diff --git a/Cargo.lock b/Cargo.lock
index d7bcb99..969a145 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -358,6 +358,7 @@ dependencies = [
  "cortex-m-rt",
  "hash2hwaddr",
  "log",
+ "nb",
  "nom",
  "num-traits",
  "panic-abort",
diff --git a/Cargo.toml b/Cargo.toml
index fbee09d..9553af0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -29,6 +29,7 @@ bit_field = "0.10"
 byteorder = { version = "1", default-features = false }
 nom = { version = "5", default-features = false }
 num-traits = { version = "0.2", default-features = false, features = ["libm"] }
+nb = "0.1"
 
 [patch.crates-io]
 # TODO: pending https://github.com/stm32-rs/stm32f4xx-hal/pull/125
diff --git a/src/main.rs b/src/main.rs
index 84a2b00..3cd87a6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -32,6 +32,7 @@ mod init_log;
 use init_log::init_log;
 mod pins;
 use pins::Pins;
+mod softspi;
 mod ad7172;
 mod ad5680;
 mod net;
@@ -89,6 +90,8 @@ fn main() -> ! {
     wd.start(WATCHDOG_INTERVAL.ms());
     wd.feed();
 
+    timer::setup(cp.SYST, clocks);
+
     let pins = Pins::setup(
         clocks, dp.TIM1, dp.TIM3,
         dp.GPIOA, dp.GPIOB, dp.GPIOC, dp.GPIOE, dp.GPIOF, dp.GPIOG,
@@ -98,8 +101,6 @@ fn main() -> ! {
     let mut channels = Channels::new(pins);
     channels.calibrate_dac_value(0);
 
-    timer::setup(cp.SYST, clocks);
-
     #[cfg(not(feature = "generate-hwaddr"))]
     let hwaddr = EthernetAddress(NET_HWADDR);
     #[cfg(feature = "generate-hwaddr")]
diff --git a/src/pins.rs b/src/pins.rs
index ff4545f..dd1c773 100644
--- a/src/pins.rs
+++ b/src/pins.rs
@@ -1,6 +1,6 @@
 use stm32f4xx_hal::{
     adc::Adc,
-    hal::{blocking::spi::Transfer, digital::v2::OutputPin},
+    hal::{blocking::spi::Transfer, digital::v2::{InputPin, OutputPin}},
     gpio::{
         AF5, Alternate, Analog,
         gpioa::*,
@@ -20,6 +20,20 @@ use stm32f4xx_hal::{
     time::U32Ext,
 };
 use crate::channel::{Channel0, Channel1};
+use crate::softspi::SoftSpi;
+
+
+pub struct DummyInputPin;
+
+impl InputPin for DummyInputPin {
+    type Error = (); // `Void`
+    fn is_high(&self) -> Result<bool, Self::Error> {
+        Ok(false)
+    }
+    fn is_low(&self) -> Result<bool, Self::Error> {
+        Ok(true)
+    }
+}
 
 
 pub trait ChannelPins {
@@ -58,8 +72,8 @@ impl ChannelPins for Channel1 {
 /// SPI peripheral used for communication with the ADC
 pub type AdcSpi = Spi<SPI2, (PB10<Alternate<AF5>>, PB14<Alternate<AF5>>, PB15<Alternate<AF5>>)>;
 pub type AdcNss = PB12<Output<PushPull>>;
-type Dac0Spi = Spi<SPI4, (PE2<Alternate<AF5>>, NoMiso, PE6<Alternate<AF5>>)>;
-type Dac1Spi = Spi<SPI5, (PF7<Alternate<AF5>>, NoMiso, PF9<Alternate<AF5>>)>;
+type Dac0Spi = SoftSpi<PE2<Output<PushPull>>, PE6<Output<PushPull>>, DummyInputPin>;
+type Dac1Spi = SoftSpi<PF7<Output<PushPull>>, PF9<Output<PushPull>>, DummyInputPin>;
 
 pub type TecUMeasAdc = Adc<ADC3>;
 
@@ -196,14 +210,10 @@ impl Pins {
         clocks: Clocks, spi4: SPI4,
         sclk: PE2<M1>, sync: PE4<M2>, sdin: PE6<M3>
     ) -> (Dac0Spi, PE4<Output<PushPull>>) {
-        let sclk = sclk.into_alternate_af5();
-        let sdin = sdin.into_alternate_af5();
-        let spi = Spi::spi4(
-            spi4,
-            (sclk, NoMiso, sdin),
-            crate::ad5680::SPI_MODE,
-            crate::ad5680::SPI_CLOCK.into(),
-            clocks
+        let sclk = sclk.into_push_pull_output();
+        let sdin = sdin.into_push_pull_output();
+        let spi = SoftSpi::new(
+            sclk, sdin, DummyInputPin,
         );
         let sync = sync.into_push_pull_output();
 
@@ -214,14 +224,10 @@ impl Pins {
         clocks: Clocks, spi5: SPI5,
         sclk: PF7<M1>, sync: PF6<M2>, sdin: PF9<M3>
     ) -> (Dac1Spi, PF6<Output<PushPull>>) {
-        let sclk = sclk.into_alternate_af5();
-        let sdin = sdin.into_alternate_af5();
-        let spi = Spi::spi5(
-            spi5,
-            (sclk, NoMiso, sdin),
-            crate::ad5680::SPI_MODE,
-            crate::ad5680::SPI_CLOCK.into(),
-            clocks
+        let sclk = sclk.into_push_pull_output();
+        let sdin = sdin.into_push_pull_output();
+        let spi = SoftSpi::new(
+            sclk, sdin, DummyInputPin,
         );
         let sync = sync.into_push_pull_output();
 
diff --git a/src/softspi.rs b/src/softspi.rs
new file mode 100644
index 0000000..f1fc52c
--- /dev/null
+++ b/src/softspi.rs
@@ -0,0 +1,147 @@
+use stm32f4xx_hal::hal::{
+    spi::FullDuplex,
+    digital::v2::{InputPin, OutputPin},
+    blocking::spi::{Transfer, transfer},
+};
+use nb::{block, Error, Error::WouldBlock};
+use crate::timer::now;
+
+/// Bit-banged Mode3 SPI
+pub struct SoftSpi<SCK, MOSI, MISO> {
+    sck: SCK,
+    mosi: MOSI,
+    miso: MISO,
+    state: State,
+    input: Option<u8>,
+}
+
+#[derive(PartialEq)]
+enum State {
+    Idle,
+    Transfer {
+        clock_phase: bool,
+        mask: u8,
+        output: u8,
+        input: u8,
+    },
+}
+
+impl<SCK: OutputPin, MOSI: OutputPin, MISO: InputPin> SoftSpi<SCK, MOSI, MISO> {
+    pub fn new(mut sck: SCK, mut mosi: MOSI, miso: MISO) -> Self {
+        let _ = sck.set_high();
+        let _ = mosi.set_low();
+        SoftSpi {
+            sck, mosi, miso,
+            state: State::Idle,
+            input: None,
+        }
+    }
+
+    /// Call this at twice the data rate
+    pub fn tick(&mut self) {
+        match self.state {
+            State::Idle => {}
+            State::Transfer { clock_phase: false,
+                              mask, output, input } => {
+                if output & mask != 0 {
+                    let _ = self.mosi.set_high();
+                } else {
+                    let _ = self.mosi.set_low();
+                }
+                let _ = self.sck.set_low();
+
+                self.state = State::Transfer {
+                    clock_phase: true,
+                    mask, output, input,
+                };
+            }
+            State::Transfer { clock_phase: true,
+                              mask, output, mut input } => {
+                if self.miso.is_high().unwrap_or(false) {
+                    input |= mask;
+                }
+                let _ = self.sck.set_high();
+
+                if mask != 1 {
+                    self.state = State::Transfer {
+                        clock_phase: false,
+                        mask: mask >> 1,
+                        output, input,
+                    };
+                } else {
+                    self.input = Some(input);
+                    self.state = State::Idle;
+                }
+            }
+        }
+    }
+
+    pub fn run(&mut self) {
+        while self.state != State::Idle {
+            self.tick();
+            spi_delay();
+        }
+    }
+
+    fn retry<R, E, F>(&mut self, f: &F) -> Result<R, E>
+    where
+        F: Fn(&'_ mut SoftSpi<SCK, MOSI, MISO>) -> Result<R, nb::Error<E>>
+    {
+        loop {
+            match f(self) {
+                Ok(r) => return Ok(r),
+                Err(nb::Error::Other(e)) => return Err(e),
+                Err(WouldBlock) => self.run(),
+            }
+        }
+    }
+}
+
+impl<SCK: OutputPin, MOSI: OutputPin, MISO: InputPin> FullDuplex<u8> for SoftSpi<SCK, MOSI, MISO> {
+    type Error = ();
+
+    fn read(&mut self) -> Result<u8, nb::Error<Self::Error>> {
+        match self.input.take() {
+            Some(input) =>
+                Ok(input),
+            None if self.state == State::Idle =>
+                Err(nb::Error::Other(())),
+            None =>
+                Err(WouldBlock),
+        }
+    }
+
+    fn send(&mut self, output: u8) -> Result<(), nb::Error<Self::Error>> {
+        match self.state {
+            State::Idle => {
+                self.state = State::Transfer {
+                    clock_phase: false,
+                    mask: 0x80,
+                    output,
+                    input: 0,
+                };
+                Ok(())
+            }
+            _ => Err(WouldBlock)
+        }
+    }
+}
+
+impl<SCK: OutputPin, MOSI: OutputPin, MISO: InputPin> Transfer<u8> for SoftSpi<SCK, MOSI, MISO> {
+    // TODO: proper type
+    type Error = ();
+    fn transfer<'w>(&mut self, words: &'w mut [u8]) -> Result<&'w [u8], Self::Error> {
+        for b in words.iter_mut() {
+            self.retry(&|spi| spi.send(*b))?;
+            *b = self.retry(&|spi| spi.read())?;
+        }
+        Ok(words)
+    }
+}
+
+fn spi_delay() {
+    const DELAY: u32 = 1;
+
+    let start = now();
+    while now() - start < DELAY {}
+}