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rp/spi: sane prescaler calculation

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This commit is contained in:
Dario Nieuwenhuis 2021-07-11 23:47:33 +02:00
parent 61a3462f87
commit 17e31eb903
1 changed files with 26 additions and 18 deletions
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44
embassy-rp/src/spi.rs
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@ -50,26 +50,36 @@ impl<'d, T: Instance> Spi<'d, T> {
let clk_peri = crate::clocks::clk_peri_freq();
assert!(config.frequency <= clk_peri);
// TODO replace these trial-and-error loops with decent calculations.
// final SPI frequency: spi_freq = clk_peri / presc / postdiv
// presc must be in 2..=254, and must be even
// postdiv must be in 1..=256
// Find smallest prescale value which puts output frequency in range of
// post-divide. Prescale is an even number from 2 to 254 inclusive.
let presc = (2u32..=254).step_by(2).find(|&presc| {
(clk_peri as u64) < (presc as u64 + 2) * 256 * config.frequency as u64
});
fn div_roundup(a: u32, b: u32) -> u32 {
(a + b - 1) / b
}
// if this fails, frequency is too low.
let presc = unwrap!(presc);
// divide extra by 2, so we get rid of the "presc must be even" requirement
let ratio = div_roundup(clk_peri, config.frequency * 2);
if ratio > 127 * 256 {
panic!("Requested too low SPI frequency");
}
// Find largest post-divide which makes output <= baudrate. Post-divide is
// an integer in the range 1 to 256 inclusive.
// TODO figure what's up with postdiv=1, it is dividing by 0. Iterate down to 2 for now.
let postdiv = (2u32..=256)
.rev()
.find(|&postdiv| clk_peri / (presc * (postdiv - 1)) > config.frequency);
let postdiv = unwrap!(postdiv);
let presc = div_roundup(ratio, 256);
let postdiv = if presc == 1 {
ratio
} else {
div_roundup(ratio, presc)
};
p.cpsr().write(|w| w.set_cpsdvsr(presc as _));
debug!(
"SPI: requested freq {=u32}, actual freq {=u32}, presc {=u32}, postdiv {=u32}",
config.frequency,
clk_peri / (presc * 2 * postdiv),
presc * 2,
postdiv
);
p.cpsr().write(|w| w.set_cpsdvsr((presc * 2) as _));
p.cr0().write(|w| {
w.set_dss(0b0111); // 8bit
w.set_spo(config.polarity == ehnb::Polarity::IdleHigh);
@ -80,8 +90,6 @@ impl<'d, T: Instance> Spi<'d, T> {
w.set_sse(true); // enable
});
info!("SPI freq: {=u32}", clk_peri / (presc * postdiv));
if let Some(pin) = clk.pin_mut() {
pin.io().ctrl().write(|w| w.set_funcsel(1));
}