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Add flexibility to 32bit MCU designs using standard peripherals approach

Posted: 29 Aug 2012     Print Version  Bookmark and Share

Keywords:microcontroller  analogue-to-digital converters  DACs 

To implement this solution, the developer must use a current-mode IDAC. While a typical DAC provides a voltage output through a current output, an IDAC is controlled by a smart timer that can assist in generating the timing sequence for the communications protocol. In addition, a FIFO buffer on the IDAC can generate an interrupt after the FIFO has sent the last word of data, allowing the software to load the FIFO with the next packet of data to be sent, as shown in figure 3.

Input filter for noisy communications
In a typical industrial context, it is relatively common to find communication channels affected by noise. For example, a USART is a typical interface that can be impacted by a noisy environment and thus benefits from a comparator threshold filter used to filter noise from the receiver (RX) path.

Figure 3: Current-based communications using IDACs.

To implement this system, it is necessary to use a comparator with an N-bit DAC acting as a voltage reference on the negative input. If the device containing this comparator is used in a noisy environment and it has to detect a noisy USART RX signal, the comparator can be used as an RX filter to remove the noise and avoid bad USART data.

The USART RX signal coming from the slave device will connect to the selected comparator positive input pin. The N-bit DAC can be used to threshold filter the RX signal, given prior knowledge of the noise in the system. The comparator's asynchronous output is then sent from the device back into the actual USART RX input.

This input filter technique is not limited to a USART RX channel. The same technique can be used on any noisy input signal. Figure 4 provides a flow diagram of the signal path to enable a filter mechanism for a UART implementation.

Figure 4: Comparator-as-filter USART receiver.

Capacitive touch for tamper detection
Many 32bit 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 32bit 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.

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