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cbalint13 / tinnymodbus

RS485 ModBus tiny multi-sensor module
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general question: UsiUART / SoftUART #18

Closed swissiety closed 1 year ago

swissiety commented 1 year ago

Hello, thank you for providing this nice project for us to adapt and learn. While exploring the code I recognized, that the bootloader uses softuart while the main program uses usiart for communication - as im quite new to electronics I'd like to ask you to elaborate why it is that way. Thank you in advance.

cbalint13 commented 1 year ago

Hello Markus,

Thanks for your interest in this little project !

I recognized, that the bootloader uses softuart while the main program uses usiart for communication - as im quite new to electronics I'd like to ask you to elaborate why it is that way. Thank you in advance.

There was some reasons behind, the hardest was the first one:

  1. The usiart requires interrupts & fixed interrupt vectors with routines while softuart is free of such.

    • usiart use interrupts (3 of them) and must be shared somehow between main & boot (difficult & messy)
    • softuart , and anything in boot code does not use any kind of interrupts thus is completley independent.
    • independent boot means one can swap on the fly back-forth from boot<->main without issues, it is flexible.
    • boot have only one single task: to wait the octets on wire for firmware flashing so softuart is fine for this.

  2. The usiart had a limitation when used on lower speeds like 9600, a safe limit for firmware upgrading.

    • usiart is not even a complete uart it is more like a serdes needing 3 interrupts to handle e.g. 8N1 frames.
    • ironically the code of usiart is even bigger (due to many handlers) making it a bad candidate against softuart.
    • usiart or a any real uart is mandatory in main to offload comm from CPU, due to variety of tasks done here.

If one's concern is stability booth usiuart and softuart offer exact same stability except that that main clock is RC (internal) and not a real crystal since attiny85 have very limited physical pins.

Now, one can port it to a more beefy attiny having stable crystal and real `usart, but hey: this was the real challenge :) Some deployments have ~5 years of continuous readings, exceeding my expectations, they did very well in bad weather too.

Cheers, ~Cristian.