fix(pcnt): fix the accum_value missing when overflow

2026-10-02 11:10:54 +03:00 · 2025-07-02 20:14:37 +08:00
parent e2c9605f72
commit ba0a08a1d0
4 changed files with 86 additions and 15 deletions
@@ -0,0 +1,39 @@
+# PCNT Driver Design
+
+## Concurrency
+
+The count value and the overflow state of the count value are located in *different* registers, resulting in the software being unable to obtain information from both of them in the same read instruction.
+
+The race condition case is as follow:
+
+```mermaid
+sequenceDiagram
+    participant HW as PCNT Hardware
+    participant CPU0_ISR as CPU0_ISR
+    participant CPU1_Task as CPU1_Task (pcnt_unit_get_count)
+    participant REG as Reg and Soft accum counter State
+
+    CPU1_Task->>CPU1_Task: Call pcnt_unit_get_count()
+    Note over REG: intr_status = 0<br/>cnt_reg = cnt_value<br/>accum_value = old_value
+    CPU1_Task->>CPU1_Task: portENTER_CRITICAL_SAFE()
+    CPU1_Task->>REG: Read intr_status
+    Note over CPU1_Task: intr_status=0, no need to do compensation
+    HW->>REG: Overflow interrupt triggered
+    Note over REG: intr_status = 1<br/>cnt_reg = 0<br/>accum_value = old_value
+    REG->>CPU0_ISR: ISR is called
+    CPU0_ISR->>CPU0_ISR: try portENTER_CRITICAL_SAFE() but spin
+
+    CPU1_Task->>REG: Read cnt_reg(0) + accum_value(old)
+    CPU1_Task->>CPU1_Task: portEXIT_CRITICAL_SAFE()
+
+    CPU0_ISR->>CPU0_ISR: portENTER_CRITICAL_SAFE()
+    CPU0_ISR->>REG: Clear interrupt status and update accum_value
+    Note over REG: intr_status = 0<br/>accum_value = new_value
+    CPU0_ISR->>CPU0_ISR: portEXIT_CRITICAL_SAFE()
+    
+    Note over CPU0_ISR: Process events
+    Note over CPU1_Task: Return incorrect count ❌
+
+```
+
+In the software, we determine whether to perform compensation by checking whether the count value exceeds half of the limit. This can prevent counting errors when the overflow frequency is not high.