Introduction
isoSPI is a proprietary Linear Technology protocol that uses Isolated Transceivers to provide robust galvanic isolation for various digital serial communication interfaces and devices. Isolation barriers are implemented using either inexpensive Ethernet transformers, coupling capacitors or embedded inductors. The main benefit is very low EMI susceptibility and emissions, as well as high noise immunity, to preserve signal integrity.
Architecture
There are transceivers that take 4-wire SPI and translate them into pulses
| Maximum Clock Speed | 1MHz (see p.12 in LTC6820 datasheet) |
|---|---|
| Synchronous/Asynchronous | Synchronous |
| Lines | 2 (twisted pair) |
| Number of Slaves | Limited by the number of transceivers |
| Communication Direction | Bidirectional (Full-Duplex) |
| Signalling | Differential between isoSPI positive and negative |
Scope Captures
Being able to decode scope captures are pretty useful when debugging. Linear Technology has an isoSPI Analyzer board (LTC DC1907A) which automatically converts the isoSPI signal into 4-wire SPI for use with the LTC6804, but we can do the same thing manually using an oscilloscope.
Scope Settings
| Probe between | isoSPI positive and negative |
|---|---|
Logical Signals
(Taken from Table 1 in the LTC6820 datasheet)
| Pulse Type | First Level | Second Level | Ending Level |
|---|---|---|---|
| Long +1 | +Va (150ns) | -Va (150ns) | 0V |
| Long -1 | -Va (150ns) | +Va (150ns) | 0V |
| Short +1 | +Va (50ns) | -Va (50ns) | 0V |
| Short -1 | -Va (50ns) | +Va (50ns) | 0V |
(Taken from Table 2 in the LTC6820 datasheet)
| SPI Master Event | Transmitted Pulse |
|---|---|
| CS Rising | Long +1 |
| CS Falling | Long –1 |
| SCK Latching Edge, MOSI = 1 | Short +1 |
| SCK Latching Edge, MOSI = 0 | Short –1 |
Example 1: Logical High (0xFF)
Here's a scope capture with logical high being sent by the SPI master.
Example 2: Odd Alternating Bits (0xAA)
Here's a scope capture with odd alternating bits being sent by the SPI master.
Example 3: Even Alternating Bits (0x55)
Here's a scope capture with even alternating bits being sent by the SPI master.
Gotchas
When debugging the LTC6804 and the LTC2484, we ran into a few issues. Here are a couple things to watch out for:
- Check that the SPI mode your transciever is configured for matches the SPI mode of the IC
- Ensure that you are connecting the isoSPI positive on the master to positive on the slave, and isoSPI negative on the master goes to negative on the slave