A standard peripherals approach to adding flexibility to 32-bit MCU designs
Capacitive touch for tamper detection
Many 32-bit MCU-based systems used in industrial control or home automation require some level of tamper detection. For example, security cameras use detection of a known number of consecutive dark images as a tamper detection trigger. Other techniques might use external sensors to generate a tamper trigger event. Most 32-bit MCUs have fixed input pins that are either active high or active low and float or switch polarity to indicate a tamper event.
An alternative solution uses a capacitive touch-sense engine as a potential multi-channel tamper detection system. For example, the capacitive touch-sense engine built into Silicon Labs’ ARM-based Precision32® MCUs supports channel bonding and has a maximum of 16 possible input channels supporting up to 16 tamper detection channels in a single conversion.
Alternately, using a non-channel-bonded mode combined with a programmable threshold, 16 tamper detection channels with independent tamper thresholds on each channel can be implemented with minimal software overhead. This engine can be used to generate a time stamp via an interrupt to the system generating a real-time clock (RTC) trigger event.
A typical untampered system can be implemented with tamper detect channels that have a known capacitance, which will be detected. For example, if the capacitive touch-sense node is connected to a closed door monitored by a security system, you would expect the capacitance to be “X” pico farads (pf).
If the door is opened, the capacitance will be “less than X” pf. In this scenario, the capacitive touch-sense module can be configured to generate an interrupt that then triggers the system to time-stamp the event in software by reading the RTC timer register at this time. There will be a few cycles of latency, but given the frequency of the RTC and the operating frequency of the system, the human scale timing of this event will, for all intents and purposes, look like a zero-time event.
The capacitive touch-sense module also supports a “greater than threshold” event trigger, so the opposite of the “closed door” system can also be implemented (i.e., an “open door” system). Given the multi-sampling capability of the capacitive touch-sense module, multiple samples can be taken and accumulated to filter out false triggers.
The capacitive touch-sense capability can also function in sleep mode to enable a low-power system. This can be accomplished by setting up the RTC to trigger a certain number of times per second, wake up the system and perform a capacitive touch-sense port scan. Depending on whether a tamper event was detected or not, the system will either continue with post-tamper-related functions or go back to sleep.
Synchronous current to voltage converter systems
Many sensors used in factory automation, building control and medical applications, such as pressure sensors, optical sensors, transducers and biosensors, provide a current output instead of a voltage output to achieve greater noise immunity.
To properly interface with current output sensors, the design must include a current-to-voltage converter. Typically, an external current sensing system is added to the system, resulting in greater BOM cost, design complexity and challenges in synchronizing measurements based on dynamic excitation references.
For example, a biosensor used in a medical glucose meter, outputs tiny current values as a result of a dynamic excitation. A cost-effective implementation of a biosensor interface that requires full synchronization between dynamic excitation and current measurements can be achieved using a synchronized DAC-ADC mechanism.
Precision32 MCUs, for example, support this synchronization capability. In addition to providing a full synchronization path between the ADC and DAC, Precision32 MCUs also feature an internal current-to-voltage converter.
Figure 5 below illustrates the DAC-ADC synchronization mechanism using the current-to-voltage converter. The sample synchronization generator module can generate a synchronous pulse to trigger both the DAC and ADC conversion inputs.
Click on image to enlarge.
The ADC can be configured to either sample synchronously with the DAC output or in a delayed fashion by a fixed number of clock cycles to allow the sensor to process the stimulus input. This enables a fully synchronous measurement system, simplifying the system design and reducing cost by eliminating external components.
Conclusion
Innovative implementation of standard 32-bit MCU peripherals coupled with a highly flexible interconnect architecture enables a wide variety of configurations for embedded systems. The synchronous current-to-voltage converter, input filter for noisy communication and 3 to 5 V level shifting are examples of flexible interconnect architectures.
Using versatile I/Os and peripherals, such as capacitive touch, I/O pulse generation and DAC precision enhancement, developers can achieve novel solutions to real-world embedded design problems while reducing cost, simplifying design and accelerating time to market.
Thomas David is a principal design engineer for Silicon Labs’ MCU products and was the lead designer for the company’s first 32-bit MCU product, the SiM3U16X Precision32 MCU. He has been involved, either as a designer or as a chip lead, in nearly all of the MCU products released by Silicon Labs. He came to Silicon Labs as part of the company’s Cygnal Integrated Products acquisition in 2003. Prior to Cygnal, he was president of Silogix, an Austin-based silicon intellectual property (SIP) company that was acquired by Cygnal Integrated Products. He a BSEE from Purdue University and an MSEE from Penn State University.
Pedro Pachuca manages Silicon Labs’ global microcontroller (MCU) interface product business. He joined Silicon Labs in early 2010. Previously, he was a product marketing manager at Freescale Semiconductor where he developed MCU business strategies to penetrate new global markets and managed a business with an annual run rate in excess of $250 million. Mr. Pachuca holds a BSSE degree from the Instituto Politecnico Nacional at Mexico City.
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