Solar panel temperature controller
----------------------------------

4 x DS1820 type sensors
4 x PT1000 type sensors
4 x RELAYS


**** PCB IS NOT UP TO DATE ! Relay footprint is wrong (argh). ****

Communication to a controlling device (e.g. raspberry pi) is done via
I2C, default address 0x32.

Reading 144 bytes from the device will return a buffer with the
following structure (LSB):


struct ds1820 {
	uint8_t pin;
	uint8_t state;
	uint8_t data[9];
	uint8_t pad;
	uint32_t read_count;
} __attribute__ ((packed));

struct pt1000 {
	uint8_t amux;
	uint8_t converting;
	uint16_t raw[5];
	uint32_t count;
} __attribute__ ((packed));

struct i2c_registers {
	struct ds1820 ds1820[4];
	struct pt1000 pt1000[4];
	uint8_t  relays;
	uint8_t  pad[5];
	uint32_t twi_timestamp;
	uint32_t timestamp;
	uint16_t marker;
} __attribute__ ((packed));


ds1820.data contains the DS18S20 scratchpad. ds1820.read_count can be
used to check if the sensor is responding properly.

pt1000.raw[5] are the last 5 ADC values, suggest you average them.

i2c_registers.relays bits 0..3 are the relays 1..4

i2c_registers.timestamp are roughly seconds since reset.

i2c_registers.marker is 0xAA55, a very coarse check for bit errors on
I2C.

To set the relays, write 2 bytes on I2C: first byte is the command
(must be == 1) and second byte is the relays value (bits 0..3).

If no I2C communication is done for more than ~1 minute, the relays
are reset (safe-mode).