A simple I2C slave in VHDL
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I2C_slave.vhd
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README.org

This is a simple I2C slave written in VHDL. The slave supports single and repeated reads/writes, as well as quick read/writes (master can switch between read and write commands while also addressing different slaves in quick succession without having to write STOP command on the bus). Clock stretching is not implemented, so make sure that data to master is ready upon request.

Usage

The module can be used through the following interface:

read_req         : out std_logic;
data_to_master   : in  std_logic_vector(7 downto 0);
data_valid       : out std_logic;
data_from_master : out std_logic_vector(7 downto 0));

When read_req is high, the slave fetches the data data_to_master in the same clock cycle, so make sure to set the correct data in advance. When data_valid becomes high, data_from_master provides valid data in the same clock cycle.

The slave ignores all master commands directed at addresses that do not match SLAVE_ADDR generic. The 7 bits of the SLAVE_ADDR are usually given in the most significant bit (MSB) first format on the master side. For example, a 7 bit address “0000011” corresponds to decimal 3 on the slave, is actually a 6 on the master because the LSB is never sent.

Tests

A testbench is provided in the design. To run the testbench, run make, for which you need to have ghdl. If you want no error at the end of a successful simulation in ghdl, in the ghdl library you need to substitute Error ("simulation stopped by --stop-delta"); by Info ("simulation stopped by --stop-delta"); in src/grt/grt-processes.adb. The testbench should also work in ModelSim.

Here is an example of a single write: ./pics/single-write.png

And here, an example of 3 quick writes followed by a quick read. Quick means that the slave address is given only once in the beginning of the first “quick” operation. ./pics/consequtive-3xwrite-and-read.png