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spi: support zero post-divide
This allows you to use a 62.5 MHz SPI baud rate assuming the default 125 MHz peripheral clock. It would previously crash due to a division by zero.
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@ -86,7 +86,7 @@ impl<S: State, D: SpiDevice, const DS: u8> Spi<S, D, DS> {
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let freq_in = peri_frequency.into().integer();
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let baudrate = baudrate.into().integer();
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let mut prescale: u8 = u8::MAX;
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let mut postdiv: u8 = 1;
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let mut postdiv: u8 = 0;
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// Find smallest prescale value which puts output frequency in range of
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// post-divide. Prescale is an even number from 2 to 254 inclusive.
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@ -104,8 +104,8 @@ impl<S: State, D: SpiDevice, const DS: u8> Spi<S, D, DS> {
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debug_assert_ne!(prescale, u8::MAX);
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// Find largest post-divide which makes output <= baudrate. Post-divide is
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// an integer in the range 1 to 256 inclusive.
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for postdiv_option in (0..=255u8).rev() {
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// an integer in the range 0 to 255 inclusive.
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for postdiv_option in (1..=255u8).rev() {
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if freq_in / (prescale as u32 * postdiv_option as u32) > baudrate {
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postdiv = postdiv_option;
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break;
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