Space-Grade, 100-krad, Isolated Serial Peripheral Interface (SPI) RS-422 Circuit
Design Goals
| Parameter | Design Requirement |
|---|---|
| Maximum Bit Rate | ≥ 100kbit/s |
| Isolation Voltage | ≥ ±100 V |
| Maximum Cable Length | ≥ 5 m |
| Maximum Total Ionizing Dose | ≥ 100krad (Si) |
| Maximum SEL | ≥ 75MeV-cm2/mg |
Design Description
The Serial Peripheral Interface (SPI) is commonly used in embedded systems to connect microcontrollers to peripheral devices such as ADCs and DACs. It is often used in spacecraft applications, and sometimes an isolated SPI implementation is necessary to preserve the spacecraft grounding principles or to support DC offsets.
The following circuit uses the DS26F31MQML-SP and DS26F32MQML-SP RS-422 driver and receiver devices to implement one channel of an isolated SPI. Isolation is provided by the AC-coupling capacitors, whose voltage rating determines the level of isolation supported.
Complete SPI Implementation
The following image is a typical four-signal SPI implementation. The circuit shown uses the DS26F31MQML-SP and DS26F32MQML-SP devices and a 5-V supply voltage.
Design Notes
The circuit biases the negative input of each receiver channel slightly above ground. Positive feedback changes the bias point of the positive input depending on the output state of the receiver. When the output is high, the positive input is biased higher than the negative input; when the output is low, the positive input is biased lower than the negative input. In this way, the receiver always remains in the last state that it was actively driven to. The time constant of the AC-coupling circuit is irrelevant, and there is no minimum operating frequency.
Receiver Function Table
| Differential Inputs | Enables | Outputs | |
|---|---|---|---|
| VID = V(+IN) – V(–IN) | ENABLE | ENABLE | OUT |
| VID ≥ 0.2 V | H | X | H |
| X | L | H | |
| VID ≤ –0.2 V | H | X | L |
| X | L | L | |
| X | L | H | Z |
Isolation
Isolation is provided by the AC-coupling capacitors. These should be chosen with a voltage rating greater than the isolation voltage required. For improved failure tolerance, two capacitors in series can be used; however, each capacitor must be rated for the full isolation voltage.
Design Steps
In the following description the component reference designators referred to are those used in the circuit diagram on the first page of this document.
- Choose R1 to match the characteristic impedance of the cable used. In this case, with 24 AWG twisted pair cable, R1 = 100 Ω.
- Choose R2 and R3 so that when the output
is low the differential input voltage is ≤ –200 mV. When the output is low the voltage on
the positive input is 0 V and the voltage on the negative input is given by the following
equation:
This yields the following equation for the differential input voltage:
- Choose R4 and R5 so that when the output voltage
is high, the differential input voltage is ≥ 200 mV. When the output is high the voltage
on the positive input is given with the following equation:
Therefore, the following equation shows the differential input voltage:
- C1 and C2 set the time constant of the
AC-coupling network. If this time constant is long enough to cause the receiver to change
state, the absolute value of the AC-coupling time constant is unimportant. The positive
feedback network formed by R5 ensures that the receiver stays in the last state it was
actively driven to, and thus the minimum operating frequency is independent of the values
of C1 and C2. The time constant of C2,
R2, and R3 is given in the following equation:
The time constant of C1, R4, and R5 is given in the following equation:
- Choose capacitors C1 and C2 with a voltage rating that exceeds the isolation voltage required. For fail-safe circuits, two capacitors in series can be used; however, each individual capacitor must be rated for the full isolation voltage.