高精度实验室系列：MSDI

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Hello. Today we'll give an overview of TI's Multi-Switch Detection Interface, or MSDI portfolio. We'll discuss a discrete implementation of automotive switches and how MSDI devices provide a better solution. We will also discuss common MSDI applications, MSDI features, and how the MSDI devices can be used with digital and/or analog switches.

So what is MSDI? MSDI stands for multi-switch detection interface. It is a fully integrated low power switch and sensor interface device that monitors multi position analog and digital switches, connected to either the supply voltage or ground. It monitors the switches by using internal current sources and sink current to simultaneously supply the switch with a small amount of current called the wetting current.

It monitors the voltage drop across a switch, which will be different for a switch that is open or closed. Immediately following the end of the configuration, the switches will be pulled, and their default state stored. Once they switch to is detected, the device reports the status of the switches to the MCU, their status register that can be pulled and/or a hardware interrupt pin.

Texas Instruments offers three MSDI products, with all devices being capable of monitoring 24 direct switch inputs in a total system. The TIC12400-21 and the TIC10024-Q1 both are designed to detect external mechanical switches status in an automotive system, while the TIC12400 is designed to detect external mechanical switches status in an industrial system.

Applications of MSDI devices are typically seen in automotive and industrial systems such, as VCMs, automotive lighting, heating and cooling, powered seats, mirrors, signal measurement, PLCs, DCs, PACs, and instrumentation modules. Throughout the years, the automotive industry has changed, and can be seen in the features in the car you drive. Features such as headlights, turn signals, and AC provide convenience and safety while driving.

Newer features such as central locking systems, keyless ignition systems, and backup cameras have brought even more safety and convenience. These features, among others, are usually implemented into a car by multiple switches or sensors that are monitored by MCU. The switches require multiple discrete components to protect the microcontroller, and provide wetting currents from the contacts. However, this discreet approach causes many issues for both the designer and the user.

This figure is a design example of how an external ground connected switch input is implemented using multiple discrete components. Those components include transistors and resistors to create wetting currents, GPIOs to enable and disable the wetting currents, as well as diodes, resistors, and capacitors for each switch input.

This discrete implementation, however, brings out many problems. The high component count increases both the size of the physical solution and the bill of material and manufacturing costs, as seen in the figure above. There's high power consumption because the wetting current and microcontroller must remain on constantly, and are always monitoring the GPIO's inputs fast switch response time.

Variations in wetting currents are also common due to transient load changes from the shared current source and component tolerance variations, as abnormal events such as load dumps and jump starts that can cause system malfunction. And with each switch option requiring a different discrete design, it lacks design portability and reuse.

MSDI, however, is a solution to all these problems, by integrating the discrete components for multiple channels into a single device, as seen in this figure. For example, a discreet 24-channel solution requires at least 78 resistors, 27 capacitors, 24 diodes, and 6 FETs, as well as 28 GPIOs on the MCU. With MSDI, the only components needed are the MSDI device, 24 capacitors for the IO pins, 5 capacitors for decoupling, 5 GPIOs on the MCU for the SPI bus, and hardware interrupt pins that need a single pullup resistor. MSDI decreases component count, saves board space, which decreases the bill of material and manufacturing costs.

The following are some of the other features Texas Instruments MSDI devices offer. In the TIC12400 and TIC12400-Q1, the integrated ADC allows for detection of resistor coded switches, which are seen in wipers and light dimmers. The integrated ADC reduces the system complexity, and allows for a cheaper MCU. Multiple digital switches can be combined to one MSDI input, and the ADC can provide the information of the status of each switch, which reduces the cabling and number of connections in the system.

The threshold for the ADC can be set to one of the 1023 ADC codes, while the comparator seen in all devices offers four threshold levels, 2, 2.7, 3, and 4 volts. These configurable thresholds provide the capability for the system to mitigate ground shift, supply shift, and supply fluctuations. The MSDI provides integrated ESD protection on supply and switch inputs pins.

MSDI can monitor switch inputs, while the MCU's in sleep mode, and interrupt generation can be used to wake up the power regulator, which will wake up to MCU and reduce system power. All the devices also have unique wetting current settings that can be programmed for each input to support different application scenarios.

These current settings are internally monitored and controlled so they remain consistent over a wide range of battery input voltages. The MSDI device can detect abnormal supply conditions to the MCU to avoid a fall switch detection, and can also indicate a temperature warning to the MCU.

The cyclic redundancy check feature is available as a safety feature. It is a way for the system to detect any form or configuration change, the corruption of the configuration register, such as from an unexpected voltage drop or noise spur on the reset pin, or potentially even some unwanted access, such as a hacking event that could result in switches not being properly detected.

This feature enables robust SPI transactions and ensures proper registered programming in the noisy automotive market. And the TIC12400-Q1 and TIC12400, ADC self diagnostics is available to monitor the integrity of the internal ADC. Wetting current diagnostics are also available to make sure the wetting current is flowing accurately. Both are useful in safety critical applications.

As mentioned before, MSDI devices monitor the voltage drop across analog and digital switches. But what is the difference between these types of switches? Digital switches are defined to have only two states, on or off. An example of a digital switch is a seatbelt gate switch in a car.

A simple comparator can be used to detect whether the voltage drop across an open or closed switch is either above or below a set threshold. Analog switches, also called resistor coded switches, have multiple states or positions associated with a different resistor value to ground for each state, resulting in a different voltage drop across the position.

An example of a resistor coded switch is a windshield wiper, where you can change the speed setting. An ADC is needed to accurately measure the voltage drop across a switch for comparison against multiple configured thresholds to determine the switches' current position. The TIC12400-Q1 and the TIC12400 have both comparator and ADC options for each input channel and can be used with either type of switch. But the TIC12400-Q1 only have a comparator, and can only be used to detect digital type switches.

Texas Instruments MSDI devices prove to have many features that benefit customers. The MSDI design allows for a decrease in component count, saving board space, and reducing the bill of materials. The devices can be used for analog and/or digital switches, and based on the system's application needs, the MSDI operation can be configured, which is discussed more in detail in the MSDI operation Precision Labs video. To find more MSDI technical resources and search products, visit ti.com/MSDI.