From cb32e3abfe6300fb48348aa2aa1029ae556d79d0 Mon Sep 17 00:00:00 2001
From: "Earle F. Philhower, III" <earlephilhower@yahoo.com>
Date: Sat, 20 Sep 2025 10:59:43 -0700
Subject: [PATCH] Fix SerialPIO::available during wraparound

Fixes #3142
---
 cores/rp2040/SerialPIO.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cores/rp2040/SerialPIO.cpp b/cores/rp2040/SerialPIO.cpp
index 32af295ac..ddd9b0a7f 100644
--- a/cores/rp2040/SerialPIO.cpp
+++ b/cores/rp2040/SerialPIO.cpp
@@ -338,7 +338,7 @@ int SerialPIO::available() {
     if (!_running || !m || (_rx == NOPIN)) {
         return 0;
     }
-    return (_writer - _reader) % _fifoSize;
+    return (_fifoSize + _writer - _reader) % _fifoSize;
 }
 
 int SerialPIO::availableForWrite() {