A challenge faced by many commercial and industrial applications is how to connect low voltage microcontrollers” title=”controllers” > controllers and digital signal processors (DSPs) to high voltage sensor switches via the interface” title=”interface”>interface and other digital, high voltage circuits. In most cases, feedback in the form of binary (1/0, or high/low) status information is required through these interfaces.
A new generation of interface devices, known as digital input serializers (DIS), are capable of The detection range is as low as 6Vdc and as high as 300 Vdc.
This article will describe the working principle of a digital input serializer and its configuration of low, medium, and high voltage input signals.
To better understand how DIS works, we study this device in the context of a complete interface design (see Figure 1). Typically, the high voltage bus powers a set of sensor switches, S0 – S7, whose on/off status is detected by the device’s eight field inputs, IP0 – IP7. Internal signal processing converts the input signal to a low volt level and applies it to the input of the parallel-in, serial-out shift register. Since the microcontroller’s load pulse is applied to the /LD input, the internal input data is latched into the shift register. The microcontroller applies a clock signal to the CLK input, according to which the data is serially shifted from the DIS and then passed through the digital isolator into a controller register, thus completing the reading of the shift register contents.
The high-voltage interface requires the use of a digital isolator in order to electrically isolate the ground potential of the drastically changing remote sensor switch from the local ground of the controller electronics.
Figure 1 Typical structure of a digital input serializer
Some sensor switches suitable for high-voltage interfaces include proximity switches, relay contacts, limit switches, push-button switches, and more. For high input voltages, the implementation of input resistors RIN0 to RIN7 is necessary to raise the input switching threshold to higher levels, while low input voltage systems generally do not require input resistors.
Figure 1 shows that supply voltages up to 34V can be applied directly to the supply terminals and eight inputs without protection resistors. With this supply voltage, an internal linear regulator can provide a regulated 5V output to power the device’s internal circuitry and external isolators or microcontrollers. Another auxiliary function is an on-chip temperature sensor, which alerts the controller when the junction temperature reaches 150oC.
Adjustable input current limit makes it possible to use high voltages up to 34V directly at the input of the device. As far as the high-voltage interface with pure resistance input is concerned, the input voltage rises due to the increase of the input current, which leads to a sharp increase in its power consumption. In contrast, the input to the DIS greatly reduces power consumption by limiting the input current to a constant level that can be adjusted using an external precision resistor.
Additionally, each channel has its input signal checked for strength and endurance. This current, voltage detection function has some internal signal thresholds to ensure that the channel is not triggered by leakage current or residual voltage.
In the ON state (switch closed), the current comparator detects if the input current is above a predefined leakage current threshold, while the voltage comparator detects if the input voltage is above an internally set reference voltage. If both comparator outputs are logic high, a programmable debounce filter checks whether the new change in input state is caused by a noisy transient or a true input signal.
In the on state, the filter output is high and the current limiter output is connected to the signal return output (Rex). Each RE-output has a light emitting diode (LED) connected to the ground plane, allowing visual indication of the sensor’s on-off status. So if a switch is off, the LED is on. In the off state (switch open), the filter output is low and the output of the current limiter is grounded, the LED does not light.
When configuring the digital input serializer for an application, there are only two important parameters, the input current limit, IIN-LIM, and the turn-on threshold, VIN-ON. Both parameters are adjusted by external resistors RLIM and RIN0 to RIN7. Although RLIM defines the current limit for all eight input channels, it is also possible to set the turn-on threshold for each channel individually by using different RIN values.
The current limiter implements the comparator function internally, and its threshold current ITH is exactly the same as the maximum input current IIN-LIM. ITH is derived from the reference current IREF using a current mirror with a reflection coefficient of n = 72. Since IIN-LIM is the same as ITH, the maximum input current can be expressed as:
IREF is in turn calculated from the ratio of the internal 1.25V bandgap reference to the external resistor RLIM:
. Equation 2
Plugging Equation 2 into Equation 1 yields IIN-LIM as a function of RLIM:
. Equation 3
Solving Equation 3 yields RLIM, the resistor value required to set the ideal current limit:
. Equation 4
The field input turn-on threshold voltage, VIN-ON, is related to the current limit, the input resistor, and the turn-on threshold voltage VIP-ON of the device input. VIP-ON is equal to the fixed 5.2V reference voltage of the internal voltage sense comparator. Therefore, VIP-ON can be expressed as:
Inserting the value of VIP-ON and then substituting the result of the IIN-LIM calculation in Equation 3 yields:
Then solve for RIN to get the input resistor value required to set the ideal turn-on threshold at the specified current limit:
Therefore, only two equations are required to fully configure the DIS for various applications, Equation 3 for setting the current limit and Equation 7 for reaching the ideal turn-on threshold voltage. Based on these two equations, Table 1 lists various resistor combinations for different input threshold voltages and current limits.
Table 1 Various input configurations
Asterisks in Table 1 indicate that very high input voltages would exceed the maximum device voltage of 34V. In this case, a 30V Zener diode connected between IPx and ground prevents damage to the device input. Setting the switching threshold in the middle of the input voltage range, ie VIN-ON = VIN-max/2, then the maximum zener current will be equal to the input current limit, ie IZ-max = IIN-LIM, and the total input current will be twice the current limit.
To save power, set the current limit to 0.5mA. Obviously, with this low input current, it doesn’t make sense to connect the indicator LED to the Rex output as it won’t light up. Instead, we should place them on the controller side where the CMOS output can easily implement the LED driving function.
Figure 1 shows that for bus supplies up to 24V nominal, or 34V maximum, the digital input serializer can scale down the bus voltage to 5V to provide sufficient power for digital isolators or microcontrollers. However, under high voltage conditions, turning down the bus supply voltage before DIS greatly reduces the overall power efficiency. In non-isolated applications, it is more efficient to use a miniature charge pump and provide backup power to the DIS from the controller power supply. However, in isolated applications, an isolated DC-DC converter is required to provide controller power across the isolation barrier.
The reason for galvanic isolation is that digital input serializers are typically used to sense the output voltage of remotely mounted sensors and signal sources, such as the output of an AC rectifier, whose ground potential is significantly different from the local controller ground. Connecting various ground potentials to each other causes significant ground loop currents to flow. Using digital isolators prevents this from happening.
As mentioned earlier, the control of the DIS digital interface is easy to implement. The system controller simply sends a short, low activity load pulse to the /LD input of the DIS through one of its general purpose outputs, designed to latch the current field input state into the DIS shift register. After that, it applies a clock signal to the CLK line to serially shift out the register contents.
As shown in Figure 2, the shift register structure of the DIS enables multiple devices to be daisy-chained by simply connecting the serial output SOP of the preceding device to the serial input SIP of the following device. This approach allows the design of compact digital input modules with high channel counts while using only one serial interface.
When reading the contents of multiple DIS devices at a time, short read cycle times are essential, and standard microcontroller SPI interfaces already have a maximum speed of 10 MHz or 20 Mbps. However, the serial interface of the DIS can support data rates up to 300 Mbps, which even exceeds the data rates of some high-speed isolators. Therefore, to reduce the read cycle time to an absolute minimum, extremely high clock frequencies are required, and the propagation delay of the isolator must also be eliminated.
Because of this, microcontrollers are often replaced by Field Programmable Gate Arrays (FPGAs) because not only do they have a high clock frequency, but they also allow the implementation to receive a clock input (shown by the blue line in Figure 2). Then, the same clock signal sent by the FPGA, delayed by the isolator, starts to shift the register contents out of the DIS while getting feedback through another isolator channel along with the SOP signal to maintain the phase relationship between the received clock and data.
Figure 2 Isolated 32-channel digital input module
Digital input serializers are the most versatile solution for interfacing low-power controllers with high DC voltages. The SN65HVS88x family of digital input serializers supports interface design between low-voltage controllers and high-voltage applications, with a variety of features such as: brownout detection, current limit, debounce filtering, thermal protection, parity generation, and a single 5V supply Wait.
• “Digital Input Serializers Improve Performance of High-Channel-Density Input Modules,” by Kugelstadt, Thomas, TI, June 2008, Industrial Control Design.
• December 2008, TI SN65HVS880 User Guide.
About the Author
Thomas Kugelstadt is a Senior Systems Engineer at TI where he is responsible for defining new high-performance analog products and developing complete system solutions for detecting and conditioning low-level analog signals in industrial systems. During his 20 years at TI, he has been assigned to various international application positions in Europe, Asia, and the United States. Thomas graduated from the Frankfurt University of Applied Science and became a Graduate Engineer upon graduation.
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