msp430 32k

Application Report SLAA322B–August 2006–Revised April 2009 MSP430 32-kHz Crystal Oscillators Peter Spevak and Peter Forstner ............................................................................ MSP430 Applications ABSTRACT Selection of the right crystal, correct load circuit, and proper board layout are important for a stable crystal oscillator. This application report summarizes crystal oscillator function and explains the parameters to select the correct crystal for MSP430 ultralow-power operation. In addition, hints and examples for correct board layout are given. The document also contains detailed information on the possible oscillator tests to ensure stable oscillator operation in mass production. Contents 1 The 32-kHz Crystal Oscillator ..................................................................... 2 2 Crystal Selection.................................................................................... 3 3 PCB Design considerations ....................................................................... 6 4 Testing the Crystal Oscillator ..................................................................... 8 5 Crystal Oscillator in Production ................................................................... 9 List of Figures 1 Mechanical Oscillation of a Tuning-Fork Crystal ............................................... 2 2 Equivalent Circuit of a Crystal .................................................................... 2 3 Reactance of a Crystal............................................................................. 2 4 Principle Pierce Oscillator Circuit................................................................. 3 5 Frequency vs Load Capacitance for a 0-ppm Crystal ......................................... 4 6 Frequency Deviation of a Tuning-Fork Crystal Over Temperature .......................... 5 7 Layout Without and With External Load Capacitors (XIN and XOUT Neighboring Pins Are Standard Function Pins) ................................................................ 7 8 Layout With External Capacitors and Ground Guard Ring (XIN and XOUT Neighboring Pins Are NC Pins) Examples for MSP430F41x and MSP430F1232IRHB .. 7 9 Negative Resistance Method With Added Resistor RQ........................................ 9 List of Tables 1 Typical Oscillation Allowance Values for the 32-kHz Oscillator .............................. 5 2 Safety Factor ........................................................................................ 9 SLAA322B–August 2006–Revised April 2009 MSP430 32-kHz Crystal Oscillators 1 Submit Documentation Feedback 1 The 32-kHz Crystal Oscillator 1.1 The Crystal CM RM LM C0 F =S 2 L Cpi√ M M 1 F =A 1 CM C0 1 +2 L Cpi√ M M Frequency FA FS R ea ct an ce –jX +jX The 32-kHz Crystal Oscillator www.ti.com For an ultralow-power design, only low-frequency crystals are usable, because with higher-frequency oscillators, the current consumption increases significantly. Tuning-fork crystals typically have a frequency range of 10 kHz to 200 kHz in fundamental mode and a maximum drive level of 1 µW. These parameters make them the first choice for the 32768-Hz ultralow-power crystal oscillator in MSP430 microcontrollers. Every MSP430 has a built-in crystal oscillator that can be operated with a tuning-fork crystal at 32768 Hz (often called 32 kHz). The mechanical oscillation (see Figure 1) of a 32-kHz tuning fork crystal is converted into an electrical signal. The equivalent electrical circuit of a crystal (see Figure 2) gives these electrical characteristics: • CM motional capacitance • LM motional inductance • RM mechanical losses during oscillation • C0 parasitic capacitance of package and pins Figure 1. Mechanical Oscillation of a Figure 2. Equivalent Circuit of a Crystal Tuning-Fork Crystal The series-resonance circuit consisting of CM, LM, and RM represents the electrical equivalent of the mechanical resonance of the tuning fork. The frequency characteristics of a crystal’s reactance are shown in Figure 3 and give two special frequencies: • FS (series resonance frequency) solely depends on CM and LM and gives a very stable frequency value. • FA (anti-resonance or parallel-resonance frequency), in addition, also depends on C0, the parasitic capacitance of package and pins, which is not as precise as the other parameters, CM and LM. Hence, FA gives a less well-defined frequency than FS. Figure 3. Reactance of a Crystal 2 MSP430 32-kHz Crystal Oscillators SLAA322B–August 2006–Revised April 2009 Submit Documentation Feedback ESR = R 1 +M C0CL )2) (1) 1.2 The Oscillator Loop Gain = 1360° 180° Amplifier CL1 CL2 180° 2 Crystal Selection www.ti.com Crystal Selection The equivalent series resistance (ESR) can be calculated with the formula in Equation 1 from the equivalent circuit in Figure 2: C0 is shown in Figure 2 and given by the crystal’s data sheet, as is RM or ESR. CL is the required load capacitance of a crystal and is also given by the crystal’s data sheet. The principle circuit of an oscillator is shown in Figure 4. Two basic parameters must be fulfilled to enable oscillation: • Closed loop gain ≥ 1 for oscillator start up and closed loop gain = 1 for stable oscillation • Closed loop phase shift = n × 360° Figure 4. Principle Pierce Oscillator Circuit Figure 4 shows the Pierce oscillator circuit, which takes advantage of the crystal’s serial resonance frequency. The inverting amplifier gives a phase shift of approximately 180°. The feedback circuit consisting of a 32-kHz crystal and two load capacitors adds another 180° phase shift. This results in the required oscillator closed-loop phase shift of 360°. The closed-loop gain must be adjusted with the gain of the inverting amplifier. All MSP430 32-kHz crystal oscillators are Pierce oscillators. The most important parameters when choosing a crystal are: • Crystal’s required effective load capacitance (for 32-kHz crystals, typically 6 pF to 15 pF) • Crystal’s ESR (for 32-kHz crystals, typically 30 kΩ to 100 kΩ) • Tolerance (typically 5 ppm to 30 ppm) All of these crystal parameters are given by the crystal data sheet but can be also measured at the real crystal using, for example, crystal impedance bridge, a vector voltmeter, or a network analyzer. It is very important to know these parameters, because otherwise it is not possible to design a stable oscillator. SLAA322B–August 2006–Revised April 2009 MSP430 32-kHz Crystal Oscillators 3 Submit Documentation Feedback 2.1 Effective Load Capacitance C =Load C' × C'L1 L2 C' + C'L1 L2 (2) C =Load C + CL1 Parasitic 2 (3) 32760 Hz 32762 Hz 32764 Hz 32766 Hz 32768 Hz 32770 Hz 32772 Hz 32774 Hz 32776 Hz 32778 Hz 32780 Hz 1 pF 3 pF 5 pF 7 pF 9 pF 11 pF 13 pF 15 pF 17 pF 19 pF Load Capacitance Fr eq u en cy Crystal Frequency C DependentL Effective Load Capacitance Frequency Target Crystal Selection www.ti.com The Pierce oscillator (see Figure 4) uses two load capacitors, CL1 and CL2, as load for the crystal. These capacitors generate, together with the crystal’s inductance (LM) (see Figure 2), the required 180° phase shift of the feedback loop. From the view of the crystal, these capacitors are a serial connection through GND. Hence, if using two equal capacitors, the values of these capacitors must be twice the required load capacitance. It is also important to consider all parasitic capacitances, such as PCB traces and MSP430 pin capacitance, for the calculation of the necessary capacitors according to the formula in Equation 2. Where: C’L1 = CL1 + CL1Parasitic C’L2 = CL2 + CL2Parasitic When using equal capacitors for CL1 and CL2 and a symmetric layout with equal parasitic capacitance on both crystal pins, the effective load capacitance is shown in Equation 3. Example: Crystal requires 12 pF load. Parasitic capacitance per pin is 2 pF. CL1 = (2 × CLoad) – CParasitic = (2 × 12 pF) – 2 pF = 22 pF CL2 = CL1 = 22 pF One result of choosing the wrong load capacitors, which can be easily measured, is an incorrect oscillation frequency. A typical curve, showing frequency vs load capacitance, is given in Figure 5. Figure 5. Frequency vs Load Capacitance for a 0-ppm Crystal All MSP430 32-kHz oscillators have built-in load capacitors, CL1 and CL2. In some MSP430 versions, these load capacitors are fixed; in other MSP430 versions, the internal load capacitor values can be programmed or external capacitors can be used. For details, see the data sheets and MSP430 family user’s guides. The various MSP430 families have the following load capacitor configuration: • MSP430x1xx: 6 pF (fixed effective capacitance with 12 pF per pin), external capacitors are not recommended • MSP430F2xx: 0 pF to 12.5 pF (programmable effective capacitance), external capacitors are possible • MSP430F4xx: 0 pF to 10 pF (programmable effective capacitance), external capacitors are possible MSP430 32-kHz Crystal Oscillators4 SLAA322B–August 2006–Revised April 2009 Submit Documentation Feedback 2.2 ESR Value 2.3 Tolerance -160.0 PPM -140.0 PPM -120.0 PPM -100.0 PPM -80.0 PPM -60.0 PPM -40.0 PPM -20.0 PPM 0.0 PPM -40°C -20 C° 0 C° 20°C 40 C° 60 C° 80°C Temperature D F/ F T0 = 25°C ±5°C –0.035 ppm/°C × (T – T0) ±10%2 2 www.ti.com Crystal Selection The ESR value is an electrical representation of losses of the mechanical crystal oscillation. A larger crystal loses less energy during oscillation, and this results in a lower ESR value. Small crystals, especially SMD crystals, tend to have higher ESR. A higher ESR value reflects the higher losses of a crystal. The oscillator becomes unstable and stops oscillation if the ESR becomes too high. Hence, each oscillator has maximum limits of the ESR value. The lower the ESR than the recommended maximum value, the better the oscillator start up and stability. A common test for oscillator stability is the negative resistance method (see Section 4.2). For this test, ESR must be increased with an external resistor. The maximum value of this increased ESR is called the oscillation allowance (OA). With this OA value, it is possible to make a judgment of the oscillator safety factor (SF) margin. It is good practice to do the negative resistance test, to avoid oscillator problems in high-volume applications. Table 1 lists typical OA values for the 32-kHz oscillators of various MSP430 families. Note: If oscillation allowance for LF crystals (OALF) values are specified in an MSP430 data sheet, this table does not apply, and only the data sheet values are valid. Table 1. Typical Oscillation Allowance Values for the 32-kHz Oscillator MSP430x1xx MSP430x2xx MSP430x4xx CL = 6 pF CL = 6 pF CL = 12.5 pF CL = 6 pF CL = 12.5 pF VCC = 3 V 185 kΩ 500 kΩ 200 kΩ 460 kΩ 180 kΩ VCC = 2.2 V 88 kΩ 500 kΩ 200 kΩ 440 kΩ 170 kΩ Refer to crystal manufacturer recommendation for 32-kHz crystals operating with MSP430 oscillators. The ppm tolerance value given in the data sheet expresses the possible frequency deviation of the resulting oscillator frequency, assuming that all other frequency-affecting parameters, such as effective capacitive load, temperature, etc., are at recommended values. Figure 6. Frequency Deviation of a Tuning-Fork Crystal Over Temperature SLAA322B–August 2006–Revised April 2009 MSP430 32-kHz Crystal Oscillators 5 Submit Documentation Feedback 2.4 Start-Up Time 3 PCB Design considerations PCB Design considerations www.ti.com It should be considered that the amount of the frequency variation due to temperature depends very much on the crystal cut and the crystal shape. In comparison to some other crystal cuts, 32-kHz tuning-fork crystals exhibit a relative high frequency drift over temperature. Figure 6 shows the typical frequency deviation of a 0-ppm tuning-fork crystal over temperature. The ±ppm tolerance value, given in the crystal data sheet, shifts the graph of the tuning-fork crystal up and down. In case the 32-kHz crystal oscillator frequency is used for precision measurements over a wide temperature range, software can improve the measurement results by correcting the measured values according to the curve in Figure 6. In this case, the real curve for the used crystal should be obtained from the crystal manufacturer. A test for oscillator frequency and a method to adjust the oscillator frequency is explained in Section 4.1. When initially energized, the only signal in the circuit is noise. That component of noise whose frequency satisfies the phase condition for oscillation is propagated around the loop with increasing amplitude. The amplitude continues to increase until the amplifier gain is reduced either by nonlinearities of the active elements ("self-limiting Pierce", MSP430x1xx) or by some automatic level control (“controlled Pierce” with AGC circuitry, MSP430x2xx and MSP430x4xx). Start-up times between several hundred milliseconds and a few seconds are normal values for low-frequency tuning-fork crystals, like 32768-Hz crystals. The start-up time of a crystal oscillator depends on various factors: • The oscillator frequency influences the start-up time. A 32-kHz crystal oscillator starts relatively slowly, compared to a crystal oscillator with a high frequency, e.g., above 1 MHz. • High Q-factor crystal oscillators typically start slower than crystal oscillators with higher frequency tolerance. • Crystal with low load capacitance typically start faster than crystals requiring high load capacitance. • Crystals with low ESR start more quickly than high ESR crystals. • Oscillators with high OA (Oscillation Allowance) start faster than low OA crystal oscillators. The MSP430 LFXT1 32-kHz crystal oscillator is designed for ultralow-power consumption. According to the data sheets, most MSP430 derivatives consume less than 1 µA when the 32-kHz oscillator, the clock signal (ACLK), and a timer are running. Hence, the current flowing between the MSP430 pins, the crystal and, if used, the external capacitors is extremely low. Long signal lines make the oscillator very sensitive to EMC, ESD, and crosstalk. Even the best components cannot solve problems caused by a poor layout. The crystal oscillator is an analog circuit and must be designed according to analog-board layout rules: • Signal traces between the MSP430 pins, the crystal and, if used, the external capacitors must be as short as possible. This minimizes parasitic capacitance and sensitivity to crosstalk and EMI. The capacitance of the signal traces must be considered when dimensioning the load capacitors. • Keep other digital signal lines, especially clock lines and frequently switching signal lines, as far away from the crystal connections as possible. Crosstalk from digital signals may disturb the small-amplitude sine-shaped oscillator signal. • Reduce the parasitic capacitance between XIN and XOUT signals by routing them as far apart as possible. • The main oscillation loop current is flowing between the crystal and the load capacitors. This signal path (crystal to CL1 to CL2 to crystal) should be kept as short as possible and should have a symmetric layout. Hence, both capacitors' ground connections should always be as close together as possible. Never route the ground connection between the capacitors all around the crystal, because this long ground trace is sensitive to crosstalk and EMI. • Guard the crystal traces with ground traces (guard ring). This ground guard ring must be clean ground. This means that no current from and to other devices should be flowing through the guard ring. This guard ring should be connected to AVSS of the MSP430 with a short trace. Never connect the ground guard ring to any other ground signal on the board. Also avoid implementing ground loops. 6 MSP430 32-kHz Crystal Oscillators SLAA322B–August 2006–Revised April 2009 Submit Documentation Feedback GND Island isolated by a gap from the rest of the GND GND Island isolated by a gap from the rest of the GND www.ti.com PCB Design considerations • With 2-layer boards, do not route any digital-signal lines on the opposite side of the PCB under the crystal area. In any case, it is good design practice to fill the opposite side of the PCB with clean ground and also connect this ground to AVSS of the MSP430. • Connect the crystal housing to ground. Before soldering the crystal housing, contact the crystal manufacturer to make sure not to damage the crystal. Overheating the crystal housing could lead to destruction of the crystal. • In LF mode, the LFXT1 oscillator of MSP430x1xx requires a ≥5.1-MΩ resistor from XOUT to VSS when VCC < 2.5 V. This is used to increase the drive level of the MSP430 amplifier at low VCC. Refer to the data sheet for details. Making use of the MSP430 built-in capacitors gives a simple layout, with only the crystal connected to the XIN and XOUT pins of the MSP430. The traces between the MSP430 and the crystal should be as short as possible, and a ground area should be placed under the crystal oscillator area. When using external capacitors instead of the internal capacitors, the traces between the crystal and the capacitors and the trace between the two capacitors should be as short as possible. Examples for recommended layouts are shown in Figure 7. An additional ground guard ring could improve the performance. Figure 7. Layout Without and With External Load Capacitors (XIN and XOUT Neighboring Pins Are Standard Function Pins) Some of the MSP430 devices have NC (not connected) pins neighboring the XIN and XOUT crystal connection pins. In that case, it is recommended to make use of the situation and add a ground guard ring around the crystal signals. This ground guard ring should have a short connection to the MSP430 VSS pin. Layout examples for this scenario are shown in Figure 8. In all these examples, the section between crystal and the load capacitors is laid out symmetrically. NOTE: The layout on the right side includes a resistor between XOUT and VSS. The LFXT1 oscillator of MSP430x1xx (see data sheet) in LF-mode requires a resistor of ≥5.1 MΩ from XOUT to VSS when VCC < 2.5 V, to compensate for decreasing drive level with lower supply voltages. Figure 8. Layout With External Capacitors and Ground Guard Ring (XIN and XOUT Neighboring Pins Are NC Pins) Examples for MSP430F41x and MSP430F1232IRHB SLAA322B–August 2006–Revised April 2009 MSP430 32-kHz Crystal Oscillators 7 Submit Documentation Feedback 4 Testing the Crystal Oscillator 4.1 Oscillator Frequency vs Load Capacitance 4.2 Negative Resistance Method Testing the Crystal Oscillator www.ti.com The following measurements help to verify the crystal oscillator stability: • Oscillator frequency vs load capacitance • Negative resistance method (Oscillation Allowance test) – Start allowance – Stop allowance As shown in Figure 5, the crystal oscillator frequency is very much dependent on the load capacitance that is connected. Hence, measuring the oscillator frequency gives a good indication if the load capacitors that are used match the crystal requirements. This measurement also automatically includes the parasitic PCB and pin capacitances of the application. The graph in Figure 5 shows typical 32-kHz crystal characteristics. The characteristics (pullability curve) of the crystal should be provided by the crystal manufacturer. It is strongly recommended not to measure the oscillator frequency directly at the crystal pins. The capacitance at the crystal pins is in the range of 10 pF, and the impedance on this signal line is several megaohms. A typical passive probe has a capacitance in the range of 10 pF and an input impedance of about 10 MΩ. Both values are in the range of the oscillator characteristics and heavily influence the behavior of the crystal oscillators. The MSP430 internal digital ACLK clock signal always carries the clock signal of the 32-kHz crystal oscillator. All MSP430 devi