Digital input serializer principle and input signal configuration

This article describes the operation of a digital input serializer and its configuration for low, medium, and high voltage input signals.

How it works

To better understand how the DIS works, the device is examined in the context of a complete interface design (see Figure 1 ). In general, a high-voltage bus powers a set of sensor switches, S0 – S7, whose on/off states are 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 a parallel-input, serial-output shift register. The internal input data is latched into the shift register due to the microcontroller’s load pulse applied to the /LD input. The microcontroller applies a clock signal to the CLK input, which serially shifts the data from the DIS and into a controller register through a digital isolator, completing the reading of the shift register contents.

High voltage interfaces require the use of digital isolators to electrically isolate the wildly varying ground potentials of remote sensor switches from the local ground of the controller electronics.

Equation 1

I _REF is in turn calculated from the ratio of the internal 1.25V bandgap reference and the external resistor R _{LIM :}

Equation 3 ​

Solving Equation 3 yields R _LIM , the resistor value required to set the desired current limit:

Equation 5

Plugging in the value of V _IP-ON and then substituting it into the I _{IN-LIM calculation in} Equation 3 yields:

Equation 7

Therefore, only two equations are needed to fully configure the DIS for various applications, namely, Equation 3 for setting the current limit and Equation 7 for achieving the desired 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

V IN [V]	V IN-ON [V]	I IN-LIM [mA]	R IN [kΩ]	R LIM [kΩ]
12	5.2	2	0	44.8
34	5.2	2	0	44.8
34	10	2	2.4	44.8
48	24	2	9.4	44.8
100 *	50	0.5	89.6	180
350 *	150	0.5	290	180
* Requires Zener clamp

The asterisks in Table 1 indicate very high input voltages that exceed the maximum device voltage of 34 V. In this case, a 30-V Zener diode connected between IPx and ground prevents device input destruction. By setting the switching threshold in the middle of the input voltage range, V _IN-ON = V _IN-max /2, the maximum Zener current will be equal to the input current limit, I _Z-max = I _IN-LIM , and the total input current will be twice the current limit.

To save energy, the current limit should be set to 0.5mA. Obviously, at this low input current, it does not make sense to connect the indicator LEDs to the Rex outputs, since they will not light up. Instead, we should place them on the controller side where the CMOS outputs can easily implement the LED driving function.

Serial Interface

Figure 1 shows that for bus supplies up to 24V nominal, or 34V maximum, the digital input serializer can regulate the bus voltage down to 5V to provide sufficient power for the digital isolator or microcontroller. However, regulating the bus supply voltage before the DIS under high voltage conditions will greatly reduce the overall power efficiency. In non-isolated applications, using a micro charge pump and providing backup power to the DIS from the controller power supply is more energy efficient. However, in isolated applications, an isolated DC-DC converter is required to provide controller power across the isolation barrier.

The reason for implementing electrical isolation is that digital input serializers are typically used to detect output voltages from 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 the various ground potentials to each other can cause large ground loop currents to flow. Using digital isolators can prevent this from happening.

As mentioned earlier, 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 to latch the current field input state into the DIS shift register. Afterwards, it applies a clock signal to the CLK line to shift out the register contents in a serial manner.

As shown in Figure 2 , the shift register structure of the DIS allows multiple devices to be daisy-chained by simply connecting the serial output SOP of the preceding device to the serial input SIP of the succeeding device. This approach allows for compact digital input module designs with high channel counts while using only one serial interface.

When reading the contents of multiple DIS devices at once, a short read cycle time is a basic requirement, and the maximum speed of the standard microcontroller SPI interface can reach 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 rate of some high-speed isolators. Therefore, if you want to shorten the read cycle time to an absolute minimum, a very high clock frequency is required, and the propagation delay of the isolator must also be eliminated.

For this reason, microcontrollers are often replaced by field programmable gate arrays (FPGAs), which not only have high clock frequencies, but also allow for the implementation of a receive clock input (shown as the blue line in Figure 2). The same clock signal sent by the FPGA then begins shifting the register contents out of the DIS after being delayed by the isolator, while being fed back through another isolator channel along with the SOP signal, thereby maintaining the phase relationship between the receive clock and data.

Figure 2 Isolated 32- channel digital input module

in conclusion

Digital input serializers are the most versatile solution for interfacing low-power controllers with high DC voltages. The SN65HVS88x series of digital input serializers supports the interface design between low-voltage controllers and high-voltage applications, with a variety of features such as undervoltage detection, current limiting, de-jitter filtering, thermal protection, parity generation, and single 5V power supply.