Thursday, March 24, 2016

Prj 145 - BBB LX9 FPGA Board Design (Part 2)

This post summarizes the schematic of a BeagleBone Black FPGA board (overview and block diagram).  This is a minimal FPGA board intended for use with ADCs and DACs so it does not include complicating aspects like DRAM, HDMI, or high speed serial.  Having said this, Xilinx UG-380 does a good job describing the JTAG boundary scan interface, SPI flash interface and configuration interfaces (done, program_b, M0, M1).  You just have to read and study it quite a bit.  The only subtle aspect is the power-on/reset sequencing.

I wanted to use the +5V directly from the unregulated BBB input to allow for higher current draw of this board and daughter boards.  The power on sequence of the LX9 is well defined with high-Z IO pins.  It is the reset and shutdown sequence that has to be addressed.  In this scenario, the BBB and the FPGA board can be powered on, loaded, and operating normally when the user provides a shutdown command to the BBB linux which disables the on board regulated supplies but leaves the +5V unregulated input on (i.e. the wallwart is plugged in but a shutdown has been issued and the BBB is off).  In this case, the LX9 could be left powered up, configured and driving the SPI and JTAG pins while the ARM on the BBB is powered off.  To avoid this problem and allow use of the unregulated supply, a high side switcher is used to control the +5V with the enable being the BBB 3.3V regulated supply.

There are 32 IO pins at one edge of the board with a ground every 4 pins.  The very end provides +5 with 4 pins.  This nicely fits within a 2x23 header (the same used by the BBB).  Since additional pins are available those are brought out to two 2x5 headers.  These are the same used on the I board and include +5V power.  This allows interfacing with other boards using a low speed SPI or I2C.

Schematic Page 1 - LX9, Connectors, Flash, Clock.

Schematic Page 2 - Power and Bypass Capacitors

Screenshot of PCB Layout

The KiCad source material along with pdf of schematic and zip of Gerbers is available here.

The overall cost sourcing from OSHPark and Digikey is $12 for the PCB (quantity 3), LX9 $18 (quantity 1), connectors $3, oscillators, and regulators $3,  Flash and passives $3 for a rough total of $39 per board.

Related:
Prj 145 - Beagle Bone Black Simple LX9 FPGA board (Part 1)
Prj 145 - BBB LX9 FPGA Board Design (Part 2)
Prj 145 - BBB LX9 FPGA Board Construction (Part 3)
Prj 145 - BBB LX9 JTAG Boundary Scan Utilities (Part 4)
Prj 145 - BBB LX9 C++ and VHDL (Part5)

2 comments:

  1. nice design! is there a bill of materials? thanks!

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  2. I did not export the BOM, however, the source KiCad is available at: https://github.com/drone32768/BREC/tree/master/BREC_3/Boards/Fboard

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