[GitHub] utzig commented on a change in pull request #1299: [RFC] [NRF52] Remove uint64_t div in watchdog init

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utzig commented on a change in pull request #1299: [RFC] [NRF52] Remove uint64_t div in watchdog init
URL: https://github.com/apache/mynewt-core/pull/1299#discussion_r205836933
 
 

 ##########
 File path: hw/mcu/nordic/nrf52xxx/src/hal_watchdog.c
 ##########
 @@ -44,13 +44,20 @@ nrf52_wdt_irq_handler(void)
 int
 hal_watchdog_init(uint32_t expire_msecs)
 {
-    uint64_t expiration;
-
     NRF_WDT->CONFIG = WDT_CONFIG_SLEEP_Msk;
 
-    /* Convert msec timeout to counts of a 32768 crystal */
-    expiration = ((uint64_t)expire_msecs * 32768) / 1000;
-    NRF_WDT->CRV = (uint32_t)expiration;
+    if (expire_msecs >= 1048576) {
+        /* watchdog with more than 17 minutes? */
+        assert(0);
+    } else {
+        uint32_t e = expire_msecs, rtc = 32768, d = 1000;
+        while (e >= 131072) {
+            e /= 2;
+            rtc /= 2;
+            d /= 2;
 
 Review comment:
   Cool!
   
   ```
   #include <stdint.h>
   #include <stdio.h>
   
   int main(void)
   {
       uint64_t r;
       uint32_t s;
   
       for (uint32_t u = 0; u < 0xffffffff; u++) {
           r = ((uint64_t)u * 32768) / 1000;
           s = (u * 32) + ((u * 96) / 125);
   
           if (r != s) {
               printf("u=%u, r=%lu, s=%u\n", u, r, s);
               break;
           }
       }
   
       return 0;
   }
   ```
   
   ```
   $ ./test
   u=44739243, r=1466015514, s=1431655776
   ```
   
   I guess you just made it work upto 12 hours, you're a genius @wes3!

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