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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Why are some microcontrollers more reliable than others. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Microcontrollers In the article, the author considers some aspects that developers should pay attention to when choosing a microcontroller for applications that meet high reliability and safety requirements. By the nature of his professional activity in the distribution company "Eltech" LLC, the author has to discuss the problems of developing devices with many domestic electronics manufacturers. During these discussions, it turns out that Russian developers use microcontrollers from all manufacturers represented on the electronic market to solve their problems. For some manufacturers, microcontrollers of the so-called "commercial" design are quite suitable. But there are manufacturers for whom one of the most important criteria for choosing an electronic component is its reliability. First of all, these are specialists working in the field of production of medical equipment, elevator equipment, automotive electronics. Experiment In 2006, Mikhail Cherepanov, a developer of the Svey company, applied to our company (Svey is a Russian manufacturer of industrial electronics). Here is the text of his letter: "The story began with complaints from customers that our digitizers (built on the MSP430F148IPM) intermittently "freeze" and do not respond to requests until they are reset by removing and re-feeding supply voltage. There have been suggestions that the "hang" is due to the presence of impulse noise (this is a common occurrence in electrical substations). To reproduce the situation, I made a noise generator (Fig. 1).
As a result of testing, our converters were modified as follows:
After that, the crashes didn't happen anymore. Later, our products successfully passed EMC tests (for products subject to mandatory declaration of conformity) according to:
For myself, I determined the minimum requirements for the microcontroller used: 1) Interference generator test.
So our client asked for help pick up a microcontroller that is resistant to strong electromagnetic fields. We proposed NEC devices knowing that these microcontrollers are widely used in automotive electronics, where the electromagnetic environment is very difficult. Several evaluation kits were submitted. Then the customer said that he wanted to test them with a spark generator. To be honest, we were a little worried about how these tests would pass, but such conditions are quite consistent with real automotive situations when a breakdown of a high-voltage wire occurs. In this case, the electronics should continue to work properly. This test method was quite rough, because the evaluation kits are not designed for such tests. We understood that there was a certain risk in this experiment, and, perhaps, our "evaluators" could even fail after such a test. But, having sufficient experience with these devices and taking into account the experience of our customers, we decided that they are properly made and will work as expected. We have provided two evaluation kits:
The spark discharge was carried out in close proximity to the evaluation kits. The testing process is schematically shown in Fig. 2.
Both evaluation kits worked flawlessly, even when the spark was close to 5 cm away. The customer reported that he had tested more than 10 different evaluation kits in this way. We asked him to provide the results of these experiments. Further they will be given without comments, "as is". After some time, the customer conducted another, one might say, more "barbaric" experiment. However, his results are also interesting. He touched the terminals of a working quartz oscillator with his hand. Under such conditions, of all the listed microcontrollers clocked from an external generator, only one worked - uPD70F3707 (NEC). However, in fairness, it should be noted that when touched, the demo program noticeably slowed down the speed of its execution. The reason for this "behavior" of the uPD70F3707 microcontroller will be explained later. Let's try to understand why the V850ES/HG2 family (to which the uPD70F3707 microcontroller belongs) turned out to be so "tenacious". If you carefully consider some peripheral nodes, then everything gradually falls into place. Watchdog timer and clock generators The problems that our client encountered were caused by the fact that when exposed to strong electromagnetic interference, the generation of a crystal oscillator can be disrupted, and since the watchdog timer in the MSP430F148 microcontroller is clocked from the same reference oscillator, when the reference oscillator is stopped, the watchdog timer can no longer "wake up" the microcontroller [1]. To prevent this situation, in all NEC microcontrollers the watchdog timer is clocked from a separate internal ring oscillator. The ring generator is an odd number of inverters connected in a ring so that the output of one inverter goes to the input of the next. Disruption of the generation of a ring generator is practically impossible. It should be noted that in all microcontrollers of the NEC V850 family, the processor core is started from an additional built-in ring oscillator, and only after making sure that the crystal oscillator has started, you can switch the clock to "quartz". Clock monitor (Clock Monitor) The clock monitor monitors for the generation of a clock oscillator using an external crystal oscillator. If the generation fails, an internal reset signal RESCLM is generated and the flag RESF.CLMRF [2] is set. After leaving the reset mode, the microcontroller analyzes this flag and "understands" that there are problems with the external clock generator, after which the core is started from one of the internal clock generators. Depending on the family, there may be 1 or 2 oscillators, but their frequency, as a rule, is always less than the frequency of the oscillator using an external resonator. That is why, after being touched by a finger, the uPD70F3707 microcontroller continued to work, but already much "slower", which was stated by Mikhail Cherepanov from the Sway company. It is interesting that, to one degree or another, this device is implemented in other microcontrollers. However, if the clock to be used is set during the programming of the FLASH and cannot be changed by software, then the alternate internal oscillator startup scenario described above cannot be implemented. In addition to the V850ES/Hx2 family, this node also has families specifically designed for motor control applications (V850E/IA3, IA4, IF3, IG3; V850ES/IK1, IE2), for automotive dashboards (V850E/Dx3), for on-board electronics with CAN interface (V850ES/Sx2, Sx2-H, Sx3, Fx2, Fx3, Fx3-L), as well as V850ES/Kx1+, Jx2, Jx3, Jx3-L, Hx2 and Hx3. It should be noted that in some other microcontrollers (usually 8 and 16 bits), NEC engineers use a windowed watchdog timer instead of a clock monitor. It has a completely different principle of operation, however this peripheral device can be used for the same purpose as the clock monitor, that is, it can monitor the disappearance of the "external" reference oscillator clock and allow the microcontroller to switch to the internal oscillator. Separation of power rails All 32-bit NEC microcontrollers mentioned earlier, and many 8-bit ones, have separate power rails for internal peripherals, processor core, and I/O port circuits. On fig. 3 and 4 schematically show such a separation.
With proper decoupling of the core power bus and the I/O ports, noise induced on the I/O ports does not enter the peripheral and core power circuits and improves electromagnetic immunity (EMS). So, for example, both lists (Tables 1, 2) included microcontrollers with an AWP core. Table 1. Evaluation kits worked flawlessly during testing
Table 2. Evaluation kits that had test program failures when tested
The ADUC7026BSTZ62 microcontrollers worked without failures, while microcontrollers with an APM core from NXP (LPC2148) got into the "black list". If we examine the power circuits of the core, peripheral devices, and I/O ports, we can see that the microcontroller from Analog Devices, which also "stood" against a spark [3], has a power structure similar to V850ES/Hx2 from NEC. Namely, decoupled power buses for the core and I / O ports (Fig. 5, 6).
When creating the LPC2148FBD64 [4], NXP engineers limited themselves only to the separation of analog and digital power circuits (Fig. 7).
Even microcontrollers advertised for automotive applications such as the AT90CAN32/64/128; ATmega164P/324P/644P and ATmega32M1/64M1/32C1/64C1, separation of I/O port power rails and core power rails is not provided. As a result, the possibility of failure due to interference induced along the I / O circuits in critical applications increases. The MSP430F148 microcontroller, which was used in the development described by Mikhail, also does not have a separation of the core power rails and I / O ports. You can also recall another very popular chip manufacturer - Microchip. No research has been carried out with microcontrollers from this manufacturer, however, if you look at them from the point of view of separating power buses, then in a certain sense the concept of decoupling I/O ports and peripheral devices is implemented in the PIC24FJ64GA/128GA/256GA family. On fig. 8 shows that the power circuits of the VDDCORE core and the VDD I / O ports are separated. However, the VSS common wire remained not galvanically isolated for these two power circuits. According to preliminary estimates, the noise immunity of these microcontrollers will be lower than that of ADUC7026 from ADI or V850 from NEC.
Spread spectrum clock generator (sscg) Attention should also be paid to the possibility of using a spread spectrum clock generator. Such a generator has frequency-modulated oscillations. "Peak" frequency response, characteristic of the generator of harmonic oscillations, under the influence of frequency modulation "smeared" and turns into a "shelf". The depth and period of the frequency modulation of the SSCG signal can be changed. Microcontrollers of the V850E / ME2, Dx3, V850ES / Hx3, Fx3, V850E2 / ME3 families from NEC are endowed with such a generator. Its application allows to reduce by more than 10 dB the electromagnetic emission (EME) emitted by the generator, and, consequently, to reduce the sensitivity to external electromagnetic interference (EMS) at the frequencies of the clock generator (Fig. 9).
Application of the PLL circuit Another way to reduce EMS is to use a PLL-based frequency synthesizer. On fig. 10 shows that spurious high-frequency signals induced at the terminals of the quartz resonator are filtered when passing through the low-pass filter of the PLL. On fig. Figure 11 shows data that allows you to evaluate how much the EMS of the microcontroller improves when using the PLL.
Supply voltage It can be shown that the higher the supply voltage, the higher the noise immunity of the microprocessor circuit. It is also true that the lower the supply voltage, the less the microcontroller will "noise". So, LPC2129 [5] from NXP and AT91SAM7S128 [6] from Atmel, which are on the "black list", have the necessary decoupling of the core power bus and I/O port power buses. However, too low a core supply voltage (1,8 V) adversely affects the noise immunity of this microcontroller. Sometimes it is necessary to "link" 3- and 5-volt logic. In this case, the tolerance of input / output ports to different levels of logical signals, that is, the ability of the microcontroller to support various voltages of the input / output ports with a constant supply voltage of the microcontroller core and peripheral devices [7] (Fig. 12).
The chip manufacturer's support may include resources such as PCB routing recommendations, the chip manufacturer's analysis of the PCB area associated with microcontroller routing and additional components with suggestions for improving electromagnetic compatibility (EMC) (Figure 13), materials on electromagnetic radiation (EME) of microcontrollers [8] (provided upon request from the distributor). On fig. Figure 14 shows the NEC laboratory for conducting research on EMC [8]. Its peculiarity is that it should be located high in the mountains, away from sources of electromagnetic radiation.
Distributor support includes providing samples and evaluation kits for testing, technical support and other services. In some cases, as shown above, the distributor takes a risk to get the project going. More "dense" work with the distributor, as a rule, is always beneficial to the final developer and manufacturer. Table 3 lists some NEC microcontroller families recommended for use in applications with high requirements for reliability, and some characteristics that allow you to evaluate how reliable a device built on these microcontrollers can turn out. Table 3. Parameters affecting the reliability of some families of NEC Electronics microcontrollers
Conclusion The main aspects related to the choice of microcontrollers for responsible applications are considered. Tests carried out according to a rather "harsh" methodology given in the article allow the reader to solve the problem of choosing a microcontroller for his application, taking into account the requirements for the reliability of the device being developed, as well as the service received both during development and at all subsequent stages of production. In one of its new developments, the Sway company used an 8-bit microcontroller UPD78F9212GR manufactured by NEC Electronics. Literature 1 focus.ti.com/lit/ds/symlink/msp430f148.pdf
Author: Gennady Goryunov, gennady.gr@eltech.spb.ru; Publication: cxem.net
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