ESP32-C3 TWAI Data Register Access

[alexbohm]

While implementing a Rust driver, I came across some inconsistencies in the access to the TWAI_DATA_x_REG registers. It seems like they are listed as write only access in the svd, yet I believe they are used to read from the receive buffer.

Snippet of esp32c3.svd:

        <register>
          <name>DATA_0</name>
          <description>Data register 0</description>
          <addressOffset>0x40</addressOffset>
          <size>0x20</size>
          <fields>
            <field>
              <name>TX_BYTE_0</name>
              <description>In reset mode, it is acceptance code register 0 with R/W Permission. In operation mode, it stores the 0th byte information of the data to be transmitted under operating mode.</description>
              <bitOffset>0</bitOffset>
              <bitWidth>8</bitWidth>
              <access>write-only</access>
            </field>
          </fields>
        </register>

If I'm reading section 29.4.4.1 in the ESP32-C3 reference manual correctly, it states that writing these addresses is for transmission and reading accesses the receive buffer:
https://www.espressif.com/sites/default/files/documentation/esp32-c3_technical_reference_manual_en.pdf#subsubsection.29.4.4

To add to the confusion, it looks like reading these registers is undocumented in the ESP32-C3 reference manual outside of that section.

Taking a look at how the idf C code implements the drivers:
https://github.com/espressif/esp-idf/blob/867745a05c5cb1fe5eb212200580626ad0fa839d/components/hal/esp32c3/include/hal/twai_ll.h#L514
it does look like these registers are read to receive packets.

[jessebraham]

I believe that the TWAI peripheral can operate in two different modes, Operation Mode and Reset Mode. Looking at the register summary for that peripheral shows two access levels, one in black (Operation) and one in red (Reset).

Shouldn't be an issue to just set these as read-write I don't think. If you wanted to submit an SVD patch to esp-pacs then that will unblock your work at the very least. Regardless, I will update the SVD whenever I get a chance, I'm unfortunately quite busy with some other projects currently.

[alexbohm]

Thanks for taking a look at this! I just put up a pull request with a patch. I'm not too familiar with what the SVD files would look like for the two operating modes, but for now having the read-write works. Thanks!

[jessebraham]

Unfortunately the SVD specification does not seem to support multiple modes. I had discussed this briefly with a colleague, the thoughts were that we can do one of the following (or possibly another option):

1. Create two TWAI peripherals in the SVD, one for each mode
2. If able, just update the access level and/or whatever else needs updating, and the developer will need to be aware of some registers having multiple uses

Not sure which the best solution is, but it may become evident once the driver is implemented.