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mistBee

Overview

mistBeeTM is a XBee Pro plug-in compatible 802.15.4 mesh radio module which utilizes the mist® network technology to deliver superior RF performance and scalability while consuming less than 250 uA average current at 3.3V. The mist® network has demonstrated sensor networks with over 10,000 active units through a single mist mesh gateway. The mistBee™ module includes both a precision temperature sensor and 3-axis accelerometer as well as support for serial communications and an array of external sensors. The mistBee module was created as an OEM solution to allow 3rd party companies to easily evaluate and implement the mist network technology.

mistbee-1

mistBeeTM Features

The mistBeeTM module has a form-factor similar to the Digi XBee-Pro unit and fits into all of the currently available XBee adapter modules (shields). The mistBeeTM provides a serial port interface so that host devices can connect to the mistBee®. Hosts connected to the mistBee can communicate proprietary data between each other or can implement the mist protocol. By implementing the mist protocol, a host device can query any other mist device in the network. This allows a host to query the sensor values of a mist device or the pin states of any mistBee device in the network.

The mistBeeTM includes a precision temperature sensor and 3-axis accelerometer as well as twelve (12) user configurable GPIO pins, 5 of these pins can be configured as analog inputs. The mistBee® incorporates an analog to digital convertor (ADC) with 1 mV resolution.

Pin Assignments

Table 1 lists the pin assignments for the mistBeeTM module.

Pin Table:

Pin # Pin Function Direction Description
1 VCC 2.8V – 3.6V
2 Dout Output Serial Out
3 Din Input Serial In
4 GPIO Both Digital Input/Ouput
5 !RESET Input Module Reset
6 Not Associated/GPIO Both Not Associated LED drive or Digital Input/Output (20 mA)
7 I2C Data Both Reserved for I2C bus
8 GPIO/Analog Input Both Digital Input/Output or ADC input
9 GPIO Both Digital Input/Ouput
10 GND Ground
11 GPIO/Analog Input Both Digital Input/Output or ADC input
12 GPIO Both Digital Input/Ouput
13 Sensor_On/GPIO Both Module Status Indicator/GPIO
14 GPIO Both Digital Input/Ouput
15 Associate Both Associate LED drive
16 GPIO Both Digital Input/Ouput
17 GPIO/Analog Input Both Digital Input/Output or ADC input
18 GPIO/Analog Input Both Digital Input/Output or ADC input
19 I2C Clock Both Reserved for I2C bus
20 GPIO/Analog Input Both Digital Input/Output or ADC input

Table 1 Pin assignments for mistBeeTM module

 

The twelve user configurable GPIO pins can be set as digital outputs or inputs to read the state of external components. These GPIO pins can be configured as follows:

  • Digital Output low
  • Digital Output high
  • Digital Input floating, No Interrupt
  • Digital Input internal pull-up resistor applied, No Interrupt
  • Digital Input internal pull-down resistor applied, No Interrupt
  • Digital Input floating, Interrupt on falling edge
  • Digital Input internal pull-up resistor applied, Interrupt on falling edge
  • Digital Input internal pull-down resistor applied, Interrupt on falling edge
  • Digital Input floating, Interrupt on rising edge
  • Digital Input internal pull-up resistor applied, Interrupt on rising edge
  • Digital Input internal pull-down resistor applied, Interrupt on rising edge
  • Digital Input floating, Interrupt on both edges
  • Digital Input internal pull-up resistor applied, Interrupt on both edges
  • Digital Input internal pull-down resistor applied, Interrupt on both edges
  • Pin not used

All GPIO pins are based upon 3.3V logic levels. Assigning the interrupt function to a GPIO pin provides a mechanism for a pin state mistbee-2change to cause an immediate report of the state of all of the pins. This report is sent by the mistBeeTM to the Mesh Gateway. In a similar fashion, the 5 analog capable GPIO pins can be configured to trigger a report based upon a defined threshold or a change in value. Similarly, the temperature sensor, accelerometer and supply voltage (VCC) can be configured to provide alerts based upon a threshold value or a change (delta) in value.

Serial Data Lines

Pins 2 and 3 serve as the serial Dout and Din lines. The mistBeeTM uses interrupt processing on the Din line to ensure that the unit is awake in order to process serial data. Serial data support is based upon the mist® Host Protocol. The default baud rate for the mistBeeTM is 115,200 though it is configurable from 2400 to 460,800 baud.

Reset

To reset the mistBeeTM module, a low logic level applied to Pin 5 must be held low for at least 1 ms. This pin should be left in an open state for normal operation so as not to interfere with the voltage supervisory circuit on the mistBeeTM. This pin is complaint with many of the XBee shields that have a momentary contact switch connected to this pin

Associate LED

Pin 15 is used to drive an LED to indicate that the unit is associated with a network. In the default configuration, Pin 15 will be raised high for approximately 15 msec every 3 seconds. If desired, an LED with a dropping resistor (between 330 – 1K ohms) can be connected between this pin and ground. This pin is complaint with many of the XBee shields that have an Associate LED and dropping resistor connected to this pin.

Not Associated LED

Pin 6 can be used to drive an LED to indicate that the unit is not associated with a network. In the default configuration, Pin 6 will be raised high for approximately 15 msec every 3 seconds when the mistBee® is not associated with a network. If desired, an LED with a dropping resistor (between 330 – 1K ohms) can be connected between this pin and ground. This pin will drive the RSSI LED that is on many of the XBee shields, but it will not provide a measure of the RSSI. Note, since the mistBeeTM is designed for very low power operation, it does not support a Pulse Wave Modulation (PWM) output. If desired, this pin can be configured as a GPIO instead of providing the Not Associated indication.

I2C Lines

Pins 7 and 19 are reserved for the I2C bus that is used by the mistBeeTM to interface to the on board accelerometer and temperature sensor. These lines are exposed to provide for future support of external I2C sensors.

Sensor On

In the default configuration, pin 13 provides an indication or when the mistBee® unit is sampling the GPIO pins. Pin 13 is driven high for approximately 2 msec during the time in which the unit’s sensors and pins states are read. Pin 13 can be used to drive external devices that need to be powered to be sampled. As an example, pin 13 can be connected to the collector of a phototransistor to read the analog voltage at the emitter resistor follower junction. Alternative, pin 13 could be used to drive the gate of a N-channel MOSFET (such as 2N7002). This pin is complaint with many of the XBee shields that have a Sleep LED and dropping resistor connected to this pin. If desired, this pin can be configured as a GPIO instead of providing the Sensor On functionality.

System Reset

The mistBeeTM device can be reset to the default factory configuration should a configuration of the unit cause it to be non-functional. The user can cause the mistBeeTM to perform a System Reset by connecting pin 15 to pin 8 prior to powering the mistBee or forcing a reset by grounding pin 5. Pin 15 and pin 8 must remain connected for at least 3 seconds following a reset. Once the connection between the pins has been removed, the mistBeeTM will then perform the System Reset. Following the System Reset, the unit will boot up into the default factory configuration.