From 1c58f41b091805772b8aa4f30170b6860a058765 Mon Sep 17 00:00:00 2001
From: David Antliff <david@pitchblende.co.nz>
Date: Sun, 6 Aug 2017 11:50:34 +1200
Subject: [PATCH] Sample temperature from multiple devices approximately once
 per second.

---
 main/ds18b20.c      | 16 ++++++++++------
 main/ds18b20.h      |  6 +++++-
 main/ds18b20_main.c | 23 ++++++++++++++++++-----
 3 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/main/ds18b20.c b/main/ds18b20.c
index f05fe04..b84a0ad 100644
--- a/main/ds18b20.c
+++ b/main/ds18b20.c
@@ -144,8 +144,9 @@ static bool _check_resolution(DS18B20_RESOLUTION resolution)
     return (resolution >= DS18B20_RESOLUTION_9_BIT) && (resolution <= DS18B20_RESOLUTION_12_BIT);
 }
 
-static void _wait_for_conversion(DS18B20_RESOLUTION resolution)
+static float _wait_for_conversion(DS18B20_RESOLUTION resolution)
 {
+    float elapsed_time = 0.0f;
     if (_check_resolution(resolution))
     {
         int divisor = 1 << (DS18B20_RESOLUTION_12_BIT - resolution);
@@ -156,7 +157,11 @@ static void _wait_for_conversion(DS18B20_RESOLUTION resolution)
 
         // wait at least this maximum conversion time
         vTaskDelay(ticks);
+
+        // TODO: measure elapsed time more accurately
+        elapsed_time = ticks * portTICK_PERIOD_MS;
     }
+    return elapsed_time;
 }
 
 static float _decode_temp(uint8_t lsb, uint8_t msb, DS18B20_RESOLUTION resolution)
@@ -385,17 +390,16 @@ void ds18b20_convert_all(const OneWireBus * bus)
     owb_reset(bus);
     owb_write_byte(bus, OWB_ROM_SKIP);
     owb_write_byte(bus, DS18B20_FUNCTION_TEMP_CONVERT);
-
-    // wait the maximum conversion duration
-    _wait_for_conversion(DS18B20_RESOLUTION_12_BIT);
 }
 
-void ds18b20_wait_for_conversion(const DS18B20_Info * ds18b20_info)
+float ds18b20_wait_for_conversion(const DS18B20_Info * ds18b20_info)
 {
+    float elapsed_time = 0.0f;
     if (_is_init(ds18b20_info))
     {
-        _wait_for_conversion(ds18b20_info->resolution);
+        elapsed_time = _wait_for_conversion(ds18b20_info->resolution);
     }
+    return elapsed_time;
 }
 
 float ds18b20_read_temp(const DS18B20_Info * ds18b20_info)
diff --git a/main/ds18b20.h b/main/ds18b20.h
index 76dd47a..e6f0163 100644
--- a/main/ds18b20.h
+++ b/main/ds18b20.h
@@ -143,6 +143,9 @@ bool ds18b20_convert(const DS18B20_Info * ds18b20_info);
 
 /**
  * @brief Start temperature conversion on all connected devices.
+ *
+ * This should be followed by a sufficient delay to ensure all devices complete
+ * their conversion before the measurements are read.
  * @param[in] bus Pointer to initialised bus instance.
  */
 void ds18b20_convert_all(const OneWireBus * bus);
@@ -150,8 +153,9 @@ void ds18b20_convert_all(const OneWireBus * bus);
 /**
  * @brief Wait for the maximum conversion time according to the current resolution of the device.
  * @param[in] bus Pointer to initialised bus instance.
+ * @return An estimate of the time elapsed, in milliseconds. Actual elapsed time may be greater.
  */
-void ds18b20_wait_for_conversion(const DS18B20_Info * ds18b20_info);
+float ds18b20_wait_for_conversion(const DS18B20_Info * ds18b20_info);
 
 /**
  * @brief Read last temperature measurement from device.
diff --git a/main/ds18b20_main.c b/main/ds18b20_main.c
index e8615e2..892e5ef 100644
--- a/main/ds18b20_main.c
+++ b/main/ds18b20_main.c
@@ -137,18 +137,31 @@ void app_main()
     {
         while (1)
         {
-            printf("\nTemperature readings (degrees C):\n");
+            TickType_t start_ticks = xTaskGetTickCount();
+
             ds18b20_convert_all(owb);
 
-            // we know all devices use the same resolution, so use the first device to determine the delay
+            // in this application all devices use the same resolution,
+            // so use the first device to determine the delay
             ds18b20_wait_for_conversion(devices[0]);
 
+            // read the results immediately after conversion otherwise it may fail
+            // (using printf before reading may take too long)
+            float temps[MAX_DEVICES] = { 0 };
+            for (int i = 0; i < num_devices; ++i)
+            {
+                temps[i] = ds18b20_read_temp(devices[i]);
+            }
+
+            // print results in a separate loop, after all have been read
+            printf("\nTemperature readings (degrees C):\n");
             for (int i = 0; i < num_devices; ++i)
             {
-                float temp = ds18b20_read_temp(devices[i]);
-                printf("  %d: %.3f\n", i, temp);
+                printf("  %d: %.3f\n", i, temps[i]);
             }
-            vTaskDelay(1000 / portTICK_PERIOD_MS);
+
+            // make up delay to approximately 1 second per measurement
+            vTaskDelay(1000 / portTICK_PERIOD_MS - (xTaskGetTickCount() - start_ticks));
         }
     }