Software UGN Demo

This chapter describes the specific hardware setup used to perform a test where each node is able to determine the UGNs to its neighbors without communicating with a host PC that is able to read all of the information from each of the nodes in the system. This is accomplished through the firmware on the management unit (MU).

Architecture

Initialization sequence

1. “Boot” CPU
   1. Gets programmed by the host
   2. Programs the clock boards
   3. Gets transceiver block out of reset (enabling the bittide domain / other CPUs)
   4. Activates each transceiver channel and waits until they’ve negotiated a link with their neighbors.
   5. Prints “all done” message to UART.
2. Clock control CPU
   1. Gets programmed by the host
   2. Calibrates clocks
   3. Prints “all done” message to UART.
   4. (Keeps calibrating clocks.)
3. Management unit CPU
   1. Gets programmed by the host
   2. Sets the channels to “user” mode. This makes the channels accept data from the outside world instead of their negotiation state machinery.
   3. Waits for hardware UGN to be captured
   4. Start auto-centering elastic buffers
   5. Waits for clocks to be considered stable
   6. Stops auto-centering the elastic buffers
   7. Prints UGNs captured by hardware component to UART.
   8. Initializes the scatter/gather calendars.
   9. Calls the “c_main” and runs the software UGN discovery protocol

Domain related

Components:

• Boot CPU (BOOT)
• Transceivers
• Domain difference counters
• Clock control (CC)
• Elastic buffers (one per incoming transceiver link)
• Hardware UGN capture (for comparison)

Node related

Components:

• Management unit (MU)
• 7 scatter and gather units, one of each per elastic buffer

Management unit

Connected components:

• Timer
• UART (for debugging)
• FPGA DNA register

The management unit has access to and is responsible for all scatter/gather calendars in the node. In this demo, it programs the calendars with increasing values (0, 1, 2, …), effectively creating a transparent link for the MU to access the scatter/gather units directly. It also centers the elastic buffers to ensure stable communication. Finally it runs a distributed protocol to discover the Uninterpretable Garbage Numbers (UGNs) of the network links. For a detailed description of the procedure, see Software UGN Discovery Procedure. It uses the scatter/gather units (enabled by the MU) to exchange timestamped messages with neighbors, calculating the propagation delays in software.

To change the binary run on this CPU, one may either:

• Edit bittide-instances/src/bittide/Instances/Hitl/SoftUgnDemo/Driver.hs, line 215 (at time of writing) to use another binary instead of soft-ugn-mu
• Edit the source files in firmware-binaries/soft-ugn-mu/ to change the binary pre-selected by the driver function

Debugging related

• UART arbiter
• JTAG interconnect
• Integrated logic analyzers
• SYNC_IN/SYNC_OUT

Running tests

One may specifically run the software UGN demo test by making a .github/synthesis/debug.json with the following contents:

[
  {"top": "softUgnDemoTest",       "stage": "test", "cc_report": true}
]

At the time of writing, the clock control CPU stabilizes system. The driver running on the host (bittide-instances/src/bittide/Instances/Hitl/SoftUgnDemo/Driver.hs) then releases the reset of the management unit CPU. In turn, this CPU will center the elastic buffers, initialize the scatter/gather calendars, and print out the UGNs captured using the hardware UGN capture component over UART. Finally, the general purpose processing element is started. It executes the software UGN discovery protocol and prints the results over UART. The host driver then compares the hardware-captured UGNs with the software-discovered UGNs to verify correctness.

Tests are configured to run the following binaries on the system’s CPUs:

• Boot CPU: switch-demo1-boot (firmware-binaries/demos/switch-demo1-boot)
• Clock control CPU: clock-control (firmware-binaries/demos/clock-control)
• Management unit: soft-ugn-mu (firmware-binaries/demos/soft-ugn-mu)

One may change this by either:

1. Changing the driver function so that it loads different binaries onto the CPUs. This may be accomplished by changing which binary name is used with each of the initGdb function calls.
2. Changing the source code for the binaries. The locations for them are listed above.