mhz19_internals.h File Reference

Internal addresses, registers, constants for the MH-Z19 CO2 sensor. More...

Detailed Description

Internal addresses, registers, constants for the MH-Z19 CO2 sensor.

Author

Koen Zandberg koen@.nosp@m.berg.nosp@m.zand..nosp@m.net

Definition in file mhz19_internals.h.

Go to the source code of this file.

MH-Z19 Baud rate
Fixed at 9600 by design.
#define	MHZ19_UART_BAUDRATE   9600

MH-Z19 Buffer size
A transmission from the MH-Z19 is 9 bytes long: 1 Start byte 1 Sensor number 2 bytes data 4 bytes padding 1 byte checksum The start byte is not stored because it is not used in the checksum calculation.
#define	MHZ19_BUF_SIZE   8

MH-Z19 Timeout in milliseconds
20 ms gives a decent margin on top of the UART transmission time. The datasheet does not specify any timings beside the UART baud rate. A single byte takes 10 bits effectively: a start bit, 8 bits data and stop bit. 9 bytes are transmitted, thus 10 bits * 9 / 9600bps = 9.3 ms.
#define	MHZ19_TIMEOUT_READ   20
#define	MHZ19_TIMEOUT_CMD   (MHZ19_TIMEOUT_READ * 5)

MH-Z19 transmission constants
#define	MHZ19_READ_START   0xff
	Start bytes.
#define	MHZ19_READ_SENSOR_NUM   0x01
	Sensor number.

MH-Z19 commands
#define	MHZ19_CMD_AUTO_CALIBRATION   0x79
	Auto calibration command.
#define	MHZ19_CMD_GAS_CONCENTRATION   0x86
	Gas concentration command.
#define	MHZ19_CMD_CALIBRATE_ZERO   0x87
	Zero calibration command.
#define	MHZ19_CMD_CALIBRATE_SPAN   0x88
	Span calibration command.

MH-Z19 transmission data positions
#define	MHZ19_RX_POS_PPM_HIGH   1
	Measurement high byte.
#define	MHZ19_RX_POS_PPM_LOW   2
	Measurement low byte.
#define	MHZ19_RX_POS_CHECKSUM   7
	Checksum position.