MAX232 Replacement

Question

I have a circuit where a 12V square wave signal (a Profile Ignition Pickup signal from a Ford EDIS ignition module) was fed to an MCU via a MAX232 chip. My understanding of the MAX232 is that the signal polarity is inverted and level shifted to 5V. In the original circuit design, the MAX232 was also used to convert RS232 to TTL for comms with the MCU.

I have omitted the COM port and the MAX232 from my design. I only have the PIP signal which I need to convert and want to use minimum components to do so. My thinking is that a CMOS inverter MOSFET arrangement would do this.

Is this a valid solution?

Answer 1

Profile Ignition Pickup processing for an MCU input

You circuit will work but I'd make the following comments:

1. It is subject to some minor shoot through (when both FETs are ON). With your circuit the problem is very minor (10's uA) at 25degC and worst case probably less than 1mA at 100degC, so you can probably ignore it.

2. It has very poor static (ESD) capability and you are very likely to blow a FET when the input cables are disconnected. Even a simple series 1k Ohm and 100k Ohm resistor from the FET Gates to Gnd would help control this.

3. The voltage switching point is quite low (less than 2V) for a 12V system.

4. There is no hysteresis in the input circuit, so it may be prone to noise.

^{simulate this circuit – Schematic created using CircuitLab}

In the schematic above I've included R2 and R3 to provide some pulse protection, but you really should have an ESD diode (20V) across R2 to provide real clamping.
The waveform would be as below:

A more effective circuit is shown below:

^{simulate this circuit}

In the schematic above I've shown a TLV6001 opamp used as a comparator. This device is effective as it is designed with an inbuilt clamp to the power supply rated at 10mA.
Since the R'Pi 3V3 power supply is consuming much more than 10mA (and has significant capacitance), there is no problem in sinking current into the supply during pulse conditions. This circuit could withstand 100V on the PIP line without damage.

Note: D1 is only shown in place for the simulation, the clamp diode is actually inside the TLV6001.

The threshold points are set to 3.3V rising and 2.8V falling, so giving about 500mV of hysteresis.

The waveforms are as shown below, though I used a triangle wave for the simulator.

Update: Considering the need for small risetime values.
I particularly selected this device as the slew rate for the TLV6001 is 0.5V/us, which seems more than adequate for this sort of low frequency application. In a high noise electrical environment such as an automobile you don't want or need very sharp edges. For a Crankshaft sensor application the sensor frequency is just 200Hz at 6000rpm with a sensor output in the ms pulse width range. One rotation of the crank at 6krpm is 10ms or about 27us/deg. With a 10us risetime you can expect position uncertainty less than 0.3deg of rotation. Since I assume you are trying to calculate spark advance in the 10-12deg range I'd consider this more than adequate. You already have software delays which are going to far exceed this level of uncertainty.

Answer 2

Your design is essentially a CMOS logic inverter: when PIP is logic HIGH, PIP_UC is logic LOW and vice versa.

The first concern is that the +5V supply could have a low impedance path to ground through Q1 and Q2 when these transistors are both conducting for a brief time as they change logic states. The +5V power supply and any circuits connected to the +5V supply must be able to tolerate this. See regions A, B, and C, and the graph of IDD vs. Vin on page 18 in this PDF.

If you perform an Internet search using the keywords "logic level converter" you'll find some simple circuits that can do what you want. The circuit that's commonly used is based upon a single NMOS transistor with two resistors (one resistor between the source pin and +5V (the lower voltage side), and the other resistor between the drain pin and +12V (the higher voltage side)), and the gate pin connected to +5V. And the circuit I'm thinking of supports bidirectional communication. (Unfortunately I cannot get CircuitLab to export a schematic, otherwise I'd provide an example schematic here.)

If you do not need bidirectional communication, and your goals are a) voltage level conversion and b) logic level inversion (e.g., a logic HIGH input produces a logic LOW output), you can accomplish this with a single NMOS transistor and a resistor. Connect the NMOS transistor's source to ground, connect the resistor between the NMOS transistor's drain and the +5V supply, connect the PIP signal to the NMOS's gate, and the output signal is taken at the NMOS's drain.

Other concerns are power system noise, overcurrent protection, and perhaps galvanic isolation between the +5V and +12V systems. If you're planning to use this circuit in an automotive application, overcurrent protection is a must. Also, the vehicle's +12V power (battery + alternator) is very noisy with voltage spikes, and voltage rises/droops that must be carefully managed. Galvanic isolation between the +5V and +12V systems (e.g., an optocoupler) would also be worth considering.

Answer 3

If you use the single transistor approach and your transistor is a MOSFET, and if you directly connect the +12V signal to the MOSFET's gate, you'll likely see large voltage spikes on the +5V logic side during each logic state transition on the 12V side.

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EXAMPLE 1

Figure 1 shows the voltage at the drain pin of an IRF530N NMOS for the circuit topology shown in fig. 2, but without resistor R2, and with M1's drain connected directly to a digital input pin on the microcontroller. (n.b. For this discussion, assume the microcontroller's input pin circuit does not provide voltage clamping protection diodes.)

The preshoot pulses observed at the corners of the voltage signal at M1's drain pin (fig. 1) occur because the MOSFET capacitively couples the edge of each state transition on the +12V side through its gate-drain capacitance to to drain pin on +5V logic side. The amplitude of these voltage spikes can be large enough to damage the circuitry of the microcontroller's digital input pin. For example, if a microcontroller's digital input pin has a minimum rated input voltage of -0.5 V, and a maximum rated voltage of 5.5 V, the signal shown in fig. 1 approaches the -0.5 V minimum, and it clearly exceeds the +5.5 V maximum.

Figure 1.

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There are various ways to manage this problem—e.g., see this EE.SE message thread: Schottky diodes or regular ones for input protection circuit.

Note that modern microcontrollers usually provide the two voltage clamping protection diodes shown in the linked page; they are part of the circuitry that implements a digital input pin. If this is the case, you provide resistor R2 connected in series between the MOSFET's drain pin and the microcontroller's digital input pin as shown in fig. 2.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2.

If the addition of R2 does not reduce the amplitude of the preshoot voltage spikes to acceptable levels at the digital input pin, you can try adding a small value resistor in series with M1's gate, and adding a small capacitance (e.g., 2 nF) between M1's drain and ground. Note, however, that adding these components—and in particular, adding the capacitor—will slow down M1's switching times.