e---_q-------
When External Voltage Source is relative to VDD:
asin


V_-----D_---D_{-V---–-P--}V_-E---c_A---p_K---i_--n_----v_{-}
TOFFSET = ------------2----_-----_----_F----_r---_e---_q-------------
This offset time can be compensated for by adding a
pull-up or pull-down biasing resistor to the ZCD pin. A
pull-up resistor is used when the external voltage
source is varying relative to VSS. A pull-down resistor is
used when the voltage is varying relative to VDD. The
resistor adds a bias to the ZCD pin so that the target
external voltage source must go to zero to pull the pin
voltage to the VCPINV switching voltage. The pull-up or
pull-down value can be determined with the equations
shown in Equation 21-3 or Equation 21-4.
EQUATION 21-3: ZCD PULL-UP/DOWN
DS40001824A-page 316
Preliminary
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