From 767aa140862eb02f31e1706ea1567459b022fa64 Mon Sep 17 00:00:00 2001
From: edisondeng <edisondeng@gmail.com>
Date: Fri, 31 Jul 2026 15:52:50 +0800
Subject: [PATCH] fix(timer): compensate Timer0_GetTimestamp for pending TF0
 overflow

TH0/TL0 torn-read protection didn't cover the case where the hardware
counter already wrapped but Timer0_Isr hasn't run yet to bump
timer0_ovf_count, making the timestamp appear to jump backward by one
overflow period (~32.768ms). Read TF0 under a brief EA guard and treat
a pending-but-unserviced overflow as ovf+1 without clearing it.
---
 Drivers/timer.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Drivers/timer.c b/Drivers/timer.c
index 2efa5e2..b92a08a 100644
--- a/Drivers/timer.c
+++ b/Drivers/timer.c
@@ -35,6 +35,8 @@ u32 Timer0_GetTimestamp(void)
 {
     u16 ovf1, ovf2;
     u8 h1, h2, l;
+    bit tf, ea_bak;
+
     do {
         ovf1 = timer0_ovf_count;
         h1 = TH0;
@@ -42,6 +44,21 @@ u32 Timer0_GetTimestamp(void)
         h2 = TH0;
         ovf2 = timer0_ovf_count;
     } while (h1 != h2 || ovf1 != ovf2);
+
+    // 上面的撕裂读保护只处理了 TH0/TL0 两字节配对读取时的撕裂，但还有另一种更隐蔽的竞态：
+    // 硬件计数器已经物理溢出归零 (TH0/TL0 已经是溢出后的小数值)，但 Timer0_Isr 因为中断延迟
+    // 还没来得及把 timer0_ovf_count 加一，导致算出来的时间戳比真实值凭空小了一整个溢出周期
+    // (65536 tick ≈ 32.768ms)，看起来像是"时间倒流"。这里读一次硬件溢出标志位 TF0 做补偿：
+    // 若 TF0 已经置位但还未被服务，本次读数临时按 ovf+1 计算，不清 TF0，留给真正的
+    // Timer0_Isr 之后正常执行、更新持久计数，不影响后续调用。
+    ea_bak = EA;
+    EA = 0;
+    tf = TF0;
+    EA = ea_bak;
+    if (tf) {
+        ovf1++;
+    }
+
     return ((u32)ovf1 << 16) | ((u16)h1 << 8) | l;
 }