AN402 F-RAM™ RTC Oscillator Design Guide Author: Harsha Medu Associated Part Family: FM31278, FM31276, FM31L278, FM31L276, FM31256, FM3164, and FM33256B Related Application Notes: click here AN402 describes the real-time clock (RTC) oscillator and provides design considerations for using the RTC in the F-RAM™ processor companions. 1 Overview The FM31xxx and FM33xxx are integrated processor companion devices that feature a real-time clock or RTC. The RTC provides the date and time information for the system. The RTC operates on VDD power and switches to a backup supply when the VDD power is removed. Under backup power, the RTC draws very little current, which allows a long operating time. The accuracy of the RTC is dependent mainly on the accuracy of the crystal oscillator. The crystal frequency is affected by the capacitive rating of the crystal and the operating temperature. This application note focuses on the RTC oscillator and provides design considerations for a system designer using these devices. 2 Oscillator and Crystals The heart of the RTC is the oscillator, which uses a 32.768-kHz crystal. It provides an accurate, low-power time base for the divide-by counters that generate seconds, minutes, hours, and more. The oscillator must work properly and be undisturbed for the clock/time to be accurate over long periods of time. An F-RAM processor companion with crystal is shown in Figure 1. Figure 1. Crystal Hookup to F-RAM RTC X1 32.768 kHz Crystal X2 F-RAM Processor Companion with RTC The RTC oscillator is designed to use a 32.768-kHz crystal (having 6 pF/12.5 pF load capacitance specification) without the need for any external components. If additional components, such as capacitors or resistors, are connected to the X1 or X2 pins, the oscillator will not operate properly because the DC operating point and the oscillator frequency will be shifted. It is possible that the oscillator will not even start at power up. Passive 10X oscilloscope probes with 10 pF and 10 MΩ impedance will also upset the oscillator. The simplified schematic of the oscillator in Figure 2 shows a Pierce oscillator with C1 and C2 loading capacitors inside the chip. Figure 2. Simplified Oscillator Circuit VBAK 150 nA X1 32.768-kHz Crystal C1 X2 C2 www.cypress.com Document No. 001-87418 Rev. *B 1 F-RAM™ RTC Oscillator Design Guide The capacitors operate in series with the crystal, so the CLOAD is C1 * C2 / (C1 + C2). For all F-RAM parts, C1 and C2 are 12 pF; hence, CLOAD is 6 pF. The F-RAM RTC oscillator is optimized for using the 6-pF crystal to achieve the lowest operating current under backup power. A 12.5-pF crystal will also work equally well but draws almost twice as much current. When using a 12.5-pF crystal, consider the loading mismatch and take appropriate measure to offset the frequency shift. Note: All 32.768-kHz crystals have a load capacitance specification. There are two common crystals in the market: a “6 pF” type and a “12.5 pF” type. This is the recommended capacitive load that the crystal must see across the X1 and X2 pins while operating. That is, the X1/X2 pins must present a 6-pF load to a 6-pF crystal. The load capacitance specification is not the actual capacitance of the crystal. The actual capacitance of the crystal (shunt capacitance) is about 1 pF. 3 Oscillator Frequency Shift Typically, RTC calibration is required primarily to compensate for the crystal tolerance, whether ±10 ppm, ±20 ppm, or ±50 ppm. The use of a 12.5-pF crystal introduces a loading mismatch factor when calibrating the RTC. A 12.5-pF crystal expects a capacitive load of 12.5 pF to ensure accurate frequency. However, because the F-RAM processor companions are designed for 6-pF crystals, the load presented to the crystal is roughly half the rated value. Therefore, this lighter load presented across the crystal shifts the oscillation frequency. The shift has been measured and is typically about +90 ppm or 2.9 Hz. The actual crystal frequency will be 32.7709 kHz. In calibration mode, this frequency shift can be measured directly on the CAL pin (ACS pin on FM33256B) as 512.046 Hz. Figure 3 shows the parallel resonance area as the bold line where the RTC oscillator operates. A 12.5-pF crystal (with 6-pF load) operates higher on the curve. A +90 ppm error translates to nearly four minutes of clock deviation per month (the clock runs fast), so calibrating the RTC is highly desirable to achieve clock accuracy. You must note that the effect of the calibration setting to minimize the frequency error will not be observed on the CAL pin (ACS pin on FM33256B). The RTC applies a digital correction to the counter logic downstream of the 512-Hz output. Figure 3. Higher Oscillator Frequency for 12.5 pF Crystal 12.5 pF + reactance 6 pF _ f frequency S If you use a 12.5-pF crystal, Cypress recommends that the specified tolerance be ±20 ppm (or better) to ensure that the total error (tolerance + mismatch) remains within the ±135 ppm calibration range. If you use a 6-pF crystal, the tolerance can be as high as ±100 ppm, because this is well within the calibration range. Precautionary note: The oscillator’s 32.768-kHz frequency cannot be monitored directly by probing the X1 and X2 pins. Do not attach a scope probe or meter to the X1 and X2 pins. In calibration mode, the CAL pin is used to check the frequency. Moreover, the calibration code can be entered only in calibration mode. 4 Calibration Procedure The following sequence shows how to measure the oscillator frequency and apply a calibration code. On two-wire devices, remember to use the slave ID 1101b to access the RTC registers. 1. Apply VDD. 2. Apply VBAK. 3. Cycle VDD off then on again. 4. Turn on the oscillator by setting O C RTC Register 00h). www.cypress.com bit low. Write 00h to RTC Register 01h (For FM33256B, write 00h to Document No. 001-87418 Rev. *B 2 F-RAM™ RTC Oscillator Design Guide 5 5. Set CAL bit high. Write 04h to RTC Register 00h. 6. Measure the 512-Hz output on CAL pin (ACS pin on FM33256B) with a frequency counter. 7. Determine the calibration code setting from the table in Calibration Table (This table is also available in the device datasheet). 8. Apply the calibration code. For example, a 12.5-pF crystal makes the oscillator run fast and the CAL output is 512.046 Hz. Write 16h to RTC Register 01h. 9. Reset CAL bit. Write 00h to RTC Register 00h. Temperature Effects A key factor that contributes to timekeeping error is the crystal's temperature – even after the RTC is calibrated. A calibration code can compensate for timekeeping errors due to capacitive load mismatch and crystal tolerance, but not for temperature. Figure 4 shows a typical temperature curve for 32.768-kHz crystals. As temperature rises or drops from +25 °C, the oscillation frequency shifts lower and therefore the clock slows down. Figure 4. Crystal Frequency Change vs. Temperature +25 -40 +85 Temp (°C) -50 -100 ∆f/f (ppm) If your system is at room temperature most of the time and has few excursions above and below 25 °C, then you should calibrate to “zero out” any frequency error by complying with the table in Calibration Table. On the other hand, if the system frequently spends time above or below room temperature, you will achieve improved clock accuracy by intentionally creating a slightly positive frequency error. For example, assume the system spends half the time at 25 °C and half at 50 °C, and the frequency error at 50 °C is -30 ppm. Applying a calibration code that will shift clock by +15 ppm will compensate for temperature changes. Consult the crystal manufacturer for a temperature dependence curve. 6 External Oscillator If a 32.768-kHz crystal is not used, an external oscillator may be connected to the F-RAM RTC oscillator. Apply the oscillator to the X1 pin. Its high and low voltage levels can be driven rail-to-rail or to amplitudes as low as approximately 500 mV p-p. To ensure proper operation, a DC bias must be applied to the X2 pin. It should be centered between the high and low levels on the X1 pin. This can be accomplished with a voltage divider as shown in Figure 5. Figure 5. External Oscillator www.cypress.com Document No. 001-87418 Rev. *B 3 F-RAM™ RTC Oscillator Design Guide In the above example, R1 and R2 are chosen such that the X2 voltage is centered on the X1 oscillator drive levels. If you wish to avoid the DC current, you may choose to drive X1 with an external clock and X2 with an inverted clock using a CMOS inverter. 7 Layout Recommendations The X1 and X2 crystal pins are high-impedance pins, which must be treated with care: Ensure that the crystal lead length to the X1 and X2 pins is short, less than 5 mm. Also the X1 and X2 trace lengths should be less than 5 mm. Ensure that the VDD pin has good decoupling (0.1 uF with return to ground plane). Do not route other signals close to the X1/X2 pins, even if the signal is routed on an inner board layer. Use a guard ring (ground) around the crystal pins. Use a ground plane on the back or inner board layer. An FM31xx device and SMD crystal are shown in the example layout shown in Figure 6. Note the plated-through holes that tie the guard ring down to the ground plane. Figure 6. Layout for Surface Mount Crystal (red = top layer, green = bottom layer) 8 Summary This application note described the oscillator selection, frequency shift, calibration procedure, and layout design guidelines for F-RAM processor companion devices. 9 Related Application Notes You can refer to the following application notes for better understanding of the F-RAM processor companion devices. AN407 - A Design Guide to I2C F-RAM Processor Companions – FM31278, FM31276, FM31L278, and FM31L276 AN408 - A Design Guide to SPI F-RAM Processor Companion - FM33256B AN400 - Generating a Power-Fail Interrupt using the F-RAM Processor Companion AN401 - Charging Methods for the F-RAM RTC Backup Capacitor AN404 - F-RAM RTC Backup Supply (VBAK pin) and UL Compliance www.cypress.com Document No. 001-87418 Rev. *B 4 F-RAM™ RTC Oscillator Design Guide A Calibration Table Positive Calibration for Slow Clocks (Calibration will achieve ±2.17 PPM after calibration) Measured Frequency Range Error Range (PPM) Program Calibration Register to: Min Max Min Max 0 512.0000 511.9989 0 2.17 000000 1 511.9989 511.9967 2.18 6.51 100001 2 511.9967 511.9944 6.52 10.85 100010 3 511.9944 511.9922 10.86 15.19 100011 4 511.9922 511.9900 15.20 19.53 100100 5 511.9900 511.9878 19.54 23.87 100101 6 511.9878 511.9856 23.88 28.21 100110 7 511.9856 511.9833 28.22 32.55 100111 8 511.9833 511.9811 32.56 36.89 101000 9 511.9811 511.9789 36.90 41.23 101001 10 511.9789 511.9767 41.24 45.57 101010 11 511.9767 511.9744 45.58 49.91 101011 12 511.9744 511.9722 49.92 54.25 101100 13 511.9722 511.9700 54.26 58.59 101101 14 511.9700 511.9678 58.60 62.93 101110 15 511.9678 511.9656 62.94 67.27 101111 16 511.9656 511.9633 67.28 71.61 110000 17 511.9633 511.9611 71.62 75.95 110001 18 511.9611 511.9589 75.96 80.29 110010 19 511.9589 511.9567 80.30 84.63 110011 20 511.9567 511.9544 84.64 88.97 110100 21 511.9544 511.9522 88.98 93.31 110101 22 511.9522 511.9500 93.32 97.65 110110 23 511.9500 511.9478 97.66 101.99 110111 24 511.9478 511.9456 102.00 106.33 111000 25 511.9456 511.9433 106.34 110.67 111001 26 511.9433 511.9411 110.68 115.01 111010 27 511.9411 511.9389 115.02 119.35 111011 28 511.9389 511.9367 119.36 123.69 111100 29 511.9367 511.9344 123.70 128.03 111101 30 511.9344 511.9322 128.04 132.37 111110 31 511.9322 511.9300 132.38 136.71 111111 www.cypress.com Document No. 001-87418 Rev. *B 5 F-RAM™ RTC Oscillator Design Guide Negative Calibration for Fast Clocks (Calibration will achieve ±2.17 PPM after calibration) Measured Frequency Range Error Range (PPM) Program Calibration Register to: Min Max Min Max 0 512.0000 512.0011 0 2.17 000000 1 512.0011 512.0033 2.18 6.51 000001 2 512.0033 512.0056 6.52 10.85 000010 3 512.0056 512.0078 10.86 15.19 000011 4 512.0078 512.0100 15.20 19.53 000100 5 512.0100 512.0122 19.54 23.87 000101 6 512.0122 512.0144 23.88 28.21 000110 7 512.0144 512.0167 28.22 32.55 000111 8 512.0167 512.0189 32.56 36.89 001000 9 512.0189 512.0211 36.90 41.23 001001 10 512.0211 512.0233 41.24 45.57 001010 11 512.0233 512.0256 45.58 49.91 001011 12 512.0256 512.0278 49.92 54.25 001100 13 512.0278 512.0300 54.26 58.59 001101 14 512.0300 512.0322 58.60 62.93 001110 15 512.0322 512.0344 62.94 67.27 001111 16 512.0344 512.0367 67.28 71.61 010000 17 512.0367 512.0389 71.62 75.95 010001 18 512.0389 512.0411 75.96 80.29 010010 19 512.0411 512.0433 80.30 84.63 010011 20 512.0433 512.0456 84.64 88.97 010100 21 512.0456 512.0478 88.98 93.31 010101 22 512.0478 512.0500 93.32 97.65 010110 23 512.0500 512.0522 97.66 101.99 010111 24 512.0522 512.0544 102.00 106.33 011000 25 512.0544 512.0567 106.34 110.67 011001 26 512.0567 512.0589 110.68 115.01 011010 27 512.0589 512.0611 115.02 119.35 011011 28 512.0611 512.0633 119.36 123.69 011100 29 512.0633 512.0656 123.70 128.03 011101 30 512.0656 512.0678 128.04 132.37 011110 31 512.0678 512.0700 132.38 136.71 011111 www.cypress.com Document No. 001-87418 Rev. *B 6 F-RAM™ RTC Oscillator Design Guide Document History Document Title: AN402 – F-RAM™ RTC Oscillator Design Guide Document Number: 001-87418 Revision ECN Orig. of Change Submission Date Description of Change ** 4018188 MEDU 06/07/2013 New Spec. *A 4559478 MEDU 11/04/2014 Changed title from “F-RAM RTC Oscillator Guide” to “F-RAM RTC Oscillator Design Guide” Included content from AN403, “F-RAM RTC Crystals – 6 pF vs. 12.5 pF” Updated 6-pF RTC oscillator load for all F-RAMs Added the Related Application Notes section Added Appendix A for the calibration table *B 5293268 www.cypress.com MEDU 06/02/2016 Updated template Document No. 001-87418 Rev. *B 7 F-RAM™ RTC Oscillator Design Guide Worldwide Sales and Design Support Cypress maintains a worldwide network of offices, solution centers, manufacturer’s representatives, and distributors. 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