From fd3f33880b42bb3901acbee83e7cb9b03598e0bd Mon Sep 17 00:00:00 2001
From: Hyungsin <hskim@berkeley.edu>
Date: Wed, 11 Oct 2017 18:17:04 -0700
Subject: [PATCH] drivers/hdc1000: add caching capability for hdc1000 driver

---
 drivers/hdc1000/hdc1000.c      | 36 ++++++++++++++++++++++++++++++++++
 drivers/hdc1000/hdc1000_saul.c |  4 ++--
 drivers/include/hdc1000.h      | 16 +++++++++++++++
 3 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/drivers/hdc1000/hdc1000.c b/drivers/hdc1000/hdc1000.c
index eddd894b88..40eae9466e 100644
--- a/drivers/hdc1000/hdc1000.c
+++ b/drivers/hdc1000/hdc1000.c
@@ -33,6 +33,13 @@
 
 #define I2C_SPEED                  I2C_SPEED_FAST
 
+#ifndef HDC1000_RENEW_INTERVAL
+#define HDC1000_RENEW_INTERVAL     (1000000ul)
+#endif
+
+static int16_t temp_cached, hum_cached;
+static uint32_t last_read_time;
+
 int hdc1000_init(hdc1000_t *dev, const hdc1000_params_t *params)
 {
     uint8_t reg[2];
@@ -72,6 +79,12 @@ int hdc1000_init(hdc1000_t *dev, const hdc1000_params_t *params)
     }
     i2c_release(dev->p.i2c);
 
+    /* initial read for caching operation */
+    if (hdc1000_read(dev, &temp_cached, &hum_cached) != HDC1000_OK) {
+        return HDC1000_BUSERR;
+    }
+    last_read_time = xtimer_now_usec();
+
     /* all set */
     return HDC1000_OK;
 }
@@ -132,3 +145,26 @@ int hdc1000_read(const hdc1000_t *dev, int16_t *temp, int16_t *hum)
     xtimer_usleep(HDC1000_CONVERSION_TIME);
     return hdc1000_get_results(dev, temp, hum);
 }
+
+
+int hdc1000_read_cached(const hdc1000_t *dev, int16_t *temp, int16_t *hum)
+{
+    uint32_t now = xtimer_now_usec();
+
+    /* check if readings are outdated */
+    if (now - last_read_time > HDC1000_RENEW_INTERVAL) {
+        /* update last_read_time */
+        if (hdc1000_read(dev, &temp_cached, &hum_cached) != HDC1000_OK) {
+            return HDC1000_BUSERR;
+        }
+        last_read_time = now;
+    }
+
+    if (temp) {
+        *temp = temp_cached;
+    }
+    if (hum) {
+        *hum = hum_cached;
+    }
+    return HDC1000_OK;
+}
diff --git a/drivers/hdc1000/hdc1000_saul.c b/drivers/hdc1000/hdc1000_saul.c
index 7971686fec..dca43277a9 100644
--- a/drivers/hdc1000/hdc1000_saul.c
+++ b/drivers/hdc1000/hdc1000_saul.c
@@ -25,7 +25,7 @@
 
 static int read_temp(const void *dev, phydat_t *res)
 {
-    if (hdc1000_read((const hdc1000_t *)dev, &(res->val[0]), NULL) != HDC1000_OK) {
+    if (hdc1000_read_cached((const hdc1000_t *)dev, &(res->val[0]), NULL) != HDC1000_OK) {
         return -ECANCELED;
     }
     memset(&(res->val[1]), 0, 2 * sizeof(int16_t));
@@ -37,7 +37,7 @@ static int read_temp(const void *dev, phydat_t *res)
 
 static int read_hum(const void *dev, phydat_t *res)
 {
-    if (hdc1000_read((const hdc1000_t *)dev, NULL, &(res->val[0])) != HDC1000_OK) {
+    if (hdc1000_read_cached((const hdc1000_t *)dev, NULL, &(res->val[0])) != HDC1000_OK) {
         return -ECANCELED;
     }
     memset(&(res->val[1]), 0, 2 * sizeof(int16_t));
diff --git a/drivers/include/hdc1000.h b/drivers/include/hdc1000.h
index 58addd354c..32644c14de 100644
--- a/drivers/include/hdc1000.h
+++ b/drivers/include/hdc1000.h
@@ -153,6 +153,22 @@ int hdc1000_get_results(const hdc1000_t *dev, int16_t *temp, int16_t *hum);
  */
 int hdc1000_read(const hdc1000_t *dev, int16_t *temp, int16_t *hum);
 
+/**
+ * @brief   Extended read function including caching capability
+ *
+ * This function will return cached values if they are within the sampling
+ * period (HDC1000_RENEW_INTERVAL), or will trigger a new conversion, wait for
+ * the conversion to be finished and the get the results from the device.
+ *
+ * @param[in]  dev          device descriptor of sensor
+ * @param[out] temp         temperature [in 100 * degree centigrade]
+ * @param[out] hum          humidity [in 100 * percent relative]
+ *
+ * @return                  HDC1000_OK on success
+ * @return                  HDC1000_BUSERR on I2C communication failures
+ */
+int hdc1000_read_cached(const hdc1000_t *dev, int16_t *temp, int16_t *hum);
+
 #ifdef __cplusplus
 }
 #endif