The core device is a FPGA-based hardware component that contains a softcore or hardcore CPU tightly coupled with the so-called RTIO core, which runs in gateware and provides precision timing. The CPU executes Python code that is statically compiled by the ARTIQ compiler and communicates with peripherals (TTL, DDS, etc.) through the RTIO core, as described in :ref:`artiq-real-time-i-o-concepts`. This architecture provides high timing resolution, low latency, low jitter, high-level programming capabilities, and good integration with the rest of the Python experiment code.
While it is possible to use the other parts of ARTIQ (controllers, master, GUI, dataset management, etc.) without a core device, many experiments require it.
The core device contains some flash storage space which is used to store configuration data. It is one sector (typically 64 kB) large and organized as a list of key-value records, accessible by using ``artiq_coremgmt`` (see :ref:`core-device-management-tool`). The core device IP and MAC addresses, as well as, if present, the startup and/or idle kernels (see :ref:`miscellaneous_config_core_device`) are stored here.
`Kasli <https://github.com/sinara-hw/Kasli/wiki>`_ and `Kasli-SoC <https://github.com/sinara-hw/Kasli-SOC/wiki>`_ are versatile core devices designed for ARTIQ as part of the open-source `Sinara <https://github.com/sinara-hw/meta/wiki>`_ family of boards. All support interfacing to various EEM daughterboards (TTL, DDS, ADC, DAC...) through twelve onboard EEM ports. Kasli-SoC, which runs on a separate `Zynq port <https://git.m-labs.hk/M-Labs/artiq-zynq>`_ of the ARTIQ firmware, is architecturally separate, among other things being capable of performing much heavier software computations quickly locally to the board, but provides generally similar features to Kasli. Kasli itself exists in two versions, of which the improved Kasli v2.0 is now in more common use, but the original Kasli v1.0 remains supported by ARTIQ.
Kasli can be connected to the network using a 10000Base-X SFP module, installed into the SFP0 cage. Kasli-SoC features a built-in Ethernet port to use instead. If configured as a DRTIO satellite, both boards instead reserve SFP0 for the upstream DRTIO connection; remaining SFP cages are available for downstream connections. Equally, if used as a DRTIO master, all free SFP cages are available for downstream connections (i.e. all but SFP0 on Kasli, all four on Kasli-SoC).
The DRTIO line rate depends upon the RTIO clock frequency running, e.g., at 125MHz the line rate is 2.5Gbps, at 150MHz 3.0Gbps, etc. See below for information on RTIO clocks.
With the NIST CLOCK and QC2 adapters, for safe operation of the DDS buses (to prevent damage to the IO banks of the FPGA), the FMC VADJ rail of the KC705 should be changed to 3.3V. Plug the Texas Instruments USB-TO-GPIO PMBus adapter into the PMBus connector in the corner of the KC705 and use the Fusion Digital Power Designer software to configure (requires Windows). Write to chip number U55 (address 52), channel 4, which is the VADJ rail, to make it 3.3V instead of 2.5V. Power cycle the KC705 board to check that the startup voltage on the VADJ rail is now 3.3V.
The QC2 hardware uses TCA6424A I2C I/O expanders to define the directions of its TTL buffers. There is one such expander per FMC card, and they are selected using the PCA9548 on the KC705.
To avoid I/O contention, the startup kernel should first program the TCA6424A expanders and then call ``output()`` on all ``TTLInOut`` channels that should be configured as outputs.
The core device generates the RTIO clock using a PLL locked either to an internal crystal or to an external frequency reference. If choosing the latter, external reference must be provided (via front panel SMA input on Kasli boards). Valid configuration options include:
*``int_100`` - internal crystal reference is used to synthesize a 100MHz RTIO clock,
*``int_125`` - internal crystal reference is used to synthesize a 125MHz RTIO clock (default option),
*``int_150`` - internal crystal reference is used to synthesize a 150MHz RTIO clock.
*``ext0_synth0_10to125`` - external 10MHz reference clock used to synthesize a 125MHz RTIO clock,
*``ext0_synth0_80to125`` - external 80MHz reference clock used to synthesize a 125MHz RTIO clock,
*``ext0_synth0_100to125`` - external 100MHz reference clock used to synthesize a 125MHz RTIO clock,
*``ext0_synth0_125to125`` - external 125MHz reference clock used to synthesize a 125MHz RTIO clock.
As of ARTIQ 8, it is now possible for Kasli and Kasli-SoC configurations to enable WRPLL -- a clock recovery method using `DDMTD <http://white-rabbit.web.cern.ch/documents/DDMTD_for_Sub-ns_Synchronization.pdf>`_ and Si549 oscillators -- both to lock the main RTIO clock and (in DRTIO configurations) to lock satellites to master. This is set by the ``enable_wrpll`` option in the JSON description file. Because WRPLL requires slightly different gateware and firmware, it is necessary to re-flash devices to enable or disable it in extant systems. If you would like to obtain the firmware for a different WRPLL setting through ``awfs_client``, write to the helpdesk@ email.
If phase noise performance is the priority, it is recommended to use ``ext0_synth0_125to125`` over other ``ext0`` options, as this bypasses the (noisy) MMCM.
Bypassing the PLL ensures the skews between input clock, downstream clock outputs, and RTIO clock are deterministic across reboots of the system. This is useful when phase determinism is required in situations where the reference clock fans out to other devices before reaching the master.
