BoB2 is a nice idea but there are a few things not quite right with it:
1N3595 or 1N4001 are poor choices for D2. The forward voltage drop is too high and does not leave enough voltage to power the mbed module. You need to use a Schottky diode.
You need to connect the MagJack transformer primary centre taps to +3.3V. These pins are not shown on your schematic symbol, but are pins 3 and 4 on the Tyco datasheet. Check out the DP83848 datasheet, Section 5.1, Figure 11. You also need to decouple these pins with 100nF capacitors to GND located close to the pins. If you don't make these connections the Ethernet signal amplitudes will be wrong.
The Ethernet tracks should be routed as differential pairs with a differential impedance of 100 ohms. Not doing this will introduce reflections which increase the jitter on the signals.
The USB tracks should also be routed as a differential pair. However, this is less critical than Ethernet.
USB interfaces can be susceptible to damage through ESD. It would be prudent to put some protective circuitry on BoB2. NXP do some low capacitance diode arrays that are suitable.
I recommend you thicken the power track from the USB connector to D2 to minimise the voltage drop. You might like to do the same on the VIN track to the PWM headers and the VIN_MBED track.
The present connection of the buttons and switches conflicts with the second SPI interface. I would like to keep that available for connection to LCDs such as the Nokia 6610. I think it would be better to connect the buttons and switches to p27 - p30.
A push button reset switch connected to NR would be useful. I think there is just room to squeeze one in.
Incidentally, why did you specify an expensive POE MagJack when you are not using the POE function? A plain vanilla MagJack would do the job and costs around half the price.
I look forward to seeing the Mk2 BoB2. If you make these changes, I will definitely buy one!
Paul
Mohamed Yousry.
7
replies
BoB2 is a nice idea but there are a few things not quite right with it:
1N3595 or 1N4001 are poor choices for D2. The forward voltage drop is too high and does not leave enough voltage to power the mbed module. You need to use a Schottky diode.
You need to connect the MagJack transformer primary centre taps to +3.3V. These pins are not shown on your schematic symbol, but are pins 3 and 4 on the Tyco datasheet. Check out the DP83848 datasheet, Section 5.1, Figure 11. You also need to decouple these pins with 100nF capacitors to GND located close to the pins. If you don't make these connections the Ethernet signal amplitudes will be wrong.
The Ethernet tracks should be routed as differential pairs with a differential impedance of 100 ohms. Not doing this will introduce reflections which increase the jitter on the signals.
The USB tracks should also be routed as a differential pair. However, this is less critical than Ethernet.
USB interfaces can be susceptible to damage through ESD. It would be prudent to put some protective circuitry on BoB2. NXP do some low capacitance diode arrays that are suitable.
I recommend you thicken the power track from the USB connector to D2 to minimise the voltage drop. You might like to do the same on the VIN track to the PWM headers and the VIN_MBED track.
The present connection of the buttons and switches conflicts with the second SPI interface. I would like to keep that available for connection to LCDs such as the Nokia 6610. I think it would be better to connect the buttons and switches to p27 - p30.
A push button reset switch connected to NR would be useful. I think there is just room to squeeze one in.
Incidentally, why did you specify an expensive POE MagJack when you are not using the POE function? A plain vanilla MagJack would do the job and costs around half the price.
I look forward to seeing the Mk2 BoB2. If you make these changes, I will definitely buy one!
Paul