The current meter probe interface uses a form of buffered I2C over a cable with a 6-pin Mini-DIN connector. This has the advantage of being able to leverage cables left over from the era when PS/2 keyboards and mice were common in PCs, but it also has a number of disadvantages:
Attaching the cable inside the device requires separating many wires, carefully punching them into the connector, and trimming the ends. This is a process prone to human error.
The available stock of suitable cables tend to be rather thick
Thick cables make it a little more annoying to manipulate the meter probe, especially when the meter probe is on the lighter side
Thick cables cannot easily be strain-reliefed by simply kinking them inside the enclosure, and necessitate some sort of cable clamp of which a suitable option has not been found
Despite many real-world examples of it being done, I2C is technically not considered to be a suitable protocol for off-board connections due to cable capacitance and noise susceptibility issues
Mini-DIN connectors are technically not designed for hot-plug, and its not too difficult to fail to plug them in all the way if enough insertion force is not used
The proposed solution to this is to switch the meter probe interface to simply using USB, as it solves many of these issues:
USB device cables are widely available as a surplus/replacement product intended for use in computer mice
These "mouse cables" are very flexible, usually have a rubber strain relief as part of their construction, and also typically come with a board connector (often JST PH compatible) already attached
USB is explicitly designed for hot-plug, and uses differential signalling for better noise immunity
The chosen approach for this change is to integrate the FTDI FT260 USB bridge IC, which provides a HID-compatible USB 2.0 FS interface on one side, and provides I2C and several supporting signals on the other side.
This IC does have a few drawbacks that will need to be managed, but it is hopefully worth it for the benefits. These basically revolve around the 1ms SOF interval of USB, which limits our interrupt response time and transaction rate to much lower than we'd have with a direct I2C interface. However, testing has shown that these can be sufficiently managed so as to not be a serious problem.
The integration plan involves using the "Bus Powered Configuration with +1.8V I/O Voltage" from the datasheet, to enable the use of a simpler LDO, and a configuration EEPROM that is large enough to serve as both the configuration memory and the sensor calibration memory.
dkonigsberg
commented
7 months ago
The current meter probe interface uses a form of buffered I2C over a cable with a 6-pin Mini-DIN connector. This has the advantage of being able to leverage cables left over from the era when PS/2 keyboards and mice were common in PCs, but it also has a number of disadvantages:
The proposed solution to this is to switch the meter probe interface to simply using USB, as it solves many of these issues:
The chosen approach for this change is to integrate the FTDI FT260 USB bridge IC, which provides a HID-compatible USB 2.0 FS interface on one side, and provides I2C and several supporting signals on the other side.
This IC does have a few drawbacks that will need to be managed, but it is hopefully worth it for the benefits. These basically revolve around the 1ms SOF interval of USB, which limits our interrupt response time and transaction rate to much lower than we'd have with a direct I2C interface. However, testing has shown that these can be sufficiently managed so as to not be a serious problem.
The integration plan involves using the "Bus Powered Configuration with +1.8V I/O Voltage" from the datasheet, to enable the use of a simpler LDO, and a configuration EEPROM that is large enough to serve as both the configuration memory and the sensor calibration memory.