Include first driver: SHT31

Make I2C reads blocking w/ timeout
Add SHT31 driver, roughly :)
Fix mg_time() to return a double.

src/mgos_sht31.c

@@ -0,0 +1,132 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "mgos.h"
+#include "mgos_sht31_internal.h"
+#include "mgos_i2c.h"
+
+// Private functions follow
+static bool mgos_sht31_cmd(struct mgos_sht31 *sensor, uint16_t cmd) {
+  uint8_t data[2];
+
+  if (!sensor || !sensor->i2c)
+    return false;
+
+  data[0]=cmd>>8;
+  data[1]=cmd&0xFF;
+  if (!mgos_i2c_write(sensor->i2c, sensor->i2caddr, data, 2, true)) {
+    LOG(LL_ERROR, ("I2C=0x%02x cmd=%u (0x%04x) write error", sensor->i2caddr, cmd, cmd));
+    return false;
+  }
+  LOG(LL_DEBUG, ("I2C=0x%02x cmd=%u (0x%04x) write success", sensor->i2caddr, cmd, cmd));
+
+  return true;
+}
+
+static uint8_t crc8(const uint8_t *data, int len)
+{
+  const uint8_t POLYNOMIAL=0x31;
+  uint8_t crc=0xFF;
+
+  for (int j=len; j; --j ) {
+    crc ^= *data++;
+    for ( int i = 8; i; --i )
+      crc = ( crc & 0x80 ) ? (crc << 1) ^ POLYNOMIAL : (crc << 1);
+  }
+  return crc;
+}
+// Private functions end
+
+// Public functions follow
+struct mgos_sht31 *mgos_sht31_create(struct mgos_i2c *i2c, uint8_t i2caddr) {
+  struct mgos_sht31 *sensor;
+
+  if (!i2c) return NULL;
+
+  sensor=calloc(1, sizeof(struct mgos_sht31));
+  if (!sensor) return NULL;
+  sensor->i2caddr=i2caddr;
+  sensor->i2c=i2c;
+  sensor->last_read_time=0;
+
+  mgos_sht31_cmd(sensor, MGOS_SHT31_SOFTRESET);
+  return sensor;
+}
+
+void mgos_sht31_destroy(struct mgos_sht31 **sensor) {
+  if (!*sensor) return;
+  free (*sensor);
+  *sensor=NULL;
+  return;
+}
+
+bool mgos_sht31_read(struct mgos_sht31 *sensor) {
+  double now = mg_time();
+
+  if (!sensor || !sensor->i2c)
+    return false;
+
+  if (now - sensor->last_read_time < MGOS_SHT31_READ_DELAY) {
+    return true;
+  }
+  // Read out sensor data here 
+  //
+  uint8_t data[6];
+  float humidity, temperature;
+
+  mgos_sht31_cmd(sensor, MGOS_SHT31_MEAS_HIGHREP);
+
+  mgos_usleep(500);
+
+  if (!mgos_i2c_read(sensor->i2c, sensor->i2caddr, data, 6, true))
+    return false;
+
+  // Check CRC8 checksums
+  if ((data[2]!=crc8(data, 2)) || (data[5]!=crc8(data+3, 2)))
+    return false;
+
+  temperature = data[0]*256 + data[1];
+  temperature *= 175;
+  temperature /= 0xffff;
+  temperature -= 45;
+  sensor->temperature = temperature;
+
+  humidity = data[3] * 256 + data[4];
+  humidity *= 100;
+  humidity /= 0xFFFF;
+  sensor->humidity = humidity;
+
+  LOG(LL_DEBUG, ("temperature=%.2fC humidity=%.1f%%", sensor->temperature, sensor->humidity));
+  sensor->last_read_time=now;
+  return true;
+}
+
+float mgos_sht31_getTemperature(struct mgos_sht31 *sensor) {
+  if (!mgos_sht31_read(sensor)) return NAN;
+
+  return sensor->temperature;
+}
+
+float mgos_sht31_getHumidity(struct mgos_sht31 *sensor) {
+  if (!mgos_sht31_read(sensor)) return NAN;
+
+  return sensor->humidity;
+}
+
+bool mgos_sht31_i2c_init(void) {
+  return true;
+}
+// Public functions end