SENSORS and CONTROL BOARD

Reference Documents:

• Requirements
• Schematic
• Layout: Actual Size , Top , Bottom , Traces Only .
• Signal List
• Parts List

Introduction

• CPU
• Display and Basic I/O
• Sensor and Control Board (Described here)
• Remote Sensor and Control

The Sensor and Control Board (SENSOR) section is one of the daughter boards of the Modular Embedded Experimenter's PIC System (MEEPS). The SENSOR section is designed to connect to the expansion connector on the CPU section. The SENSOR section is intended as a testbed for the Remote Sensor and Control Section. The SENSOR section should be easier to work with because it uses through hole parts (for most devices) and is connected to a more capable microcontroller than the one on the Remote Sensor and Control Section.

The remainder of this document describes the subsections of the SENSOR, configuration options, and variations from the requirements.

Circuit Description

Connectors

• J301 is a 40 pin connector designed to mate with the expansion connector on the CPU. The signals match those on the CPU and are also documented in Signal List file.
• J303 is a three pin connector for the 1-Wire bus. The signals on this connector are:
  1. Vcc
  2. 1-Wire I/O
  3. GND
• J310 provides access to the Parallel Slave Port module in the 18F452 (Ports D and E). The signals are arranged as follows:
  1. RE0/RD\
  2. RD7
  3. RE1/WR\
  4. RD6
  5. RE2/CS\
  6. RD5
  7. RD0
  8. RD4
  9. RD1
  10. RD3
  11. GND
  12. RD2
• J311 has the high current digital output signals.
  1. Q301 Drain
  2. GND
  3. Q302 Drain
  4. GND
  5. Q303 Drain
  6. GND
  7. Q304 Drain
  8. GND

Sensors

There can be as many as four sensors in the SENSOR section. U306 is a dual sensor for temperature and humidity. U301 is a dual axis accelerometer and U310 can be a temperature sensor, time keeper or external memory.

The SHT71 is a relative humidity and temperature sensor with a synchronous serial interface. The serial interface on the SHT71 is not an SPI or I2C interface. Even so, the clock signal SCK, U306-1, is connected to the SPI clock pin RC3 and the data signal, U306-4 DATA, is connected to the SPI data input pin RC4. The DATA signal is bi-directional, so RC4 will have to be switched from input to output as necessary to implement the STH71 communications protocol. Resistor R306 pulls the open drain signal from the SHT71 high when pin RC4 is configured as an input.

U310 is a 3-pin TO-92 1-Wire device such as the DS1822 or DS1821 temperature sensors. Some devices, such as the DS1822 but not the DS1821, can be used in a multi-drop 1-Wire bus. Connector J303 provides for off board expansion of the 1-Wire bus. The pinout for J303 is identical to the pinout for J202 on the Display and Basic I/O board.

U306 is the ADXL202E dual-axis accelerometer. This device will measure accelerations up to 2G. The acceleration measurements are reported by pulse width modulation (PWM). The PWM signals are connected to the Capture and Compare input signals (RC1 and RC2). The PWM period and the measurement bandwidth for U306 are set by R307, C301 and C302. With the values listed on the schematic (1.25 mega-ohms, and 0.47uf) the period and bandwidth are set for 10Hz operation.

Power for U301 is provided by setting signal RA5 high. This provides programmable control of the power to the ADXL202E. The data sheet for the ADXL202E provides details on how it can be operated in a power cycled mode to reduce the average power requirements.

Digital Outputs

High Power

Four (4) high current, open drain, digital outputs are provided on connector J311. The outputs are driven by N-channel MOSFET transistors; Q301-Q304. Each output is rated for 30 volts, output to ground, and 62 amps (limited by power dissipation).

Resistors R301-R304 are included to allow bipolar transistors to be substituted for the MOSFET transistors. When MOSFET transistors are installed, R301-R304 should be zero ohm devices (jumpers) as noted on the schematic.

Parallel Slave Port

Connector J310 provides access to the parallel slave port of the MEEPS CPU. The parallel slave port can be used to create an 8-bit bidirectional interface to a remote computer. Details on configuring and using the Parallel Slave port are included in the PIC data

Warning: Signals RD4 and RD5 are also used as the secondary serial channel on the SENSOR board. This operation is mutually exclusive with the Parallel Slave Port operation. Jumpers between pins 1 and 2 of J305 and J304 and between pins 2 and 3 of J306 and J307 should not be installed if the Parallel Slave Port is to be used.

The pins on J310 can also be used as individual I/O signals.

Wireless Communications

Two wireless communications channels are available on the SENSOR module: radio frequency (RF) and infrared data signaling (IRDA). Jumpers J304-J307 allow either channel to be connected to the UART signals from the MEEPS CPU. A secondary channel can be implemented using RD4 and RD5 combined with suitable bit-banging software. Use of this secondary channel would make it possible to relay RF and IRDA signals using the SENSOR module.

RF

There are two RF receivers and two RF transmitters that can be installed on the SENSOR module. They are not independent and only one receiver and one transmitter should be installed at one time. U307 and U303 are a TX and RX pair from the same manufacturer. Likewise U308 and U304 are a pair from another manufacturer. Both pairs operate on the same frequency, 433MHz. The U308/U304 pair have a higher bandwidth but are more expensive than the U307/U303 pair.

U307 is limited to 5v operation. If the SENSOR module is to be operated from a 3.3 volt supply, or batteries, U308 should be used rather than U307.

U308 and U304 have antenna connections on the radio module. Antenna connections for U307 and U303 are provided on the SENSOR module (J309 and J308 respectively).

The power supplies for U308 and U304 are filtered by R312, R311, R313, C307, and C305. These components may be omitted (replace the resistors with jumpers) if Vcc is provided by a battery or another suitably low noise supply.

Resistor R309 limits the transmitted power from U304. The 430 ohm value listed on the schematic is suitable for 5v operation but may need to be change to comply with FCC regulations. R309 may be omitted for 3.3 volt operation, but the transmitter will still work (at a lower power) if R309 is left in the circuit when running at 3.3 volts.

IRDA

The IRDA section of the SENSOR module also has two mutually exclusive configurations. U309 implements several of the IRDA standards and is a relatively complete IRDA implementation. U305 is a much simpler interface that only implements the IRDA modulation. It may also be possible to establish a IR communications connection by connecting some of the MEEPS CPU signals directly to the ZHX1820 LED and IR sensor.

U302 is a LED, IR sensor combination designed for IRDA. It can be used over distances as great as 3 feet and at speeds up to 115.2K Baud.

U309 provides communications at a range of rates from 9600 to 115.2K Baud. The internal rate serial speed to the CPU is independent of the IRDA rate. The IRDA rate will be negotiated with the remote device by U309. To avoid dropped date it is important that the flow control signals (RTS, CTS, DTR and DSR) be monitored and driven by the MEEPS CPU. The MEEPS CPU can also set the internal serial rate using RC0 and RC5 and enable or disable IRDA communications using RB4.

U305 is limited to a single communications rate, 115.2K baud. U305 does not provide any internal buffering and the internal serial data from the MEEPS CPU must be the same as the IRDA rate (115.2K baud). U311 provides a 16x clock used by U305 to produce and decode the modulated IRDA signal.

Using U305 it may be possible to create simpler IR communication links that do not use the full IRDA stack. However, such communications links may not be able to communicate with IRDA standard devices such as the Palm computer.

Configuration

This board provides a wide range of configurations. Some of the possibilities include:
• Sensors connected to a computer using a parallel interface.
• An IRDA or RF interface for a computer using ta parallel interface.
• An IRDA to RF buffer to allow extended communications.
• A temperature and humidity monitoring device with stored measurements retrieved by IRDA or RF. (Use a 1-wire memory device for additional non-volatile memory.)
• A remote sensor that periodically broadcasts the sensor values over RF (or for very short distances using IR in a non-IRDA compliant connection).
A stabilizer that uses the accelerometer to measure tilt and the high powered digital outputs to drive actuators to keep a platform level.

Variation from Requirements

The only requirement that was not met, wat the optional requirement for a prototyping area.



$Date: 2008-10-21 19:17:34 $ $Revision: 1.4 $ $Source: /home/cvsroot/MEEPSTop/hardware/sensor.html,v $

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