AN61546 Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices.pdf

AN61546
Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC)
Design Guidelines and Best Practices
Author: Shivendra Singh
Associated Part Family: CY14xxxxx (nvSRAM RTC)
Related Application Notes: AN53313
To get the latest version of this application note, or the associated project file, please
visit http://www.cypress.com/go/AN61546.
AN61546 describes the RTC functionality, component selection criteria, and best layout design practices for the
nvSRAM RTC design. The guidelines and best practices covered in this application note are intended to assist you in
designing nvSRAM with RTC functions in your system design and to minimize timing errors, which mostly occur due to
improper layout design and component selection.
Contents
1
2
3
Introduction ...............................................................1
Crystal Basics ...........................................................2
Crystal Resonant Frequencies .................................3
3.1
Equivalent Series Resistance ..........................3
3.2
Crystal Quality Factor ......................................4
3.3
Drive Level .......................................................4
4
nvSRAM RTC Clock Oscillator Circuit ......................5
4.1
Load Capacitance ............................................5
5
nvSRAM RTC Circuit Design ....................................7
5.1
RTC Backup Power Options ............................8
5.2
Calculation of RTC Backup Time .....................9
6
PCB Design Considerations ................................... 11
1
6.1
Signal Routing ............................................... 11
6.2
RTC Clock Calibration ................................... 12
6.3
Troubleshooting Guide for nvSRAM RTC ...... 12
7
Summary ................................................................ 13
8
Related Documents ................................................ 13
Document History............................................................ 14
Worldwide Sales and Design Support ............................. 15
Products .......................................................................... 15
®
PSoC Solutions ............................................................. 15
Cypress Developer Community....................................... 15
Technical Support ........................................................... 15
Introduction
The nvSRAM RTC integrates standard real time clock functions and nonvolatile SRAM functions. Applications such
as servers, security and surveillance systems, industrial controllers, data loggers, and single-board computers are
just a few system examples that require RTC functions to operate the system reliably and accurately. The integrated
RTC functions along with the nvSRAM provide multiple advantages such as enabling infinite write into SRAM of
nvSRAM, automatic data saving during power down, and allowing the systems to log critical information continuously
into nonvolatile memory with time stamping.
The RTC block uses a 32.768-kHz crystal to produce a reference clock for timekeeping functions. It maintains system
timing information regardless of whether it is in active mode or power-down mode. To keep the RTC block active
during power-down mode requires a backup power source that keeps the clock oscillator running even though the
VCC supply is switched OFF.
The accuracy of the RTC clock mainly depends upon the accuracy of the components used, layout design,
component placement, and operating temperature. The clock accuracy can be improved further by enabling the onchip clock calibration option. The RTC Clock Calibration section of this application note provides a brief description of
RTC clock calibration.
This application note discusses the crystal operation, nvSRAM RTC component selection criteria, and layout design
guidelines.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
2
Crystal Basics
Crystals are made of quartz material, which contains silicon dioxide and exhibits piezoelectric properties. Quartz
generates an electrical potential when pressure is applied on the surfaces of the quartz crystal. Inversely, when an
electrical potential is applied to the surfaces of a quartz crystal, mechanical deformation or vibration is generated.
These vibrations occur at a frequency determined by the following:




Physical dimension of the piece of quartz crystal
Cut of the crystal in relation to the crystalline axes of the quartz
Operating temperature
Oscillator circuit
The natural oscillation frequency is stable. Additionally, the resonance has a high quality factor (Q) ranging from tens
of thousands to several hundred thousand.
Crystals have several fundamental characteristics that are important to the design of an oscillator circuit. A typical
crystal symbol is shown in Figure 1 and its equivalent circuit is shown in Figure 2. The circuit consists of series
components that include motional inductance L1, motional resistance R1, and motional capacitance C1. The parallel
component C0 is the shunt capacitance of the crystal.
The equivalent series resistance (ESR), also known as motional resistance, is the impedance of the crystal when the
reactive components of the crystal cancel at the resonant frequency. ESR and Q are inversely proportional. The
lower the ESR, the less energy that is lost in the crystal. A crystal with a high ESR requires more power to operate
and takes longer to start.
Figure 1. Crystal Symbol
Figure 2. Crystal Equivalent Circuit
C0
R1
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L1
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
3
Crystal Resonant Frequencies
Figure 3 shows the reactance frequency plot of the crystal. A crystal has two resonant frequencies: series resonance
and parallel resonance.
Figure 3. Crystal Resonant Frequencies
When a crystal is operating at series resonance, it looks purely resistive, and the inductive reactance of L1 equals the
capacitive reactance of C1. Since these impedances are 180° out of phase, they cancel out each other, leaving R1 as
the impedance between the crystal terminals.
The series resonance frequency (fs) is determined by Equation 1.
Equation 1
𝑓𝑓𝑓𝑓 =
1
2𝜋𝜋�𝐿𝐿1 C 1
When the series resonance occurs, the effect of reactive impedance is minimal, and the circuit behaves as a resistive
circuit with minimum equivalent impedance, thus drawing maximum current.
When the crystal is operating in parallel resonant mode, the inductor L1 reacts with the total capacitance between its
terminals. This is also known as the antiresonant frequency and is defined by Equation 2.
Equation 2
𝑓𝑓𝑓𝑓 =
2𝜋𝜋�𝐿𝐿1
1
𝐶𝐶 1 (𝐶𝐶 0 +𝐶𝐶 𝐿𝐿 )
𝐶𝐶 1 +(𝐶𝐶 0 +𝐶𝐶 𝐿𝐿 )
This equation combines the parallel capacitance of C0 and CL, where CL is the load capacitance specified by the
crystal manufacturer as shown in Figure 5.
When a crystal is operating at its antiresonant frequency, the impedance is at its maximum, and current flow is at its
minimum. Crystals are mostly designed to resonate between fs < f < fa, and this range of frequencies between fs and
fa is called the “area of usual parallel resonance” or “parallel resonance.”
3.1
Equivalent Series Resistance
The ESR is the resistance exhibited by the crystal at series resonant frequency (fs). This is not necessarily the R1
value shown in Figure 2. The ESR of the oscillator circuit can be calculated using Equation 3.
Equation 3
𝐸𝐸𝐸𝐸𝐸𝐸, 𝑅𝑅𝑆𝑆 = 𝑅𝑅1 �1 +
𝐶𝐶0 2
𝐶𝐶𝐿𝐿
�
This value is typically monitored when tuning the quartz crystal to the specified resonant frequency. RS is sometimes
specified as a maximum resistance and should be used when determining the drive level of the oscillator (see Drive
Level).
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
3.2
Crystal Quality Factor
Due to the piezoelectric effect of the crystal, a physical displacement occurs when an electric field is applied. The
reverse effect happens when the crystal is deformed and electrical energy is produced across the crystal electrodes.
A mechanically resonating crystal is seen from its electrodes as an electrically resonating circuit. Therefore, the
crystal behaves like a tuned circuit and can store energy. You can quantify the amount of stored energy by stating the
quality factor (Q) of the crystal. Crystal Q is defined as shown in Equation 4.
Equation 4
Where,
𝑄𝑄 =
𝜔𝜔𝜔𝜔 1
𝑅𝑅1
=
1
ω𝐶𝐶1 𝑅𝑅1
𝜔𝜔𝜔𝜔1 = Reactance due to inductor (L1)
3.3
1/𝜔𝜔𝜔𝜔1 = Reactance due to capacitor (C1) at the operating frequency of the crystal
Drive Level
Drive level refers to the power dissipated in the crystal. The crystal specification document defines the maximum
drive level that the crystal can sustain. Overdriving the crystal can cause excessive aging, frequency shift, quartz
fracture, and eventual failure. The designer should ensure that the maximum rated drive level of the crystal is not
exceeded. Again, the drive should be maintained at the minimum level that is necessary for the oscillator to start up
and maintain steady state operation.
You can compute power dissipation of the crystal by using Equation 5.
Equation 5
Where,
𝑃𝑃 = 2𝑅𝑅1 [ π f (𝐶𝐶0 + 𝐶𝐶𝐿𝐿 )𝑉𝑉𝑅𝑅𝑅𝑅𝑅𝑅 ]²
VRMS = Root mean square (RMS) value of the voltage across the crystal
f = Nominal frequency of oscillation for the crystal
Most of the RTC crystals specify a maximum power dissipation of 1 μW.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
4
nvSRAM RTC Clock Oscillator Circuit
The oscillator used in the nvSRAM RTC is a CMOS inverter-based Pierce-type oscillator as shown in Figure 4. The
inverter acts as a class AB amplifier and provides approximately 180° phase shift from the input to the output,
whereas the pi (π) network formed by crystal, CINT1, and CINT2 provides an additional 180° phase shift from the output
to the input. Therefore, the total phase shift around the loop is 360°. This satisfies one of the conditions required for
sustained oscillation. The other condition for sustained oscillation is the closed loop gain, which must be greater than
one. The resistor RF around the inverter provides negative feedback and sets the bias point of the inverter near midsupply voltage, thus operating the inverter in the high gain linear region. The value of the RF resistor is high and is
measured generally in the range of a few megaohms.
Figure 4. Pierce-Type Oscillator
Driver
Ripple
Counter
CINT2
CINT1
RF
RTC Registers
XIN
XOUT
Inside Chip
Outside Chip
CEXT2
Crystal
CEXT1
The Pierce-type oscillator uses a crystal that operates in parallel mode. The nominal frequency of oscillation for the
crystal is defined under the specific load condition. To oscillate the crystal at its specified frequency, you should
design the application board in such a way that the load capacitance across the crystal pads (XIN and XOUT) on the
PCB is equal to the specified load for the crystal.
4.1
Load Capacitance
The load capacitance (CL) is the capacitive load of the oscillating circuit as seen from the pins of the crystal. Figure 5
depicts CL as a capacitance measured across the XIN and XOUT pins without crystal. CINT1, CINT2, and any stray
capacitance in the circuit are combined to create overall load capacitance.
Figure 5. Oscillator Load Capacitance
Driver
Ripple
Counter
C INT2
C INT1
RF
RTC
Registers
XOUT
XIN
Inside Chip
Outside Chip
CTRACE2
Crystal
CTRACE1
CEXT2
C PARASITIC
CEXT1
An equivalent load capacitance CEQ2 on XIN pin can be defined as shown in Equation 6.
Equation 6
CEQ2 = CINT2 + CTRACE2 + CEXT2
Similarly, an equivalent load capacitance CEQ1on XOUT pin can be defined as shown in Equation 7.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
Equation 7
CEQ1 = CINT1 + CTRACE1 + CEXT1
The resultant load capacitance (CL) is calculated as shown in Equation 8.
Equation 8
Where,
𝐶𝐶𝐿𝐿 =
𝐶𝐶𝐸𝐸𝐸𝐸 1 𝐶𝐶𝐸𝐸𝐸𝐸 2
𝐶𝐶𝐸𝐸𝐸𝐸 1 +𝐶𝐶𝐸𝐸𝐸𝐸 2
+ 𝐶𝐶𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃
CINT1 = Input capacitance on the XOUT pin
CINT2 = Input capacitance on the XIN pin
CTRACE1 = Capacitance of the trace connecting pin XOUT, crystal, and capacitor CEXT2
CTRACE2 = Capacitance of the trace connecting pin XIN, crystal, and capacitor CEXT1
CEXT1 = External capacitor connected on XOUT pin
CEXT2 = External capacitor connected on XIN pin
CPARASITIC = Parasitic capacitance due to the crystal mount on the board
CL = Total capacitive load of the circuit that must be applied across the crystal pins to oscillate at its nominal
frequency
Any change in the load capacitance of the crystal circuit has an effect on the frequency of oscillation. The drift in
frequency of the operation from nominal operation frequency can be characterized as follows.

If the specified load capacitance of the crystal is larger than the load capacitance of the crystal circuit (CL) on the
PCB across the crystal pads, then this configuration causes the oscillator to run faster than specified nominal
frequency.

Reciprocally, using a crystal with a specified capacitive load smaller than the CL causes the oscillator to run
slower than the nominal frequency.

If the specified capacitive load of a crystal equals the load capacitance of the CL, then the crystal oscillates at its
nominal frequency.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
5
nvSRAM RTC Circuit Design
The nvSRAM RTC requires connecting an external 32.768-kHz crystal and C1, C2 load capacitance to build a RTC
circuit as shown in Figure 6.
Figure 6. nvSRAM RTC Design
VRTCcap
VRTCbat
The load capacitances C1 and C2 are inclusive of the PCB parasitic components, including the capacitance due to
the land pattern of crystal pads/pins, XIN/XOUT pads, and copper traces connecting crystal and device pins. Figure 7
illustrates the PCB parasitic components.
Effective capacitance that should be mounted additionally onto the board is calculated by using the following
equations. These external load capacitors are defined as CEXT1 and CEXT2 in Figure 5.
Equation 9
Equation 10
Where,
CEXT1 = C1 – CPARASITIC1
CEXT2 = C2 – CPARASITIC2
CPARASITIC1 = Parasitic capacitance due to the land pattern of crystal pads/pins, the trace connecting the crystal pads
and device pads XOUT, and the land pattern of XOUT
CPARASITIC2 = Parasitic capacitance due to the land pattern of crystal pads/pins, the trace connecting the crystal pads
and device pads XIN, and the land pattern of XIN
Figure 7. PCB Parasitic Components
nvSRAM RTC
XOUT
XIN
CPARASITIC1
CPARASITIC2
Pad 1
Pad 2
Table 1 provides the recommended component values for the nvSRAM RTC design.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
Table 1. nvSRAM RTC Circuit Components
CY Part Number
CY14B101P
CY14B512P
CY14B256P
CY14B101KA/MA
C1 (ext)
(Typ)
C2 (ext)
(Typ)
Crystal Freq, CL
(Typ)
10 pF
68 pF
±5%
±5%
10 pF
68 pF
±5%
±5%
12 pF
68 pF
±5%
±5%
Production Status
32.768 kHz, 12.5 pF
Not Recommended for New Designs
32.768 kHz, 12.5 pF
In Production
32.768 kHz, 12.5 pF
In Production
CY14B064I/PA
CY14B256I/PA
CY14B512I/PA
CY14B101I/PA
CY14B104K/M
CY14B108K/M
CY14B116K/M
Table 2 provides a typical RTC crystal specification for the nvSRAM RTC.
Table 2. Crystal Specifications
Parameter
Symbol
Value (Typ)
Units
f
32.768
kHz
Equivalent Series
Resistance (ESR)
RS
50
kΩ
Frequency Stability
K
-0.035
PPM/(Δ °C)²
Drive Level
1
µW
Aging (first year at 25 °C)
±3
PPM
pF
Frequency
Load Capacitance
CL
12.5
Q Factor
Q
60000
Operating Temp Range
5.1
-40 °C to +85 °C
RTC Backup Power Options
The nvSRAM RTC supports both capacitor and battery options as the RTC power backup in the absence of VCC
supply. The nvSRAM RTC design recommends using either a battery or a capacitor option for the RTC power
backup. You should avoid mounting both of them together in an application.
The choice of battery or capacitor entirely depends on the application requirements. As a rule, if the system expects
power down for less than 30 days, then a capacitor may be more suitable than a battery because of its smaller
footprint and the reliability of capacitors at higher ambient temperatures. However, if the system’s expected powerdown period is longer than 30 days at a stretch, then battery is generally preferred because supercapacitors with a
value greater than 1 farad and a voltage rating above 3.6 V are not easily available in small-footprint package
options. If the application can allow larger capacitor packages, the RTC backup time for more than 30 days can be
obtained with a capacitor value larger than 1 farad. The calculation for nvSRAM RTC backup time is shown in
Example 1 for the capacitor option and Example 2 for the battery option. These two examples are valid for any rated
value of a capacitor or battery used in applications.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
When using a battery for the RTC power backup option, the VRTCbat pin is directly connected to the lithium coin cell
battery. Similarly, if a capacitor is used for the RTC power backup, then the VRTCcap pin is directly connected to the
supercap. The nvSRAM RTC has an internal automatic switching that disconnects itself from the backup power
source and switches to the main VCC whenever the main supply is available. It switches to the backup power source
when power goes off. To determine the backup time duration achievable by different capacitor values or different
battery ratings, use the equations provided in the following section.
5.2
Calculation of RTC Backup Time
This section describes equations that you can use to calculate battery and capacitor backup time for the RTC. The
nvSRAM draws a constant current (IBAK) to keep the oscillator running using a backup power supply source (either
capacitor or battery). This constant current gradually discharges the capacitor or battery over the time period.
5.2.1
Discharge Time for Capacitor
Equation 11
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 =
𝐶𝐶𝐶𝐶𝐶𝐶
𝐼𝐼
Where,
Time = Total discharge time in seconds
C = Value of supercap on VRTCcap pin in farad
ΔV = VCC – VRTCcap (min)
I = Supercap discharge current in amperes
Supercap discharge current (I) comprises nvSRAM RTC backup current (IBAK) and supercap self-leakage current (iL),
as shown in Equation 12.
Equation 12
5.2.2
I = IBAK + iL
Example 1
If the application uses a 1-F capacitor on the VRTCcap pin, with a typical power supply at 3 V (VCC) and drawing a
typical IBAK of 350 nA at room temperature, then the capacitor can power the RTC for 49.6 days without recharging.
To illustrate the backup time calculation, the following example considers the supercap discharge current (I) as the
nvSRAM RTC backup current IBAK.
Calculation
The minimum capacitor voltage required on the VRTCcap pin to run the RTC is 1.5 V.
Therefore, Δ V is 1.5 V (3 V – 1.5 V).
Equation 13
5.2.3
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇(ℎ𝑟𝑟) =
1.0 𝑥𝑥 1.5
350 𝑥𝑥(10)−9 𝑥𝑥 60 𝑥𝑥 60
= 1,190 hours or 49.6 days
Discharge Time for Battery
Battery manufacturers provide the battery spec in milliampere hours (mAh). The RTC backup timing calculation for
the battery is simpler than for the capacitor.
Equation 14
Where,
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 (ℎ𝑟𝑟𝑟𝑟) =
𝑚𝑚𝑚𝑚 ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜
𝐼𝐼 (𝑚𝑚𝑚𝑚 )
I (mA) = IBAK in milliamperes
5.2.4
Example 2
If the application uses a 48-mAh BR1225 coin cell battery, then the battery can run the nvSRAM RTC for
approximately 15.6 years without battery replacement. The nvSRAM RTC circuit draws a typical IBAK of 350 nA at
room temperature.
Calculation
Equation 15
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇(ℎ𝑟𝑟) =
48 𝑥𝑥(10)−3
350 𝑥𝑥(10)−9 𝑥𝑥 24 𝑥𝑥 365
= 137,142 hours or 15.6 years
The nvSRAM RTC specifies 1.8 V as the minimum voltage at the battery pin (VRTCbat). That is, the battery output
voltage must be greater than 1.8 V to provide the sustained RTC oscillation throughout the life span of the system.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
5.2.5
Example 2
If the application uses a 48-mAh BR1225 coin cell battery, then the battery can run the nvSRAM RTC for
approximately 15.6 years without battery replacement. The nvSRAM RTC circuit draws a typical IBAK of 350 nA at
room temperature.
Calculation
Equation 15
𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇(ℎ𝑟𝑟) =
48 𝑥𝑥(10)−3
350 𝑥𝑥(10)−9 𝑥𝑥 24 𝑥𝑥 365
= 137,142 hours or 15.6 years
The nvSRAM RTC specifies 1.8 V as the minimum voltage at the battery pin (VRTCbat). That is, the battery output
voltage must be greater than 1.8 V to provide sustained RTC oscillation throughout the life span of the system.
5.2.6
Charging Time for Capacitor on VRTCcap
The charging of the backup capacitor is also a critical parameter to consider in RTC circuit design.
If VRTCcap > 0.5 V or if no capacitor is connected to the VRTCcap pin, then the oscillator start time (tOCS) is 1 to 2 seconds
as defined in the datasheet.
If the capacitor on VRTCcap is discharged to a voltage level lower than 0.5 V (VRTCcap < 0.5 V), then after applying VCC,
the nvSRAM RTC does not start the oscillator immediately; it waits until the RTC capacitor on VRTCcap is charged to
0.5 V. After the voltage on VRTCcap reaches 0.5 V, the nvSRAM starts oscillation after tOCS time.
The time nvSRAM takes to charge the RTC capacitor to 0.5 V depends upon the initial residual voltage on the RTC
capacitor and the charging path resistance (RBKCHG) of nvSRAM, which is typically 650 Ω.
The capacitor charging equation in a given RC network is defined as follows:
Equation 16
Where,
𝑉𝑉𝑉𝑉(𝑡𝑡) = 𝑉𝑉𝑉𝑉𝑉𝑉(1 − 𝑒𝑒 −𝑡𝑡/𝑅𝑅𝑅𝑅 )
Vc(t) = Voltage across the capacitor at a time t
VCC = Supply voltage in volts
R = Charging path resistance (RBKCHG) in Ω
C = Value of supercap on the VRTCcap pin in farads
5.2.7
Example 3
A fully discharged capacitor of value 1.0 F is connected to the VRTCcap pin. When 3.0 V is applied to the nvSRAM VCC,
it starts charging the capacitor on VRTCcap to a minimum voltage level of 0.5 V before it starts oscillation in tOCS time.
The typical time nvSRAM takes to charge the capacitor from 0 V to 0.5 V is 118 sec.
Calculation
Equation 17
0.5 = 3.0 (1 − 𝑒𝑒 −𝑡𝑡/(650∗1) )
t (sec) = 118 seconds (approximately 2 minutes)
You can apply this formula to calculate the actual charging time for the RTC capacitor that is connected on the
VRTCcap pin of nvSRAM.
Refer to the device datasheet for VRTCcap/VRTCbat (min/max), IBAK (min /max), and RBKCHG (min/max) values.
Note: If a battery is applied to the VRTCbat pin prior to VCC, the chip will draw high IBAK current. This occurs even if the
oscillator is disabled. To maximize battery life, VCC must be applied before a battery is applied to the VRTCbat pin.
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
6
PCB Design Considerations
The RTC crystal oscillator is a low-current circuit with high impedance nodes on the crystal pins. Due to the lower
timekeeping current of RTC, the crystal connections are very sensitive to noise on the board. Hence, it is necessary
to isolate the RTC circuit from other signals on the board.
It is also critical to minimize the stray capacitance on the PCB board. Stray capacitances add to the overall crystal
load capacitance and therefore cause oscillation frequency errors. Proper bypassing and careful layout are required
to achieve optimum RTC performance.
6.1
Signal Routing
The board layout must adhere to (but is not limited to) the following guidelines while routing nvSRAM RTC circuitry.
Following these guidelines can help you achieve optimum performance from the nvSRAM RTC design.

Place the crystal as close as possible to the XIN and XOUT pins. Keep the trace lengths between the crystal and
the RTC equal in length and as short as possible to reduce the probability of noise coupling by reducing the
length of the antenna.

Keep the XIN and XOUT trace width less than 8 mils. A wider trace width leads to larger trace capacitance. The
larger these bond pads and traces are, the more likely it is that noise can couple from adjacent signals.

Shield the XIN and XOUT signals by providing a guard ring around the crystal circuitry. This guard ring prevents
noise coupling from neighboring signals.

Take care while routing any other high-speed signal in the vicinity of the RTC traces. The more the crystal is
isolated from other signals on the board, the less likely it is that noise is coupled into the crystal. Maintain a
minimum 200-mil separation between the XIN, XOUT traces and any other high-speed signal on the board.


Do not route any signals underneath crystal components on the same PCB layer.
Create an isolated solid copper plane on the adjacent PCB layer and underneath the crystal circuitry to prevent
unwanted noise coupled from traces routed on the other signal layers of the PCB. The local plane should be
separated by at least 40 mils from the neighboring plane on the same PCB layer. The solid plane should be in
the vicinity of RTC components only, and its perimeter should be kept equal to the guard ring perimeter.
Figure 8 shows the recommended layout for the nvSRAM RTC circuit.
Figure 8. Recommended Layout for nvSRAM RTC
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Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
6.2
RTC Clock Calibration
The RTC is driven by a quartz-controlled oscillator with a nominal frequency of 32.768 kHz. The clock accuracy
depends on the quality of the crystal, usually ±20 ppm limits at 25 °C. This error could equate to 1.53 minutes per
month. The nvSRAM RTC employs a digital calibration circuit, which can improve the accuracy to +1/–2 ppm at
25 °C. The calibration circuit adds or subtracts counts from the oscillator divider circuit. The number of times the
pulses are suppressed (subtracted, negative calibration) or split (added, positive calibration) depends upon the value
loaded into the five calibration bits defined in the calibration control register. Adding counts speeds up the clock;
subtracting counts slows down the clock. The calibration bits occupy the five lower order bits in the control register.
These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a sign bit, where “1” indicates
positive calibration and “0” indicates negative calibration. Calibration occurs within a 64-minute cycle.
The first 62 minutes in the cycle may, once per minute, have 1 second either shortened by 128 or lengthened by 256
oscillator cycles. If a binary “1” is loaded into the register, only the first 2 minutes of the 64-minute cycle is modified; if
a binary “6” is loaded, the first 12 is affected and so on. Therefore, each calibration step has the effect of adding 512
or subtracting 256 oscillator cycles for every 125,829,120 actual oscillator cycles, that is, +4.068 or –2.034 ppm of
adjustment per calibration step.
To determine the calibration value, set the CAL bit in the flag register to one. This causes the INT pin to toggle at
512-Hz nominal frequency. Any deviation measured from the 512 Hz indicates the degree and direction of the
required correction. For example, a reading of 512.01024 Hz indicates a +20-ppm error, requiring a –10 (001010) to
be loaded into the calibration register. Note that setting or changing the calibration register does not affect the test
output frequency. Refer to AN53313 for more information on nvSRAM RTC clock calibration techniques.
6.3
Troubleshooting Guide for nvSRAM RTC
The Cypress nvSRAM troubleshoot guide KBA94279 addresses common nvSRAM RTC issues that can occur in
systems and provides resolution on majority of these.
www.cypress.com
Document No. 001-61546 Rev.*F
12
Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
7
Summary
To get the desired accuracy from the RTC circuit, you must consider various factors that can directly affect RTC
performance. These factors are crystal and component selection, layout design rules, and operating conditions. This
application note highlights the key design rules that should be followed to get the desired RTC performance from the
nvSRAM RTC.
8
Related Documents
Application Note

AN53313 - Real Time Clock Calibration in Cypress nvSRAM
Datasheets














CY14B101P: 1-Mbit (128 K × 8) Serial SPI nvSRAM with Real Time Clock
CY14B512P: 512-Kbit (64 K × 8) Serial (SPI) nvSRAM with Real Time Clock
CY14B256P: 256-Kbit (32 K x 8) Serial (SPI) nvSRAM with Real Time Clock
CY14B104K, CY14B104M: 4-Mbit (512 K × 8/256 K × 16) nvSRAM with Real Time Clock
CY14C064I, CY14B064I, CY14E064I: 64-Kbit (8 K × 8) Serial (I2C) nvSRAM with Real Time Clock
CY14C064PA, CY14B064PA, CY14E064PA: 64-Kbit (8 K × 8) SPI nvSRAM with Real Time Clock
CY14C256I, CY14B256I, CY14E256I: 256-Kbit (32 K × 8) Serial (I2C) nvSRAM with Real Time Clock
CY14C256PA, CY14B256PA, CY14E256PA: 256-Kbit (32 K × 8) SPI nvSRAM with Real Time Clock
CY14C512I, CY14B512I, CY14E512I: 512-Kbit (64 K × 8) Serial (I2C) nvSRAM with Real Time Clock
CY14C101I, CY14B101I, CY14E101I: 1-Mbit (128 K × 8) Serial (I2C) nvSRAM with Real Time Clock
CY14C101PA, CY14B101PA, CY14E101PA: 1-Mbit (128 K × 8) Serial (SPI) nvSRAM with Real Time Clock
CY14B104K, CY14B104M: 4-Mbit (512 K × 8/256 K × 16) nvSRAM with Real Time Clock
CY14B108K, CY14B108M: 8-Mbit (1024 K × 8/512 K × 16) nvSRAM with Real Time Clock
CY14B116K, CY14B116M: 16-Mbit (2048 K × 8/1024 K × 16) nvSRAM with Real Time Clock
Knowledge Base Articles

Troubleshooting Guide for nvSRAM and FRAM-KBA94279
About the Author
Name:
Shivendra Singh
Title:
Applications Engineer Principal
www.cypress.com
Document No. 001-61546 Rev.*F
13
Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
Document History
Document Title: AN61546 - Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines
and Best Practices
Document Number: 001-61546
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
2930446
ZSK
05/11/2010
• New application note.
*A
3283741
ZSK
06/15/2011
• Table1 is modified to include a few new nvSRAM RTC parts.
• Table1 has added a note (Note1) for a few devices which are not recommended for
new designs now.
*B
3638383
ZSK
06/08/2012
• Added note in Discharge Time for Capacitor.
• Updated template.
*C
3721292
ZSK
08/23/2012
• Added following paragraph in RTC Backup Power Options.
“Because Super capacitors with value greater than 1 Farad and voltage rating above
3.6V are not easily available in small foot print package options. If users’ application
can allow larger capacitor packages, the RTC back up time for more than 30 days can
be obtained with capacitor value larger than 1 Farad. Calculation for nvSRAM RTC
backup time is shown through Example 1 for capacitor and Example 2 for battery
options. These two examples are valid for any rated value of a capacitor or a battery
used in applications.”
*D
4675902
ZSK
03/04/2015
• Update the following in Table 1:


Replaced the obsolete part CY14B101K/M with its drop-in-replacement part
CY14B101KA/MA
Added 16-Mbit nvSRAM RTC part CY14B116K/M
• Added Note 2 on battery installation in Example 3.
• Added Troubleshooting Guide for nvSRAM RTC section.
• Added Related Documents section.
• Updated the entire document with minor changes.
*E
4717021
ZSK
04/08/2015
• Update the application note landing page link:
http://www.cypress.com/go/AN61546.
• Fixed the broken links in the datasheet category, in Related Documents.
*F
4790158
ZSK
06/08/2015
• Updated Table 1.
• Formatted the document as per new template.
www.cypress.com
Document No. 001-61546 Rev.*F
14
Nonvolatile Static Random Access Memory (nvSRAM) Real Time Clock (RTC) Design Guidelines and Best Practices
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Document No. 001-61546 Rev.*F
15