I am trying to power a Mikroelectronica TFT LCD display (3.3 volts, 7 ma) from the P22 pin (configured as a digital o/p pin on the Mbed. The display fails to power up when the MBED is reset.The display functions normally when powered from P40 (3.3 v regulated o/p).What am I doing wrong? The backlight is PWM powered from P21 with a series resistor to limit the LED current.

The digital output from the LPC1768 current limits at 4mA. The display probably draws a lot more than that, during startup.

Hmmm, I was sure it was in the order of 20 mA. Maybe I can parallel 8 digital outputs to produce 32 mA?

I shall give it a go.

mbed pins default to inputs after a reset. You need to define it as output and drive it high.

Power limit still applies. http://mbed.org/handbook/mbed-NXP-LPC1768 says 40mA per output pin. You could use a driver transistor to improve that.

Are you sure the voltage on the pin is high enough. An output pin may not reach 3V3. It seems that your display is rated at 3V7. So the p40 pin may be more or less OK, but a regular pin is critical.

No go, back to the drawing board.

I want to use the display with the new low power M0 MBED and a LiPo battery.

I have also tried a fully charged LiPo (4 volts) connected between the power i.c. tab on the MBED and ground.disconnected the usb. Still no go.

Ahhh, regardless of the battery voltage, the digital o/p pin only reaches 2.91 volts.

I have tried the Neutronics B/W N3310LCP lcd display, powered it from a single digital o/p line P24 and with a total current consumption of 6.69 mA. and works brill after a hardware reset on MBED.

So it's obvious that the TFT display needs a higher voltage so any ideas how to simply translate the 2.9 v from the digital i.o line to 3.3 v using hardware?

Hmmm, maybe I will use the B/W display for my project since I have not tested the readability of the TFT display in bright sunlight.

Now I understand why my little hobby brings me great joy - much fun in debugging a project !

Time to go to pub, when I get back will try MAX756 (20 uA shutdown current) to convert MBED I/O pin to 3.3 volts for TFT Vcc.

gurgle gurgle

As anticipated TFT not readable in strong sunlight- there must be filters available?

Still in pub

When will the new M0 mbed be available at Farnells Simon? and will the I/O pins be able to drive my peripherals ?

or maybe I shall find one in my Christmas stockings ?

still in pub ..................

bits for max756 arrived today from farnells,

still in pub.............

Back from pub.Problem unsolvable.Decided to use a simple ON/OFF S.P.S.T power switch.

I don't really understand..

Are you trying to turn on and off the display? If so, can't you just use a mosfet or a transistor as a switch controlled by the pin..?

If it was so simple.I have tried using a BS170 FET to switch the power to the TFT display,and yes it works,but a few milliamps is still being consumed by the display,no doubt via the I/O control lines.???

The problem is that Mikrolectronica does not have a technical data sheet detailing the software instruction required to switch off the display.

So, considering the project is to be battery powered,I have no other option but to use a switch between the battery and the circuitry.

Back to pub.

sTILL IN PUB

i HAVE used the nokia 3310,3510i and 6610,(NOKIA) with a PIC and have never had a problem.

staying in pub fore-ever

the end

And, I am working on the CAN BUS, utilizing the MBED (can bus interface,which works brill) and 'FUTERLECS' CAN NODE BOARD'S,(IT TAKES TWO)to produce a canbus network just like the one's they are using for radio-telescopes in the aticama desert in Chile.

Will keep you posted,

Cheers Richard xxx