This is an implementation of the OCP TAP TimeCard for personal use based on the AMD Artix 7 XC7A100T FPGA, Symmetricom X72 rubidium atomic clock, and an RCB-F9T GNSS module. The card will either be half-height PCI Express (PCIe) x1 or full-height PCI utilizing PI7C9X111SL as a reverse PCI-PCIe bridge. The card is meant to interface with a PCI 33MHz 5V slot directly or via a StarTech PCI1PEX1 adapter card. The X72 oscillator may be chassis-mounted or card-mounted if feasible.
The use case of this card is to make use of the single PCI slot in the Supermicro X13SAE workstation motherboard for hosting a virtualized NTP and PTP server as well as serve as a frequency standard for hobbyist test equipment and amateur radio applications. Accurate timestamping of the IQ stream from one or more software-defined radios (SDRs) is one forseen application.
This project is also meant to help the auther develop her electronics development skills.
The outcomes of this project shall include:
- Working PCI card compatible with the Linux TimeCard driver
- Extended FPGA design which can configure the atomic clock at startup/reset and control its frequency
- Adequate cooling for the atomic clock in order to maintain frequency lock
StarTech PCI1PEX1 already implements a reverse PCI-PCIe bridge utilizing the PI7C9X111SL. A half-height PCIe card can mount to this adapter and would enable using PCIe in other systems instead of requiring the obsolete PCI bus.
If a half-height PCIe card is implemented, the atomic clock may need to be mounted to an optional heatsink/bracket attached to the backside of the card to 'overhang' the X72 in front of the StarTech PCI1PEX1 adapter. In other systems, the X72 may instead be chassis-mounted. A cabled connection between the atomic clock and the TimeCard is then required.
Symmetricom X72 firmware 5.04 is required for the necessary 1 PPS input. Analog frequency control of the X72 may require configuration over UART at reset depending on factory settings. Changed settings are reverted on power-off.
Startup/reset configuration of the X72 may be implemented with the FPGA's configuration AXI bus controller. This IP core is meant to take control of the AXI bus on startup/reset and write configuration data to the registers of the AXI agents. The controller can be programmed with a script and be instructed to read data, write data, or wait. Writing data through a UART agent to the X72's serial interface appears feasible.
For debugging or user-space configuration the atomic clock, /dev/ttyS6 is exposed by the TimeCard driver for the clock's serial port.
The X72 has the following serial port specification:
- Logic level: 5V
- Baud rate: 57.6k
- 8 data bits
- 1 stop bit
- No local echo
- No flow control