Hardware

Mbed board

The purpose of the interface board is to give the user a common set of interfaces to the MBED board, produced by MBED a project was initiated by two ARM employees, and later became an official research project within ARM. It is now run and maintained by ARM to help MCU Partners provide their customers with the best way to prototype designs using their micro controllers. They can be contacted via support@mbed.org, or there web site.

The first diagram shows the primary elements of the interface board, and the connections to the MBED board. The MBED is installed in two dual row sockets, so the user can test signals on any pin of the MBED board, some of these pins do not appear on the external connection at the bottom of the interface board.

Starting at the lower left hand corner of the diagram we see the power input to the interface board, the board should be run at 5 volts only, as although the MBED board can be run at upto 9 volts, there is a lot of interface devices on the board that will not handle more than 5 volts. Basic protection is available for reverse polarity, and over current. This diagram also shows how the USB and Ethernet is connected to the MBED boards dedicated connections. interfacing for the CAN bus and RS232 interface are via driver chips (IC3 and IC4 respectively). A useful note here is the inclusion of an SN75240, this is a USB snubber, added to the board design after the first production run. dependent upon how the board is driven, the RS232 interface can be disconnected, with the removal of the jumpers in JP5, also there is the option of terminating the CAN bus with JP2, this should only be needed on the devices at each end of the CAN bus.

Other interfaces here are the Micro SD card slot, and the GPIO interface on the I2C bus, this GPIO interface is hard coded to address 0x40 on the bus. Con1 and Con2 provide the analog in and out, using JP11 and JP12 these can be configured as either DC or AC coupled.

Keyboard and seven segment display

Here we can see how the keyboard is connected up, using a PCF8574(IC5) one nibble is setup as output, with the other setup as input, and scanning the rows and columns, the default address on the I2C for this device is 0x42, but can be set to anything it the address range of the PCF8574 by setting the addres on the jumpers of JP13.

IC6 SAA1064 is a four digit display controllers, added to the board to as a clock type display. The inner two digits of the display can have there decimal points re-routed to the two LED's between the display, by changing the position of the jumpers in JP4.

IC7 24AA256 EEPROM, the device is on the board to let the user save data over a re-start, power off etc, also we can save state information in the device for the users to use, things like CAN bus address, IP address etc. This device can be write protected by changing the state of JP8. JP7 provides the I2C bus termination, these jumpers may be removed, if the I2C bus is extended by connecting something to the I2CBUS connector.

Primary Board Notes

The final sheet of the schematic is just some basic notes for the board layout, showing the position of the jumpers on the main board, the order of the off board connections, and where the connections for external I/O lines, CAN, Network, RS232 and USB.

Also shown here are the layouts for the seven segment displays, which segment maps to which data bit, and what the order of the displays, in the data word of the SAA1064 device.

Finally there is a list of the primary I2C addresses for the on-board I2C devices.

Mbed board layout

Board layout, showing the component positions, the board was designed using the Eagle CAD system, and the boards designes can be found here.


pcc@cs.york.ac.uk
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