From 85a2491634f3836935be1227e43ed53058ad91b1 Mon Sep 17 00:00:00 2001
From: Abhijit Kshirsagar <kabhijit@iitdh.ac.in>
Date: Mon, 4 Sep 2023 17:49:23 +0530
Subject: [PATCH] Quick Aside to debug GPIO ISRs

Problem: It is seen that the GPIO ISR was getting called more than once
for every button press.

Diagnosis:

The last instruction in the GPIO Interrupt handler is a write to the ICR
register which clears the interrupt. However, this write takes multiple
cycles, depending on the caching involved. As a result, after the
instruction to write to ICR, the microcontroller exits from the ISR
function but the write has not yet completed. The controller sees this
as a pending interrupt and goes into the ISR again. However, by the time
the controller enters the ISR this second time the ICR write has indeed
happened, and it appears to be a spurious interrupt call.

If the code is run in single-step debug mode, it gives the code enough
time to write to the ICR register and therefore the ISR is not called a
second time.

Solution:

Wait for a few cycles *after* the ICR write instruction and before
exiting from the ISR. The official documentation recommends a DSB
instruction but that may not be enough to account for vendor-specific
write-caching. In such a case a simple write to a register location
should be enough.

Additional Info:
https://developer.arm.com/documentation/ka003795/latest/
---
 Blink/main.c           | 21 +++++++++++----------
 GPIO_Interrupts/main.c | 15 +++++++++------
 2 files changed, 20 insertions(+), 16 deletions(-)

diff --git a/Blink/main.c b/Blink/main.c
index 848362b..52c61e6 100644
--- a/Blink/main.c
+++ b/Blink/main.c
@@ -31,15 +31,15 @@ x   GPIODEN -- GPIO Digital Enable -- 0x00011111 >> 0x1F
     register uint32_t x = ZERO_DEF;
 
 
-    GPIO_PORTF_DATA_R = 0xE;
-    while(1)
-    {
-        GPIO_PORTF_DATA_R = 0x0;
-        GPIO_PORTF_DATA_R = 0x2;
-        GPIO_PORTF_DATA_R = 0x4;
-        GPIO_PORTF_DATA_R = 0x8;
-        GPIO_PORTF_DATA_R = 0xE;
-    }
+    GPIO_PORTF_DATA_R = 0x4;
+//    while(1)
+//    {
+//        GPIO_PORTF_DATA_R = 0x0;
+//        GPIO_PORTF_DATA_R = 0x2;
+//        GPIO_PORTF_DATA_R = 0x4;
+//        GPIO_PORTF_DATA_R = 0x8;
+//        GPIO_PORTF_DATA_R = 0xE;
+//    }
 
 
 
@@ -47,6 +47,7 @@ x   GPIODEN -- GPIO Digital Enable -- 0x00011111 >> 0x1F
     {
         x++;
         counter_loop();
+        GPIO_PORTF_DATA_R ^= 0x2;
     }
 }
 
@@ -60,7 +61,7 @@ void GPIO_PORTF_Init(void)
 
 void counter_loop (void)
 {
-    register uint32_t count=10;
+    uint32_t count = 0xFFFFFF;
     while(count>0)
         count--;
 
diff --git a/GPIO_Interrupts/main.c b/GPIO_Interrupts/main.c
index db97222..653bc6d 100644
--- a/GPIO_Interrupts/main.c
+++ b/GPIO_Interrupts/main.c
@@ -10,14 +10,12 @@ void SystickHandler(void);
 void Systick_Init(void);
 
 volatile uint32_t systick_count = 1;
-
+volatile uint32_t global_counter = 0;
 
 
 int main(void)
 {
 
-
-
     GPIO_PORTF_Init();
     //Systick_Init();
      while(1);
@@ -26,8 +24,13 @@ int main(void)
 
 void SW2_Handler(void)
 {
-    GPIO_PORTF_DATA_R ^= 0x8;
-    GPIO_PORTF_ICR_R = 0xff;
+    //Start with some nops and then try some dela
+
+    //GPIO_PORTF_DATA_R ^= 0x2;
+    global_counter ++;
+    //GPIO_PORTF_ICR_R = 0x10;
+    GPIO_PORTF_ICR_R = 0x10;
+    __asm(" DSB");
 }
 
 
@@ -76,7 +79,7 @@ void GPIO_PORTF_Init(void)
     //GPIOIEV -- Interropt Event  --> 0 for falling edge      --> 0x00
     GPIO_PORTF_IEV_R = 0x0;
 
-    //GPIOICR -- INterrpt clear -- must be cleared by writing 1 to corresponding bit --> 0xff
+    //GPIOICR -- Interript clear -- must be cleared by writing 1 to corresponding bit --> 0xff
     GPIO_PORTF_ICR_R = 0xff;
 
     //GPIOIM -- Interrupt Mask    --> 1 for unmasked (enabled)--> 0x01