1. Could I solder in a 4 MHz quartz to lower power consumption even more? What would be the lowest possible and sensible frequency?
3. I have some 5V digital signals I would like to monitor. What's the easiest way to input these into a 3.3V Arduino?
4. Blue and white LEDs often have a forward voltage of 3.2V or similar. Will the 3.3V in combination with the ICs be enough to light the LEDs?
5. The board will be powered via USB and the FT232RL. Will this chip provide enough current for 3.3V (up to 400mA for all LEDs)
6. Is it a good choice to use 3.3V if the power supply is 5V anyway?
External crystals with frequencies 0.4-20MHz or 32.768kHz can be used. There are also internal oscillators of 8MHz and 128kHz, or you can generate your own clock signal with no lower frequency limit. All these clock sources can be divided by 1-256x to lower the CPU speed even further.However at low frequencies you'll run into problems with serial communication and uploading sketches.
A better approach to saving power would be to put the cpu into one of the low-power modes whenever the LEDs are not being used.
Ah, no. With a linear regulator you don't save any power by reducing the voltage. You only save power by reducing the current. Since the current is dominated by the LEDs, you adjust power usage by changing the LEDs' series resistor. No need for a voltage regulator.
Is there a rule of thumb which frequencies are sane choices to allow serial communication?
QuoteIs there a rule of thumb which frequencies are sane choices to allow serial communication?Baud rates?
Unfortunately at least one LED will always be powered on, so that will never happen. But the µC can go to low power states while the states of the LEDs are not changing, right? The ICs will do the job of keeping the LED lit?
See Wormfood's baud-rate calculator here: http://www.wormfood.net/avrbaudcalc.phpI find that 57600 baud is the maximum at 8MHz, and 115200 at 16MHz. If you're using the sleep modes then there's little value in reducing the clock frequency, as the clock is stopped anyway.
Since I'm working on a battery-powered project I measured the effectiveness of the sleep modes on a 16MHz Arduino Uno and on a 8MHz Pro-Mini:ModeUno [mA]Pro-Mini [mA]Active443.5Idle381.5Power-down320.3Running the timers adds about 0.4mA to these numbers.So it looks like the USB interface on the Uno continuously draws about 30mA. The 0.3mA used by the Pro-Mini during power-down is due to the power LED - this could easily be disabled by cutting the PCB track.
The Uno's voltage regulator is very efficient, and the current used hardly depends on whether the regulator is used or bypassed.The Pro-Mini's voltage regulator is less efficient (perhaps due to the low currents being used), and the current needed rises by 20-50% if provided via the regulator.
Have you thought about just using an ATTiny or ATMega directly on a simpler board without all of the additional Arduino circuitry?
I actually started out with an Uno but rarely use it any more, having moved on to simpler circuits with ATTiny85, 2313, and the occasional ATMega328. The main benefit here is that you only add what you need for your project; you can even use the Arduino dev environment if you're so inclined. I'm rather fond of the runtime board from nkc electronics (http://www.nkcelectronics.com/arduino-runtime-board-rev-b.html: $1.99 for the PCB) for the '328 and the tiny dev boards from evilmadscientist.com (http://evilmadscience.com/productsmenu/tinykitlist/112-tiny2313; $2.30 ea in quantities of 5). Or skip the pre-made boards and just protoboard. Or solder directly to the chip <g>. I buy ATTinys in bulk from Mouser ('85s are $1.30 ea in quantities of 25, '2313s $1.53 in same qty) so I always have some on hand. You can make some really neat devices for a couple of bucks and learn quite a bit about the AVR in the process.
For me the Uno was a gateway device - I'm now hooked on the AVR <g>. For the price of a single Uno I can build quite a few devices... My biggest challenge is finishing one project before jumping into the next. This forum is excellent for generating new ideas!