Due to its unique circuitry, the RX input functions differently than the other digital inputs on the Teridian 71M6511, 71M6511H, 71M6513, and 71M6515H family of single- and 3-phase energy-metering ICs. This application note explains what unique considerations apply when interfacing signals that exceed the 0 to 3.6V range to the RX pin. The article also describes the pin summary and considerations when connecting 5V-based circuitry to the RX pin.
Rx Pin Summary
The Rx pin (serial port receive pin) of the 71M6511, 71M6513 and 71M6515 is internally clamped to the V3P3 supply as shown in Figure 1. This means, the voltage of signals applied to this pin will be clamped to V3P3D + 0.6V, i.e. nominally 3.9V. Note that this clamp voltage exceeds the 3.6V Absolute Maximum Rating of the RX input. Inputs above 1.6V (VIL) are guaranteed to be recognized as logic 1. Inputs below 0.8V (VIL) are guaranteed to be recognized as logic 0. Input voltages between 0.8V and 1.6V must be avoided.
Considerations when Connecting 5V-Based Circuitry to the RX Pin
If inputs higher than 3.6V are expected at the RX pin, e.g. when interfacing to 5V-based driving circuitry such as RS-232 transceivers/receivers, TTL or CMOS logic, a resistor attenuator should be used in order to restrict the RX input voltage.
Figure 2 shows the recommended resistor network consisting of R1 (17kΩ) and R2 (33kΩ). This network scales the input voltage VIN of 5.5V to 3.6V, and an input voltage of 2.5V will be scaled to 1.6V. For the low voltage level, VIN voltages below 1.2V will be scaled to 0.8V. The maximum current at 5.5V input voltage is 5.5V/(50kΩ) + IIH = 110µA + 1µA = 111µA.
An alternative circuit is shown in Figure 3. Here, a Schottky diode prevents higher voltages from the driving circuit to reach the RX pin, while a logic zero from the driving circuit pulls RX down. The pull-up resistor R1 ensures that the voltage at RX is driven to logic 1 when the driving circuit is not actively pulling RX down.
Assuming a forward voltage drop at the Schottky diode of 0.4V, any voltage VIN > 1.6V - 0.4V = 1.2V will guarantee a logic 1 at the RX pin. To generate a logic 0 at the RX pin, VIN must be below 0.8V – 0.4V = 0.4V. The maximum drive current at VIN = 0V will be (3.3V – 0.4V)/ 100kΩ = 29µA. If the driving circuitry offers an open-collector output, the RX pin should be connected to this opencollector output and a pull-up resistor equivalent to R1 in Figure 3 should be used.