atsamd: Add i2c_read() implementation

Signed-off-by: Simon Kühling <mail@simonkuehling.de>

src/atsamd/i2c.c

@@ -102,5 +102,47 @@ void
 i2c_read(struct i2c_config config, uint8_t reg_len, uint8_t *reg
          , uint8_t read_len, uint8_t *read)
 {
-    shutdown("i2c_read not supported on samd21");
+    SercomI2cm *si = (SercomI2cm *)config.si;
+
+    // start in write mode and write register if provided
+    if(reg_len) {
+        // start in write mode
+        si->ADDR.reg = config.addr;
+        while (!(si->INTFLAG.reg & SERCOM_I2CM_INTFLAG_MB));
+
+        // write registers
+        while (reg_len--){
+            si->DATA.reg = *reg++;
+            while (!(si->INTFLAG.reg & SERCOM_I2CM_INTFLAG_MB));
+        }
+    }
+
+    // start with read bit enabled
+    si->ADDR.reg = (config.addr | 0x1);
+
+    // read bytes from slave
+    while (read_len--){
+        while (!(si->INTFLAG.reg & SERCOM_I2CM_INTFLAG_SB));
+
+        if (read_len){
+            // set ACK response
+            si->CTRLB.reg &= ~SERCOM_I2CM_CTRLB_ACKACT;
+            while (si->SYNCBUSY.bit.SYSOP);
+
+            // execute ACK succeded by byte read
+            si->CTRLB.reg |= SERCOM_I2CM_CTRLB_CMD(2);
+            while (si->SYNCBUSY.bit.SYSOP);
+        } else {
+            // set NACK response
+            si->CTRLB.reg |= SERCOM_I2CM_CTRLB_ACKACT;
+            while (si->SYNCBUSY.bit.SYSOP);
+
+            // execute NACK succeded by stop condition
+            si->CTRLB.reg |= SERCOM_I2CM_CTRLB_CMD(3);
+            while (si->SYNCBUSY.bit.SYSOP);
+        }
+
+        // read received data byte
+        *read++ = si->DATA.reg;
+    }
 }