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Document baud rate error management for urboot:...

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stefanrueger
2025-04-26 11:52:24 +02:00
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@@ -4250,7 +4250,7 @@ must be a multiple of flash pages or minimal boot section. These can be
selected with the @code{best} feature. The @code{show} feature displays
the properties of the bootloader that would be written without writing it;
removing @code{show} will then write the bootloader to flash
and, using @code{-U}, also set the fuses:
and, using @code{-U}, also set the necessary fuses:
@smallexample
@cartouche
@@ -4310,25 +4310,24 @@ The following table lists possible features.
@noindent
Notes: Features can also be specified like in elements of a canonical file name.
For details on urboot bootloaders and their features see @url{https://github.com/stefanrueger/urboot}.
@multitable {*10} {Only works with UART I/O where the RX port pin is bit-addressable lorem ipsum}
@multitable {*10} {Only works with UART IO where the RX port pin is bit-addressable lorem ipsum}
@item *1 @tab Some parts do not provide @w{4 s} or @w{8 s} watchdog timeout
@item *2 @tab Only works with UART I/O where the RX port pin is bit-addressable
@item *3 @tab From classic parts, only ATtiny441/841 have alternate UART signals
@item *4 @tab There is a subtle difference between external oscillators (@code{x}) that are
usually reasonably accurate and internal oscillators (@code{a}...@code{h},
@code{i}, @code{j}...@code{q}) that, for classic parts, tend to be very
inaccurate. The former can afford higher baud errors (up to 2.2%) whilst for the
latter AVRDUDE warns of a too high baud error at a much lower threshold of 0.7%.
Most classic hardware UARTs only have integer baud rate divisors, which can lead
to high baud rate quantisation errors. Used with the @code{best} option AVRDUDE
can automagically replace UART I/O code with @code{swio} I/O code that usually
leads to smaller baud rate errors. For that, AVRDUDE better knows the type of
oscillator (@code{x} or @code{i}) that drives a board. For lowest baud errors on
an individual board that runs on internal oscillators it is best to measure F_CPU
and use the measured value with the @code{i} prefix. Alternatively, one can use
the nominal internal F_CPU (say @w{8 MHz}) and use prefix letters that make the
bootloader work: depending on the letter AVRDUDE subtracts from or adds to the
nominal F_CPU multiples of 1.25%.
@item *4 @tab There is a subtle difference between external oscillators (@code{x}), which are
reasonably accurate, and internal oscillators (@code{a}...@code{h}, @code{i},
@code{j}...@code{q}), which tend to be inaccurate for classic parts. The former can
afford higher baud errors up to 2.2% while for the latter AVRDUDE warns of a too
high baud error at the lower threshold of 0.7%. Cassic UARTs have integer baud rate
divisors, which can lead to high baud rate quantisation errors. Used with the
@code{best} option AVRDUDE can automagically replace UART I/O code with @code{swio}
I/O code that usually leads to smaller baud rate errors. For that, AVRDUDE better
knows the type of oscillator (@code{x} or @code{i}) that drives a board. For lowest
baud errors on an individual board that runs on internal oscillators it is best to
measure F_CPU and use the measured value with the @code{i} prefix. Alternatively,
one can use the nominal internal F_CPU (say @w{8 MHz}) and use prefix letters that
make the bootloader work: depending on the letter AVRDUDE subtracts from or adds to
the nominal F_CPU multiples of 1.25%. See examples below.
@item *5 @tab Not supported for parts lacking standard SPI communication
@item *6 @tab Not all parts provide hardware bootloader support
@item *7 @tab Only available when flash size is a power of 2 (not ATmega406)
@@ -4337,6 +4336,54 @@ For details on urboot bootloaders and their features see @url{https://github.com
@item *10 @tab The bootloader file is still written to disk in connection with @code{configs}, @code{show} or @code{list}
@end multitable
@*@noindent
Baud rate quantification errors are displayed with @code{-v}. 0.79% baud
rate error is considered OK for external oscillators but too high for
internal oscillators. Hence, AVRDUDE selects software I/O when neither
@code{uart} nor @code{swio} are explicitly requested:
@smallexample
@cartouche
$ avrdude -pm328p -U urboot:x8mhz_56kbaud -qv |& grep -i baud.error
Baud error -0.79% for external oscillator OK
$ avrdude -pm328p -U urboot:i8mhz_56kbaud -qv |& grep -i baud.error
Switching to SWIO as baud error -0.79% too high for internal oscillator
Baud error -0.10% for internal oscillator OK
@end cartouche
@end smallexample
@noindent
There is a warning when the user requests hardware UART I/O:
@smallexample
@cartouche
$ avrdude -pm328p -U urboot:i8mhz_56kbaud_uart0 -qv |& grep -i baud.error
Warning: baud error -0.79% for internal oscillator too high: consider switching to swio
@end cartouche
@end smallexample
@noindent
Requesting @code{best} makes AVRDUDE switch to @code{swio} when UART quatisation errors are considered too high:
@smallexample
@cartouche
$ avrdude -pm328p -U urboot:i8mhz_56kbaud_uart0_best -qv |& grep -i baud.error
Switching to SWIO as baud error -0.79% too high for internal oscillator
Baud error -0.10% for internal oscillator OK
@end cartouche
@end smallexample
@noindent
Alternatively, the user can request @code{swio} on those @code{rx}/@code{tx} lines that the UART uses:
@smallexample
@cartouche
$ avrdude -pm328p -U urboot:i8mhz_56kbaud_uart0_swio -qv |& grep -i baud.error
Baud error -0.10% for internal oscillator OK
@end cartouche
@end smallexample
@page
@c