umi_gpio

umi_gpio example

This example shows how to set up bit-level interaction between Python and a running RTL simulation. Assuming that switchboard is installed in your current Python environment, you can run the example with ./test.py. The output will look like this:

Initial value: 0xcafed00d
gpio.o[7:0] = 22
gpio.o[15:8] = 77
Got gpio.i[7:0] = 34
Got gpio.i[15:8] = 43
Wrote gpio.o[:] = 0x9376b708291d2fa90c31e62a150a82ff
Read gpio.i[255:128] = 0x9376b708291d2fa90c31e62a150a82ff
Read gpio.i[383:256] = 0x6c8948f7d6e2d056f3ce19d5eaf57d00
PASS!

Here's what the interaction looks like from the Python perspective (with reference to test.py):

1. Create a UmiGpio object from a UmiTxRx object using the .gpio(...) method. It is possible to run multiple GPIO interfaces from a single UmiTxRx object if they are given different dstaddr values.
2. Let's say that the UmiGpio object is called gpio. You can write bits using statements like gpio.o[msb:lsb] = ... or gpio.o[idx] = .... Note that Verilog bit-select syntax is used, where the MSB is before the colon and the LSB is after, and both are included in the bit selection. (This is different than how Python slice notation usually works.)
3. Reading from GPIO interface works in a similar fashion: ... = gpio.i[msb:lsb] or ... = gpio.i[idx].

Under the hood, these reads and writes are implemented using UMI transactions. For example, if you write gpio.o[7:0] = 42, a UMI write request is sent to a switchboard connection, indicating that bits 7-0 should be written to the value 42.

From the RTL simulation perspective, bit-level interaction happens through instances of the umi_gpio module, which is provided by the switchboard repository and automatically included when using SbDut. Connect bit-level signals to be read from Python to the gpio_in port of umi_gpio, and similarly connect bit-level signals to be written from Python to the gpio_out port of umi_gpio. The width of the gpio_in port is set by the IWIDTH parameter and the width of the gpio_out port is set by the OWIDTH parameter; the values set for those parameters in RTL should match the values provided to the UmiTxRx.gpio(...) method when constructing a UmiGpio object.