179 SPI FPU :: Quicker, easier and cheaper to make your own chip!

How it works

This is a floating point unit accesible over SPI.

SCLK = ui[0]
NCS  = ui[1]
COPI = ui[2]
CIPO = uo[0]

Constraint frequency(SCLK) < 4*frequency(clk)

It features 4 internal floating point registers, writeable and readable over SPI.

Every SPI transaction starts with a command byte followed by the arguments to the command. If the command outputs data, enough bytes need to be sent for the entire data to be received.

+-------+---------+--- ... -+
|Command|Arguments|...      |
+-------+---------+--- ... -+

WRITE_TO_REG

Writes a floating point value (serialized on COPI) into an internal floating point register.

    +----+----+----+----+----+----+
IN: |0x00|0x0r|b[0]|b[1]|b[2]|b[3]|
    +----+----+----+----+----+----+
OUT:| und| und| und| und| und| und|
    +----+----+----+----+----+----+

r is one of 0,1,2,3 corresponding to the 4 internal registers. b[0] is the lowest byte of the floating point value. b[3] is the highest byte.

PERFORM_ADD

Performs the computation register[in_c] = fadd(register[in_a], register[in_b]) This computation takes a couple of cycles (6 I think right now). These cycles are in reference to clk not to SCLK. Nevertheless add another dummy byte after the command to ensure that the add is being performed.

    +----+----+----+----+----+
IN: |0x00|in_a|in_b|in_c|0x00|
    +----+----+----+----+----+
OUT:| und| und| und| und| und|
    +----+----+----+----+----+

in_a,in_b,in_c can be one of 0x00,0x01,0x02,0x03

READ_FROM_REG

Reads a float from the internal register and serialises it on CIPO.

    +----+----+----+----+----+----+
IN: |0x00|0x0r|xxxx|xxxx|xxxx|xxxx|
    +----+----+----+----+----+----+
OUT:| und| und|b[0]|b[1]|b[2]|b[3]|
    +----+----+----+----+----+----+

r is one of 0,1,2,3 b[0] is the lowest byte of the float b[3] is the highest byte of the float

How to test

It is best to test with the provided arduino testbench in the repository https://github.com/Qwendu/tt_float_adder in the directory src/testbenches/arduino_integration_test.

I have not managed to get it to work with the provided SPI controllers in arduino. Maybe this is because my spi_rx has some bugs.

External hardware

For integration test:

Arduino

Known Bugs

[1] Denormalized numbers do not always add correctly.
[2] When testing on an fpga it was observed that it sometimes worked flawlessly and othertimes the output was always 0. to what extent that was a fault of the testsetup or the tester or the actual code has yet to be determined.

#	Input	Output	Bidirectional
0	SPI_CLK	SPI_OUT	Unused
1	SPI_NCS	Unused	Unused
2	SPI_IN	Unused	Unused
3	Unused	Unused	Unused
4	Unused	Unused	Unused
5	Unused	Unused	Unused
6	Unused	Unused	Unused
7	Unused	Unused	Unused

179 SPI FPU

179 : SPI FPU