687 Register bank accessible through SPI and I2C :: Quicker, easier and cheaper to make your own chip!

How it works

Register bank accessible throught two different serial interfaces: SPI and I2C. Use digital input to select prefered interface.

Digital input ui_in[7] = 0 selects SPI and ui_in[7] = 1 selects I2C.

Block diagram

                +----------------------------------------+
                |                                        |
                |   +-----------+        +-----------+   |
SPI Pins <----> |   | SPI Slave |        | I2C Slave |   | <----> I2C Pins
                |   | Interface |        | Interface |   |
                |   +-----+-----+        +-----+-----+   |
                |         ^                    ^         |
                |         |                    |         |
                |         |                    |         |
                |         v                    v         |
                |      +--+--------------------+--+      |
                |      |        Bus Arbiter       |      |
                |      +-------------+------------+      |
                |                    ^                   |
                |                    |  Internal         |
                |                    |  Register         |
                |                    |  Access Bus       |
                |                    v                   |
                |      +-------------+------------+      |
                |      |       Register Bank      |      |
                |      |                          |      |
                |      |   +------------------+   |      |
                |      |   |       REG0       |---|------|------> 7 segments display
                |      |   +------------------+   |      |
                |      |                          |      |
                |      |   +------------------+   |      |
                |      |   |       REG1       |   |      |
                |      |   +------------------+   |      |
                |      |            ...           |      |
                |      |   +------------------+   |      |
                |      |   |       REG14      |   |      |
                |      |   +------------------+   |      |
                |      |                          |      |
                |      |   +------------------+   |      |
                |      |   |       REG15      |   |      |
                |      |   +------------------+   |      |
                |      +--------------------------+      |
                |                                        |
                +----------------------------------------+

There are 8 read/write 8 bit registers and 8 read only 8 bit registers.

Address 0 (first byte in read/write register space) drives the 7 segment display.

SPI peripheral design based on https://github.com/calonso88/tt07_alu_74181

See that design's docs for information about the SPI peripheral.

Small improvement done on the spi_peripheral module. There used to be two buffer counters (one for RX and one for TX). Since the counters are not used together, it was possible to remove one of them and use a single buffer counter. This has reduced 4 flip flops in total and some combinatorial logic as well.

Added logic to control driver for MISO. On previous submissions of this design, the MISO was always driven. Logic has been added to put MISO into high impedance when CS_N is driven high. Due to a 2-stage synchronizer, the MISO goes to high impedance after 2 clock cycles.

I2C peripheral design based on https://github.com/sanojn/tt06_ttrpg_dice

See that design's docs for information about the I2C peripheral.

Protocol specification

SPI Write (CPOL = 0, CPHA = 0)

SPI Write

SPI Read (CPOL = 0, CPHA = 0)

SPI Write

I2C Frame

I2C frame

Register Map

Offset	Name	Access	Reset	Description
0x00	REG0	R/W	0x00	Controls 7 segmets display on demoboard
0x01	REG1	R/W	0x00	General prupose register
0x02	REG2	R/W	0x00	General prupose register
0x03	REG3	R/W	0x00	General prupose register
0x04	REG4	R/W	0x00	General prupose register
0x05	REG5	R/W	0x00	General prupose register
0x06	REG6	R/W	0x00	General prupose register
0x07	REG7	R/W	0x00	General prupose register
0x08	REG8	RO	0xC4	Constant ID Code 1
0x09	REG9	RO	0x10	Constant ID Code 2
0x0A	REG10	RO	0xAA	Constant ID Code 3
0x0B	REG11	RO	0x55	Constant ID Code 4
0x0C	REG12	RO	0xFF	Constant ID Code 5
0x0D	REG13	RO	0x00	Constant ID Code 6
0x0E	REG14	RO	0xA5	Constant ID Code 7
0x0F	REG15	RO	0x5A	Constant ID Code 8

How to test

SPI

Use SPI1 Master peripheral in RP2040 to start communication on SPI interface towards this design. Remember to configure the SPI mode using digital inputs [0] and [1] to high (if you'd like to have CPOL=1 and CPHA=1).

Example code to initialize SPI in REPL:

spi_miso = tt.pins.pin_uio3
spi_cs = tt.pins.pin_uio4
spi_clk = tt.pins.pin_uio5
spi_mosi = tt.pins.pin_uio6
spi_miso.init(spi_miso.IN, spi_miso.PULL_DOWN)
spi_cs.init(spi_cs.OUT)
spi_clk.init(spi_clk.OUT)
spi_mosi.init(spi_mosi.OUT)
spi = machine.SoftSPI(baudrate=10000, polarity=0, phase=0, bits=8, firstbit=machine.SPI.MSB, sck=spi_clk, mosi=spi_mosi, miso=spi_miso)
spi_cs(1)

Example code to write 0xF8 to address[0]:

spi_cs(0); spi.write(b'\x80\xF8'); spi_cs(1)

This should set the 7 segment LED to 0xF8 which will display "t."

Seg A - OFF, Seg B - OFF, Seg C - OFF, Seg D - ON, Seg E - ON, Seg F - ON, Seg G - ON, Seg DP - ON

Example code to read from address[0]:

spi_cs(0); spi.write(b'\x00'); spi.read(1); spi_cs(1)

The result should be 0xF8 or whatever you wrote to address[0].

I2C

Use I2C Master peripheral in RP2040 to start communication on I2C interface towards this design. Remember to configure the I2C address bits using the digital inputs [2], [3] and [4].

Example code to initialize I2C in REPL:

TO DO

Example code to write 0xF8 to address[0]:

TO DO

Example code to read from address[0]:

TO DO

External hardware

You may need to use a pull up resistors on the i2c data and i2c scl lines if not possible to configured internally on the RP2040. To be checked at a later point in time. Write to the first register to set the LEDs on the demoboard.

#	Input	Output	Bidirectional
0	cpol	spare[0]
1	cpha	spare[1]	i2c_sda
2	i2c_addr[0}	spare[2]	i2c_scl
3	i2c_addr[1}	spare[3]	spi_miso
4	i2c_addr[2}	spare[4]	spi_cs_n
5		spare[5]	spi_clk
6		spare[6]	spi_mosi
7	peripheral selector (SPI=0/I2C=1)	spare[7]

687 Register bank accessible through SPI and I2C