LPC-P2148

Development Boards MicroMod
1

Hi,

Does anybody know the role of D3 and D4 in the schematic

http://www.olimex.com/dev/pdf/lpc-h2148.pdf

Thanks

Roro

2

I can’t find them. Is that the correct schematic?

3

Sorry this is it

http://www.olimex.com/dev/pdf/LPC-P2148.pdf

thanks, roro

4

D3 clamps the base of T1 at ground for negative-going RS-232 inputs.

I can’t see why D4 is there. It would make sense if there was a capacitor to ground on the reset pin, it would discharge the capacitor when the supply was removed.

5

D4 prevents RST from being higher than 4V?

6

Thanks Leon for your help.

Denial, how does it prevent RST from going above 4 volts and when does this happen.

Also can anybody tell me why is D6 also used?

thanks roro

7

RST can’t go above 3.3V, even without D4, as I see it.

D6 disconnects the RTC battery when the board is powered, and connects it to Vbat when the supply is removed.

8

Forget what I said. The base can’t be significantly higher than 0.7V because of the base-emitter diode.

9

Leon,

I think that both Vbat and power supply are 3.3 volt so how do they block each other?

You are correct about D4.

Roro

10

It’s standard diode action. With no 3.3 V supply, the battery supplies Vbat via the upper diode. If the 3.3 V supply is present, it supplies Vbat via the lower diode, and the upper diode can’t conduct. The battery voltage is lower than 3.3 V, of course.

11

Hi All,

Can anybody see why D4 is used?

Roro

12

Seems that D4 is useless. Any comments?

Roro

13

Just a guess, but there might originally have been a capacitor in the reset circuit. A diode like D4 is often used in that situation to discharge the capacitor when power is removed, to ensure that the MCU resets properly.

14

That’s my best guess as well. It was meant to clear a capacitor in a reset circuit. Otherwise it seems to have no purpose.

15

Probably it is meant to discharge a capacitor when the power is removed. The problem is that there is no capacitor in the reset circuit unless it is internal to that MCP130T.

16

Our best guess is that the MCP130T replaced a resistor/capacitor based reset circuit and they removed the capacitor, but forgot to remove those components. The MCP130T doesn’t need any external passives on its own.