TB3138

TB3138
Zero-Cross Detection Module Technical Brief
Author:
Mike Gomez
Microchip Technology Inc.
INTRODUCTION
In earlier 8-bit PIC® microcontroller devices, the ZeroCross Detection (ZCD) of high input voltage, such as
the A/C line voltage, relies in the clamping ability of the
parasitic electrostatic discharge ESD protection diode
on the I/O pin. This method has been used successfully
for many years. However, in the advent of recent microcontroller devices with additional analog pass-gates
forbid the I/O pins voltage to exceed VDD and thus the
conduction of parasitic ESD diode. Violating this specification may cause an unexpected behavior of the
microcontroller. Refer to Microchip’s Technical Brief
TB3013 “Using ESD Parasitic Diodes on Mixed Signal
Microcontrollers” (DS90003013) for more details about
pass-gates and their roles in reducing the possibilities
of using the voltage clamping ability of the parasitic
ESD diode on the I/O pins.
FIGURE 1:
For these reasons, Microchip provides a dedicated
Zero-Cross Detection (ZCD) module on its 8-bit
microcontroller devices. This module can detect zero
crossing accurately, while preventing the parasitic ESD
diode to conduct when interfacing with the high-voltage
A/C input signals. This technical brief describes the
ZCD module features, the method of configuration and
the calculation of external components needed for the
implementation.
ZCD OPERATING CIRCUIT
Figure 1 shows a simplified schematic diagram for the
implementation of the ZCD module.The source signal
VIN can be measured by the module through a series
current-limiting resistor R1. To safely interface VIN to
the module’s input pin (ZCDxIN), R1 impedance must
be carefully chosen to limit the input current to a value
that the module can tolerate. The peak current that can
be sourced or sinked in ZCDxIN is 300 μA. Refer to
Equation 1 for selecting the R1 value.
ZCD SIMPLIFIED BLOCK DIAGRAM
Note 1: R1 should be implemented as multiple resistors to limit the voltage drop across each
resistor below the maximum rating allowed.
 2015 Microchip Technology Inc.
DS90003138A-page 1
TB3138
EQUATION 1:
ZCD CURRENT-LIMITING
RESISTOR CALCULATION
V IN  PEAK 
R 1 = -----------------------300 A
The high-frequency noise signals from the external
source can affect the module’s operation at near zerocrossing point. To prevent these unwanted signals from
causing chatter in the module’s output (ZCDxOUT), an
optional capacitor C1 can be placed across ZCDxIN to
form a simple low-pass filter with R1. However, this
additional capacitor may lag the input signal and trigger
a phase delay. Equation 2 shows the equation of the
phase delay where FC is the cut-off frequency of the
desired input signal and R1 is the calculated currentlimiting resistor. Based on the equation, the higher the
value of C1, the more it increases the phase delay (see
Example 1). Therefore, the designer should choose an
appropriate value for C1 to meet the acceptable phase
delay, based on their design tolerance.
EQUATION 2:
PHASE DELAY EQUATION
tan – 1  2  F C  R1  C 1 
T DELAY = --------------------------------------------------------------2  F C
EXAMPLE 1:
PHASE DELAY CALCULATION BASED ON THE VALUE OF C1
Time Delay of 110V AC, 60 Hz input source when C1 is 30 pF and 30 nF:
(1)
V IN  PEAK 
110V  2
R 1 = ------------------------ = -------------------------- = 518.5 k
300A
300A
When C1=30 pF:
tan – 1  2  60  518.5 k  30 nF 
T DELAY = ---------------------------------------------------------------------------------------- = 15.5 s
2  60
(2)
When C1=30 nF:
(2)
tan – 1  2  60  518.5 k  30 nF 
T DELAY = ---------------------------------------------------------------------------------------- = 3.71 ms
2  60
As stated earlier, the device I/O pin’s parasitic ESD
protection diode must not conduct while detecting the
zero crossing of the A/C input signal. In order for the
module to meet this requirement, the module applies a
current source or sink to ZCDxIN. When VIN is greater
than the zero-crossing reference voltage (VCPINV),
which is typically 0.75V above ground, the module
sinks current. When VIN is less than VCPINV, the
DS90003138A-page 2
module sources current. The current source and sink
action keep the ZCDxIN pin voltage constant over the
full-range VIN while the detection of zero crossing
happens when the current through ZCDxIN changes
direction.
 2015 Microchip Technology Inc.
TB3138
The module includes a Status bit through the
ZCDxOUT bit to determine whether the current source
or the sink is active. When the ZCDxOUT bit is set, the
ZCDxIN pin is sinking current and when cleared, the
ZCDxIN pin is sourcing current. This bit can also be
affected by the ZCD Logic Output Polarity (ZCDxPOL)
bit relative to the current source and sink output. When
ZCDxPOL is set, the polarity of ZCDxOUT is reversed,
as shown in Figure 2.
Note:
Internal weak pull-up on ZCDxIN (if
available in the device) should be disabled
so that it will not interfere with the current
source/sink action.
The ZCD interrupt can be generated based on the
ZCDxOUT bit and if the associated enable bits are set.
When the Positive Edge Interrupt (ZCDxINTP) bit is set
and the ZCDxOUT bit changes from logic low to high,
the ZCD interrupt will be triggered. Likewise, when the
Negative Edge Interrupt (ZCDxINTN) bit is set and the
ZCDxOUT bit changes from logic high to low, the ZCD
interrupt will also be triggered.
FIGURE 2:
ZCD OUTPUT WAVEFORM
+ event
- event
+ event
Input
Signal
ZCD OUTPUT
ZCDxPOL = 0
ZCD OUTPUT
ZCDxPOL = 1
ZCD EVENT OFFSET
The ZCD triggers at VCPINV and not at 0V. Assuming
that VIN is sinusoidal and relative to the VSS pin, the
voltage offset from zero to VCPINV causes the zerocross event to occur too early as the VIN waveform
falls, and too late as the VIN waveform rises. The actual
offset time produced can be calculated using
Equation 3. Refer to Example 2 for the example ZCD
event offset calculation.
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DS90003138A-page 3
TB3138
EQUATION 3:
ZCD EVENT OFFSET EQUATION
T OFFSET
EXAMPLE 2:
V CPINV – V Desired Reference
sin –1  -----------------------------------------------------------------
V PEAK
= -----------------------------------------------------------------------------------2  Frequency
ZCD EVENT OFFSET EXAMPLE CALCULATION
Input AC voltage at 110 VRMS with 60 Hz frequency and desired threshold of 0V:
(3)
T OFFSET
750 mV – 0V
sin –1  ---------------------------------
 110  2 
= ---------------------------------------------------- = 12.7886 S
2  60 Hz
Equation 3 is derived from the instantaneous voltage of
a sine wave, as seen in Equation 4. The product of
angular velocity, , and the instant time determines
the angular measurement or the position of the
instantaneous voltage
at a given time. In
detecting the zero cross, the
value of interest is
near zero voltage or approaching zero voltage,
therefore the angular measurement will be relatively
EQUATION 4:
small. Since the angle is small, the calculation of sin
will be approximately equal to
(small-angle
approximation of sine functions). In this case,
Equation 3 can be further reduced to Equation 5.
Recalculating the given values in Example 2 using
Equation 5 will arrive at approximately the same result,
as seen in Example 3.
INSTANTANEOUS VOLTAGE EQUATION
v  t  = V PEAK  sin  t 
where:  = 2ft
EQUATION 5:
SIMPLIFIED ZCD EVENT OFFSET EQUATION
VCPINV – V Desired Reference
T OFFSET = ----------------------------------------------------------------V PEAK  2   Frequency
EXAMPLE 3:
SIMPLIFIED EVENT OFFSET EXAMPLE CALCULATION
(5)
DS90003138A-page 4
750 mV – 0V
T OFFSET = --------------------------------------------------------- = 12.7885 S
110  2  2   60 Hz
 2015 Microchip Technology Inc.
TB3138
OPTIONAL BIASING RESISTOR
The ZCD event offset described in the previous section
can be compensated by adding an optional external
bias resistor (see Figure 3). The bias resistor can alter
the VCPINV detection threshold to 0V or to any desired
set point.
FIGURE 3:
OPTIONAL BIASING RESISTOR
The purpose of the external biasing resistor (RBIAS) is
to provide a current (IBIAS) that is equal to the current
flowing through the current-limiting resistor (I1) at the
desired threshold of detection (VDesired Reference).
Therefore, when the input voltage is equal to
VDesired Reference, the total input current (IIN) that will be
produced at the node will be equal to zero, thus leaving
no current entering the ZCDxIN.
equation to find R1, the R1 value is obtained (see
Equation 8). Now that R1 is already determined, the
voltage-resistor equivalent of IBIAS from Equation 7 can
be used to find IBIAS, as seen in Equation 9. Finally, the
value of RBIAS can be determined using the calculated
IBIAS value, as shown in Equation 10.
EQUATION 6:
In Figure 3, IIN is the combination of the current I1 and
IBIAS (see Equation 6). I1 and IBIAS can be replaced by
their equivalent voltage-resistor equations to calculate
the IIN, as seen in Equation 7. Simplifying Equation 7
by replacing IIN with 300 μA and rearranging the
EQUATION 7:
TOTAL INPUT CURRENT
EQUATION
I IN = I BIAS + I 1
SIMPLIFIED TOTAL INPUT CURRENT EQUATION
V CPINV – VDesired Reference V IN_PEAK – V CPINV V IN_PEAK – V Desired Reference
I IN = ----------------------------------------------------------------- + ----------------------------------------------- = ----------------------------------------------------------------------R1
R1
R1
EQUATION 8:
R1 EQUATION
V IN_PEAK – V Desired Reference V IN_PEAK – V Desired Reference
R 1 = ----------------------------------------------------------------------- = ----------------------------------------------------------------------I IN
300 A
 2015 Microchip Technology Inc.
DS90003138A-page 5
TB3138
EQUATION 9:
IBIAS EQUATION
EQUATION 10:
V CPINV – V Desired Reference
I BIAS = ----------------------------------------------------------------R1
RBIAS EQUATION
V BIAS – V CPINV
R BIAS = ------------------------------------I BIAS
For a given VIN of 110 VRMS and VBIAS of 5V,
Example 4 shows the calculation of RBIAS and R1 when
the detection threshold is set at 0V.
EXAMPLE 4:
SAMPLE EXTERNAL BIASING RESISTOR CALCULATION
VIN_PEAK = V RMS  2 = 110V  2 = 155.5V
750 mV – 0V 155.5V – 750 mV
155.5V + 0V
 7  I IN = -------------------------------- + ------------------------------------------- = -----------------------------R1
R1
R1
155.5V – 0V
 8  R 1 = ------------------------------ = 518.5 k
300 A
750 mV – 0V
 9  I BIAS = -------------------------------- = 1.44 A
518.5 k
5V – 750 mV
 10  R BIAS = -------------------------------- = 2.9 M
1.44 A
Using the calculated R1 and RBIAS values in
Example 4, the total current flowing through ZCDxIN
can be determined. Example 5 shows the input current
value based on the transition of the input source VIN
from the positive to the negative cycle. If the VIN is at
EXAMPLE 5:
the desired detection threshold, the total input current
should be equal to zero for the ZCD module to toggle
state. This is to check if the R1 and RBIAS values are
correct.
CHECKING FOR THE RESISTOR VALUES
(7)
V IN – V CPINV
V BIAS – V CPINV
I IN =  ------------------------------------- +  --------------------------------
R BIAS
R1
At VIN = 155.5 VPEAK:
5 – 0.75
155.5 – 0.75
I IN =  ------------------- +  ---------------------------  300 A
2.9 M
518.5 k
At VIN = 0V:
5 – 0.75
0 – 0.75
I IN =  ------------------- +  -----------------------  0 A (ZCD will switch state)
 2.9 M  518.5 k
At VIN = -155.5 VPEAK:
5 – 0.75
– 155.5 – 0.75
I IN =  ------------------- +  ------------------------------  – 300 A
 2.9 M  518.5 k 
DS90003138A-page 6
 2015 Microchip Technology Inc.
TB3138
Figure 4 shows the actual generated ZCD output signal
based on Example 4.
FIGURE 4:
ZCD OUTPUT
0.750V
0V
ZCD Triggers at 0.750V
110V AC Signal
ZCDxOUT
TOFFSET
(No external Biasing) VIN=110 VRMS; R1=518 k; TOFFSET=12.78 µS
0.750V
0V
ZCD Triggers at 0 V
110V AC Signal
ZCDxOUT
VIN=110 VRMS; R1=518 k; RBIAS=2.9 M
CONFIGURATION BIT
The ZCD module can be permanently enabled upon
power-up by clearing the ZCD Disable bit (ZCDDIS/
ZCD) in the Configuration Word. Clearing this bit
ensures that ZCDxIN will be kept at a regulated and
safe voltage as soon as the device is powered on.
Therefore, ZCDxIN cannot be multiplexed with any
other functionality. However, when ZCDDIS/ZCD is
set, the ZCD can be enabled or disabled during
firmware runtime by setting or clearing the Zero-Cross
Enable (ZCDxEN) bit, respectively.
 2015 Microchip Technology Inc.
DS90003138A-page 7
TB3138
IMPLEMENTING ZCD USING
MICROCHIP’S MPLAB® CODE
CONFIGURATOR (MCC)
4.
In this section, MPLAB® Code Configurator (MCC) is
utilized to easily configure the ZCD module. The MCC
is a user-friendly plug-in tool for MPLAB® X IDE which
generates drivers for controlling and driving peripherals
of PIC microcontrollers, based on the settings and
selections made in its Graphical User Interface (GUI).
Refer to the “MPLAB® Code Configurator User’s
Guide” (DS40001725) (http://www.microchip.com/
pagehandler/en_us/devtools/code_configurator/
home.html) for further information on how to install and
setup the MCC in MPLAB X IDE. The following steps
will guide on how to configure the ZCD module in
PIC16F1613 using MCC:
5.
1.
2.
3.
Navigate to: “Tools> Embedded> MPLAB Code
Configurator” to launch the MCC.
Set the desired Configuration registers and the
system clock source on the System label inside
of MPLAB X under the Project Resources
window.
Under the Device Resources panel, expand
ZCD and then double-click on ZCD::ZCD to
bring the module up to the Project Resources
panel.
FIGURE 5:
6.
7.
8.
In the center panel, after clicking the ZCD::ZCD
in the Project Resources panel, check the
Enable Zero-Cross Detection and Enable
Output checkbox.
Select non-inverted as the Logic Output
Polarity.
To enable ZCD interrupt detection on the rising
edge, check the Enable ZCD Interrupt and
Enable Positive Edge Interrupt checkbox.
To configure the ZCD input and output pins,
expand the MPLAB® Code Configurator Pin
Manager on the right side of the screen. Click
the green lock next to ZCD1OUT and ZCD1IN
to assign them as following: ZCD1OUT =
PORTA(R1), ZCD1IN = PORTA(R2).
Click the Generate Code button in the top left
corner of the center panel. This will generate a
main.c file to the project automatically. It will
also initialize the module and leave an empty
while(1) loop for custom code entry. See
Figure 5 for the User Interface of ZCD in MCC
and Example 6 for the generated initialization
code for the ZCD module.
MCC USER INTERFACE FOR ZCD
DS90003138A-page 8
 2015 Microchip Technology Inc.
TB3138
EXAMPLE 6:
MCC GENERATED INITIALIZATION CODE FOR ZCD
void ZCD_Initialize (void)
{
// Set the ZCD to the options selected in the User Interface
// ZCD1EN enabled; ZCD1POL not inverted; ZCD1INTP enabled; ZCD1OE
enabled; ZCD1INTN disabled;
ZCD1CON = 0xC2;
// Clearing IF flag before enabling the interrupt.
PIR3bits.ZCDIF = 0;
// Enabling ZCD interrupt.
PIE3bits.ZCDIE = 1;
}
void ZCD_ISR(void)
{
// Clear the ZCD interrupt flag
PIR3bits.ZCDIF = 0;
}
void PIN_MANAGER_Initialize(void)
{
LATA = 0x00;
TRISA = 0x3D;
ANSELA = 0x15;
WPUA = 0x00;
LATC = 0x00;
TRISC = 0x3F;
ANSELC = 0x0F;
WPUC = 0x00;
OPTION_REGbits.nWPUEN = 0x01;
}
CONCLUSION
This technical brief covers the Zero-Cross Detection
(ZCD) module in PIC microcontrollers. It provides ways
on how to implement and interface the modules along
with the external components needed. The calculations
of external component values such as the currentlimiting resistor and the external biasing resistor are
also provided in this technical brief to alter the detection
threshold to any set point. The configuration of ZCD is
demonstrated using the MPLAB Code Configurator
(MCC). An example initialization code is generated
using the MCC, as well.
 2015 Microchip Technology Inc.
DS90003138A-page 9
Note the following details of the code protection feature on Microchip devices:
•
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•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
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•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
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•
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•
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ISBN: 978-1-63277-462-0
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Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
Sweden - Stockholm
Tel: 46-8-5090-4654
UK - Wokingham
Tel: 44-118-921-5800
Fax: 44-118-921-5820
Taiwan - Taipei
Tel: 886-2-2508-8600
Fax: 886-2-2508-0102
Thailand - Bangkok
Tel: 66-2-694-1351
Fax: 66-2-694-1350
01/27/15
 2015 Microchip Technology Inc.
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