Distributed Real Time Input/Output (DRTIO)¶
DRTIO is a time and data transfer system that allows ARTIQ RTIO channels to be distributed among several satellite devices synchronized and controlled by a central core device.
The link is a high speed duplex serial line operating at 1Gbps or more, over copper or optical fiber.
The main source of DRTIO traffic is the remote control of RTIO output and input channels. The protocol is optimized to maximize throughput and minimize latency, and handles flow control and error conditions (underflows, overflows, etc.)
The DRTIO protocol also supports auxiliary, low-priority and non-realtime traffic. The auxiliary channel supports overriding and monitoring TTL I/Os. Auxiliary traffic never interrupts or delays the main traffic, so that it cannot cause unexpected poor performance (e.g. RTIO underflows).
Time transfer and clock syntonization is typically done over the serial link alone. The DRTIO code is organized as much as possible to support porting to different types of transceivers (Xilinx MGTs, Altera MGTs, soft transceivers running off regular FPGA IOs, etc.) and different synchronization mechanisms.
The lower layers of DRTIO are similar to White Rabbit, with the following main differences:
- lower latency
- deterministic latency
- real-time/auxiliary channels
- higher bandwidth
- no Ethernet compatibility
- only star or tree topologies are supported
From ARTIQ kernels, DRTIO channels are used in the same way as local RTIO channels.
Remote RTIO channels are accessed in the same was as local ones. Bits 16-24 of the RTIO channel number are used to select between local RTIO channels or one of the connected DRTIO satellites. Bits 0-15 of the RTIO channel number select the channel within one device (local or remote).
This scheme will be expanded later with the introduction of DRTIO switches.
Real-time and auxiliary packets¶
DRTIO is a packet-based protocol that uses two types of packets:
- real-time packets, which are transmitted at high priority at a high bandwidth and are used for the bulk of RTIO commands and data. In the ARTIQ DRTIO implementation, real-time packets are processed entirely in gateware.
- auxiliary packets, which are lower-bandwidth and are used for ancilliary tasks such as housekeeping and monitoring/injection. Auxiliary packets are low-priority and their transmission has no impact on the timing of real-time packets (however, transmission of real-time packets slows down the transmission of auxiliary packets). In the ARTIQ DRTIO implementation, the contents of the auxiliary packets are read and written directly by the firmware, with the gateware simply handling the transmission of the raw data.
At the DRTIO satellite device, the recovered and aligned transceiver clock is used for clocking RTIO channels, after appropriate jitter filtering using devices such as the Si5324. The same clock is also used for clocking the DRTIO transmitter (loop timing), which simplifies clock domain transfers and allows for precise round-trip-time measurements to be done.
RTIO clock synchronization¶
As part of the DRTIO link initialization, a real-time packet is sent by the core device to each satellite device to make them load their respective timestamp counters with the timestamp values from their respective packets.
Controlling a remote RTIO output involves placing the RTIO event into the FIFO of the remote device. The core device maintains a cache of the space available in each channel FIFO of the remote device. If, according to the cache, there is space available, then a packet containing the event information (timestamp, address, channel, data) is sent immediately and the cached value is decremented by one. If, according to the cache, no space is available, then the core device sends a request for the space available in the remote FIFO and updates the cache. The process repeats until at least one FIFO entry is available for the event, at which point a packet containing the event information is sent as before.
Detecting underflow conditions is the responsibility of the core device; should an underflow occur then no DRTIO packet is transmitted. Sequence errors are handled similarly.
The core device sends a request to the satellite for reading data from one of its channels. The request contains a timeout, which is the RTIO timestamp to wait for until an input event appears. The satellite then replies with either an input event (containing timestamp and data), a timeout, or an overflow error.