Pmod AQS Reference Manual

The Digilent Pmod AQS (Revision A) is a digital gas sensor for monitoring indoor air quality at low power. With the AMS CCS811, you can detect a wide range of Volatile Organic Compounds (VOCs) using the micro-hotplate technology enables a highly reliable solution for gas sensors with very fast cycle times.


Features

  • Optimized low-power modes
  • Integrated MCU with ADC
  • The Total Volatile Organic Compound (TVOC) output range is from 0ppb to 1187ppb.
  • 10-bit measurement resolution
  • 6-pin Pmod connector with I²C interface
  • Pass-through Pmod host port for daisy chaining

Functional Description

The Pmod AQS is designed to digitally report the Volatile Organic Compounds (VOCs) upon request by the host board. Up to 10-bits of resolution collected by allowing for longer conversion times. Micro-hotplate technology which enables a highly reliable solution for gas sensors, very fast cycle times and a significant reduction in average power consumption.


Specifications

Parameter Min Typical Max Units
Power Supply Voltage 1.8 3.3 V
Serial Clock Frequency 10 100 400 kHz
Parameter Value Units
eCO2 output range 400-8192 ppm
TVOC output range 0-1187 ppb
Sleep Mode Current Draw at 1.8V 19 uA

Interfacing with the Pmod

The Pmod AQS communicates with the host board via the I²C protocol. By first sending the 7-bit I²C address of 1011011 (0x5B) and then a read/write bit (high/low logic level, respectively), followed by the register address of interest at a maximum clock frequency of 400 kHz, users can both configure and read from the Pmod AQS. Header J2 on the Pmod AQS passes through all of the signals present on the main Header J1 to allow for the daisy chaining of multiple I²C compatible modules.

Application Register Map

Application registers for the Pmod AQS

application_register_map.jpg

Quick data acquisition

Here is the series of commands to acquire relative humidity and temperature data from the Pmod AQs in pseudo I²C code.

  1. Power on the Pmod AQS and wait at least 15 ms.
  2. Call the device ID with a write bit
    I2CBegin(0x5B); //device ID 0x81 with a write (0) bit
  3. Wait to receive an ACK from the Slave Device
  4. Write the device address that you want to talk to
    I2CWrite(0x01); //address 0x01 corresponds to the Meas_mode
  5. Wait to receive an ACK from the Slave Device
  6. Delay at least 12.85 ms (6.35 ms for the Temperature Sensor and 6.50 ms for the Humidity Sensor)
  7. Read 4 bytes from the temperature and humidity registers (two 8 byte samples from each, MSB then LSB)
    I2CReadMultiple(4); //read four bytes, sending an ACK to the slave after each byte received and a NACK after the last byte
  8. Read up to 8 bytes from the Alg_result_data register (two 8 byte samples MSB then LSB for eCO2 and two 8 byte samples MSB then LSB for VOC)
    I2CReadMultiple(8)); //read up to 8 bytes, sending an ACK to the slave after each byte received and a NACK after the last byte

Pinout Table Diagram

Header J1 Header J2 Jumper Blocks
Pin Signal Description Pin Signal Description Jumper State Description
1 IO/INT IO or Interrupt 1 IO/INT IO or INterrupt JP1 Enable Left Set to RST
2 WAKE/RST WAKE or RESET 2 WAKE/RST WAKE or RESET JP1 Enable Right Set to WAKE
3 SCL Serial Clock 3 SCL Serial Clock JP2 Both Enabled 2.2 kΩ resistors enabled on the SDA and SCL lines
4 SDA Serial Data 4 SDA Serial Data JP2 Both Disabled 2.2 kΩ resistors disabled on the SDA and SCL lines
5 GND Power Supply Ground 5 GND Power Supply Ground JP3 Enabled IO/INT enabled
6 VCC Power Supply (3.3V/5V) 6 VCC Power Supply (3.3V/5V)

The PmodAQS Jumper setting when using the Demo's are JP1: WAKE Enabled, JP2: Both Enabled, JP3: Enabled.

The Pmod AQS is an ideal Pmod to use in long term Air quality application. As a very low power Pmod between measurements, long term data to measure VOC changes in an environment can easily be collected.

Any external power applied to the Pmod AQS must be within 1.8 V and 3.3 V to ensure that the on-board chips operate correctly; however, it is recommended that Pmod is operated at 3.3 V.

Timing Diagram

A sample diagram for writing to and reading from the Pmod AQS taken from the CCS811 datasheet are provided below:

Example writing to the Pmod AQS

write_register_pmodaqs.jpg

Example reading from the Pmod AQS

read_register_pmodaqs.jpg

Physical Dimensions

The pins on the pin header are spaced 100 mil apart. The PCB is 1.25 inches long on the sides parallel to the pins on the pin header and 0.8 inches long on the sides perpendicular to the pin header.

Additional Information

The schematics of the Pmod AQS are available here. Additional information about the VOCs Gas sensor including communication modes and specific timings of the chip can be found in the datasheet here.

Example code and more specific information on how to use the Pmod AQS can be found on its Resource Center.

If you have any questions or comments about the Pmod AQS, feel free to post them under the appropriate section (“Add-on Boards”) of the Digilent Forum.