Embedded GPS NTP server

I've added a schematic of a preliminary design. It's a start, anyway.

This project has just sat around for a while, but I've taken a renewed interest in it.

I've decided to use the KSZ8081MLXCA-TR MII PHY chip. That, a 25 MHz crystal and an RJ45 jack with magnetics is all that needs to be added for the Ethernet part of this project. Along with that, the SPI0 bus will be connected to the aforementioned MAC address EEPROM chip, one of the UARTs will be connected to the serial port on the GPS module, and the PPS pin will be connected to one of the (many) remaining GPIO pins.

Glen Akins alerted me to this little chip: The Microchip 25AA02E48T-I/OT. It's a pre-programmed 256x8 SPI flash chip. The chip is available as a SOT23-6 and is a standard SPI peripheral. The top 6 bytes (0xFA-0xFF) come pre-programmed with a globally unique EUI48 address. 38¢ Q:1 pricing at DigiKey. Mischief Managed.

To use it, you do an 8 byte SPI exchange: 0x03 0xFA followed by 6 "don't care" bytes. You get the MAC address by reading the values during the "don't care" bytes.

I was talking with someone over e-mail today about this project, and they brought up the topic of a development board.

It's something I hadn't considered for this project before now, but it makes some sense, now that I think about it.

For Orthrus, I originally bought an XPLained E70 board, but I didn't actually make any use of it. Its SD card slot lacked the multiplexing hardware required for that firmware. I wound up making my own board, and it worked well enough that no further hardware iteration was strictly necessary.

But for this project, the E70 XPlained might be just the thing. It has the Ethernet interface on it, which is exactly the same as what I propose to use anyway. All I really need to do is use some jumpers to connect one of my GPS breakout boards up and write the firmware. This has the added advantage of allowing me to try different ways to wire the GPS module to get one that's optimal.

This is sort of skipping to the end, but I am going to see if I can use a 50x80 mm board for this project and the same case that I use for my GPSDO project. The big question mark is whether an RJ45 jack will fit vertically in the available space or not. Or, if not, whether a taller 50x80 extruded chassis is available.

Rather than use a 2.1mm power jack as I always have, I am leaning more towards using a USB microB jack for this project. The reason for that would be so that the same jack could be used as a firmware update port as well as to supply power. As with Orthrus, there will be an internal ERASE header that will force the chip to boot into SAM-BA to accept new firmware over USB. It may be possible to update the firmware over Ethernet as well (TBD), but this would be a fail-safe method to reclaim control of a bricked device.

The back panel will have the power and antenna jacks. I am envisioning putting the Ethernet jack on the front panel so that the integrated LEDs would be visible (and because I'm not sure there would be sufficient space on the back). I'm not yet sure about UI options on the front. I could go with just some light pipes to display rudimentary status, but for extra credit it sure would be nice to be able to actually display the time.

The hardware should wind up - again - being fairly straightforward. I intend to steal the Ethernet design directly from the E70 Xplained board. Most of the rest of the design comes over from Orthrus. The SkyTraq GPS stuff is quite mature at this point having been put to use in a number of my other projects. The only remaining design questions are the optimal wiring of the PPS signal. The intent will be to use it as a timer capture input. The datasheet is a little unclear how best to achieve that, unfortunately.