Allow configuring whether to care about the CTS line

CHANGELOG.md

@@ -4,6 +4,11 @@
 
 - **Breaking:** Changed the return type of `Uart16550::config(&self)` from
   `(&Config, &B::Address)` to `(&Config, &B)`
+- **Breaking:** `ready_to_send()` (and `send_bytes()` in turn) doesn't check
+  `MSR::CTS` anymore by default, as modern hardware tends to leave that pin
+  disconnected. This behavior is configurable through `Config::flow_control`.
+  Additionally, for manual checks, users can check
+  `if device.msr().contains(MSR::CTS) {}`.
 
 ## 0.6.0 - 2026-03-28
 

src/config.rs

@@ -151,6 +151,12 @@ pub struct Config {
     pub extra_stop_bits: bool,
     /// Whether parity bits should be used.
     pub parity: Parity,
+    /// Whether to wait for CTS before sending.
+    ///
+    /// Only activate this if your hardware connects the CTS/RTS flow control
+    /// signals and you wish to make use of them. Keep this setting disabled to
+    /// make sure that the UART works when CTS is left disconnected.
+    pub flow_control: bool,
 }
 
 impl Config {
@@ -176,6 +182,7 @@ impl Config {
         data_bits: WordLength::EightBits,
         extra_stop_bits: false,
         parity: Parity::Disabled,
+        flow_control: false,
     };
 }
 

src/lib.rs

@@ -668,8 +668,6 @@ impl<B: Backend> Uart16550<B> {
     /// an established connection.
     pub fn ready_to_send(&mut self) -> Result<(), ByteSendError> {
         let lsr = self.lsr();
-        let msr = self.msr();
-        let mcr = self.mcr();
 
         // In FIFO mode, this bit is set when the transmitter’s FIFO is
         // completely empty, being 0 if there is at least one byte in the
@@ -681,8 +679,16 @@ impl<B: Backend> Uart16550<B> {
 
         // Software flow control. TODO, what to do with hardware flow control?
         // Is this something we can and should support?
-        if !mcr.contains(MCR::LOOP_BACK) && !msr.contains(MSR::CTS) {
-            return Err(ByteSendError::RemoteNotClearToSend);
+        if self.config.flow_control {
+            // The CTS line is meaningless when in loopback mode.
+            let mcr = self.mcr();
+            if !mcr.contains(MCR::LOOP_BACK) {
+                let msr = self.msr();
+
+                if !msr.contains(MSR::CTS) {
+                    return Err(ByteSendError::RemoteNotClearToSend);
+                }
+            }
         }
 
         Ok(())