This can be done with the current AT firmware as well, with any module such as ESP-WROOM-02, ESP-12 etc. You just need to write code for accepting the command via HSPI port and not serial port.
UART is async between TX and RX, where SPI is not and master always makes transmission.
How does ESP notify user about new data to read via SPI?
How do we know how many bytes is ready to be read from ESP?
These are important facts no-where explained.
Saying "SPI is possible too" is not enough because SPI works totally other way than UART.
I hope I get my answers.
Officially, there is the SDIO firmware available with the latest nonOS SDK. An SDIO host controller would typically support that directly. You may see the documentation soon though.
As for SPI based communication, you need to implement a mechanism to interrupt the host if ESP8266 is used as SPI slave. Or you may read the status frequently using slave commands. There is no official release for this at this time, probably because that kind of setup is not very practical. I mean... if your host MCU has 512 bytes RAM, having SPI or UART makes no practical difference in most cases. So right now that is a possibility left out for developers who have such highly specific needs.
As with custom AT firmware, you may even combine SPI and UART and use SPI only for block transfers. Endless possibilities! But then it is all application specific.
When working with SDIO, I'm sure you need that capability of interrupt too, because host MCU is your master.
Anyway, if you will publish SPI based AT command communication (or not AT, maybe something else) with official software, that would be great.
By the way, I might create SPI based AT firmware as a personal project if it seems to be worth the effort (will take a week's worth of work).
By the way, SDIO specs include an interrupt mechanism.
You may post on the Free Market section of the forum regarding this and someone will surely help you out! Using SDKs is better than porting the AT firmware itself. Freelancers are the way to go for this.
I will get back to one of your posts when I find out, please allow some time.
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