(Editor's note: Signal Chain Basics is an ongoing and popular series; click here for a complete, linked list of all installments.)
Many low-voltage RS-485 bus node designs utilizing 3V microcontroller units (MCU) favor 5V over 3V transceivers (XCVR) due to higher drive capability, which ensures higher noise immunity and allows for longer transmission distance (Figure 1
). Because the output voltage levels of 3V MCUs are sufficiently high to properly drive the TTL input levels of 5V transceivers, controller outputs can be directly connected to transceiver inputs.
Figure 1. Typical interface challenge between 3V MCU and 5V transceiver.
In the opposite direction, however, the transceiver output level exceeds the MCU maximum input rating and level-shifting (LS) is required.
As a first level-shifting solution the common resistive voltage divider comes to mindfollowed by a Zener diode approach (Figure 2).
Figure 2. Level-shifting via voltage divider (left) and Zener diode (right).
The problem with both solutions is that during a bus-idling, the receiver commonly outputs a high. This causes current to flow through the level-shifter, thus, occurring at times when current isn’t actually needed but rather wasted. Of course, it is possible to increase the resistor values in order to reduce current consumption, but higher impedance in combination with the input capacitance of MCU I/O can lead to signal distortion at higher data rates. For the Zener diode approach, distortion can occur earlier due to the high diode capacitance of 400 to 500 pF.
A better solution preventing current flow during bus-idle, and only causing a small current to flow when the receiver output goes low, is implementing a reverse-biased Schottky diode in combination with a pull-up resistor (Figure 3).
Figure 3. Level-shifting with Schottky diode and pull-up resistor.
When the receiver output is high the diode is reverse-biased, and a 3.3V potential lies at the MCU’s receive data input, RxD. When the receiver output goes low, the diode is forward-biased. Now current flows from the 3.3 V supply through the pull-up resistor, and the lower transistor of the receiver’s push-pull output stage towards ground. The voltage at the MCU input is now reduced to the diode forward voltage.
Figure 4 shows a generic versus a specific design solution. Figure 4a represents a generic MCU with an external pull-up resistor and separate enable lines for driver and receiver. The circuit in Figure 4b
focuses on a specific MCU with programmable internal pull-up resistors.
Here the interface is simplified by avoiding an external resistor and
using only a single direction-control line (DIR) to configure the
transceiver for drive and receive mode.
Figure 4. Final interface solutions using a Schottky diode and an external or internal pull-up resistor.
The transceiver chosen provides a maximum data rate of 250 kbps, typically used in long-haul and e-metering applications. For low an input capacitance of about 5 pF, the combination of BAS70 diode and 10 kO
resistor provides crisp level-shifting up to approximately 1 Mbps.
Beware of those cheap current-limiting tricks that try to save you a second resistor versus the voltage-divider solution, but instead compromise the long term reliability of your controller (Figure 5). Like most logic circuits, the microcontroller inputs possess low-voltage triggered silicon-controlled rectifiers (LVTSCR) that protect the inner circuitry from over-voltages. Thus, if an over-voltage is applied to the RxD-pin, the SCR conducts and diverts excessive current straight to ground.
Figure 5. Using a single current-limiting resistor causes damage to the input circuitry.
However, when applied in conjunction with a current limiting resistor, the 5V from the transceiver output triggers the SCR in the MCU input, but the current limiting resistor, RLIM, won’t allow the SCR to reach its holding region. Because of this, the SCR drops back into high-impedance, allowing the over-voltage to reach the inner input circuitry. This oscillating between switching point and holding region continues until the over-voltage is removed, in other words, the receiver output goes low. However, during bus-idling the receiver’s high output causes an over-voltage at the MCU input just with frequent interruptions towards ground. This behavior clearly presents a long term reliability issue and can be avoided by implementing a reverse-biased Schottky diode with a pull-up resistor.
Please join us next month when we will show you how to design a two-to-four wire converter for RS-485.
For more information on interface solutions from TI, visit: www.ti.com/interface-ca.