Sunday, October 12, 2014

USB DC boost converters

I recently purchased some USB boost converters and some AA battery holders to make 5V portable power sources.  The boost converters were $6.45 for 10 from AliExpress store XM Electronic trade, and the battery holders were 16c each at Tayda.
A few of the boost converter modules had a piece cracked off the 4.7uF inductor, but otherwise they were in good order.  I trimmed the leads from the battery holder and soldered them to the boost converter input.  I slightly bent the USB connector tabs so they fit into the holes on the back of the battery holder, and hot glued the board to the battery holder.

The modules were advertised as "input voltage: 1-5V" and "Output Current: Rated 1A-1.5A (single lithium input)".  I put in 2 NiMh cells, and the read LED on the boost module lit up.  The input from the batteries was 2.6V and the output voltage with no load was 5.08V.  I measured the current at between 2 and 3 mA.  If the modules output 1-1.5A with a 3.7V lithium battery for input, then I calculated they should output at least 600mA with 2.4V in from a couple of NiMh AA cells.  Other vendors advertise specs of the same modules as, "output current of 500 ~ 600MA with two AA batteries", so my 600mA calculation seems about right.

I started load testing with a 68Ohm resistor, and the output was 5.12V for an output current of 75mA.  With a 34Ohm load the output voltage was 4.89V.  The USB voltage is supposed to be 5V +- .25V, so getting 600mA output without the voltage dropping below 4.75V was looking unlikely.  For the next load test I used an old 15W car speaker with a DC resistance of 13Ohms.  With the speaker connected the voltage was only 4.48V, giving an output current of 345mA.  At this load, the output voltage from the batteries was 2.4V.  Interpolating between the results indicates the modules would output not much more than 200mA before dropping below 4.75V.

The last thing I tried was charging a phone.  When I plugged in my wife's iPhone, nothing happened.  In order to be identified as a USB charging port, the D+ and D- pins need to be shorted, or for a high-power (over 500mA) charging port they need to have a voltage divider from the 5V power.  I checked the pins with a meter, and they were not connected.  I then soldered a small jumper to short D+ and D-, and tried plugging in my wife's iPhone again.  This time the screen indicated the phone was charging.

Since the modules did not perform as advertised, I messaged the AliExpress seller XM Electronic trade/Allen Lau with the details of my testing, and requested a partial refund.  After four days he did not respond so I opened a dispute with AliExpress.  Within 12 hours, he rejected the dispute only stating, "There was no evidence of right".  In the past I've encountered sellers on AliExpress that have even given full refunds after learning their products don't perform as advertised.  With Allen Lau it's the first time I've encountered this kind of "I don't give a shit" attitude.

Although the modules do not perform as advertised, they are good for a 5V power source up to about 250mA.  You can also find boost converters with a beefier 47uF inductor, but from testing results I've read online they're not much better; with 2.4V in, the output voltage of the bigger modules drops below 2.75V at around 300mA.  For backup power for a mobile phone, one of the mobile power banks using a lithium 18650 cell may be a better idea.

If you're just looking to boost the voltage from a battery for a MCU project, check out Sprites mods.

Saturday, October 4, 2014

nRF24l01+ reloaded

Since writing my post on nrf24l01 control with 3 ATtiny85 pins, I've realized these modules are quite popular.  Of over a dozen posts on my blog, it has the most hits.  From the comments in the blog, I can see that despite a lot of online resources about these modules, there's still some not clearly documented problems that people run into.  So I'll go into some more detail on how they work, share some of my tips for testing & debugging, and more.

The biggest source of confusion seems to be with holding CE high.   Section 6.1.6 of the datasheet specifies a 10us high pulse to empty one level of the TX fifo (normal mode) or hold CE high to empty all levels of the TX fifo.  This is borne out in testing - CE can be held high for a transmitting device.

The above is a picture of a $1.50 wireless transmitter node I made.  Sorry about the poor focus - I don't have a good macro mode on my camera.  It's made with an ATtiny88-au glued and soldered with 30AWG wire to a nRF24l01+ module.  The pin arrangement is as follows:

ATtiny88 ------ nRF module
14 (PB2/SS) --- 4 (CSN)
15 (PB3/MO) --- 6 (MOSI)
16 (PB4/MI) --- 7 (MISO)
17 (PB5/SCK) -- 5 (SCK)
29 (PC6/RST) -- 3 (CE)

Connecting reset to CE keeps CE high when the AVR is running, and it also gives me an easy way to program the ATtiny88.  By connecting the CE pin to the RST pin of my programmer (a USBasp), the existing pins on the nRF module can be used as a programming header.  As long as CSN is not grounded, the nRF will not interfere with the communication on the SPI bus.  In my testing it worked with CSN floating, but it would probably be best to tie it high or connect a pullup resistor between CSN and Vcc.  A small 0603 15K chip resistor should work nicely for this.

The module is powered by a CR2032 cell in a holder on the back of the module.  When I wrote my post about Cheap battery-powered micro-controller projects, I hadn't done much experimenting with coin cells.  Although some CR2032 batteries can output 20mA continuous, the no-name cells I bought from DX certainly cannot.  I connected a 20uF electrolytic capacitor to provide enough voltage during transmits (around 12mA), and put the module in power-down mode the rest of the time.

While keeping CE high is fine for a device that is only transmitting or only receiving, you might run into a problem if you try to switch between the two.  The reason can be found in the state diagram from the datasheet: (contrast enhanced to make the state transitions easier to see)
The state diagram shows there is no way to go directly from Rx to Tx mode or the other way around.  The module must go into the standby-I state by setting CE low, or into the power down state by setting PWR_UP=0.  With CE held high, the only way to change between Rx and Tx mode is to go through power down mode.  Another option that doesn't require a separate pin for CE control is to connect CE to CSN.  This would allow you to use some nRF libraries that rely on CE toggling.  A potential drawback to this is if you want to keep the module in receive mode while polling the status (#7) register for incoming packets.  Each time CSN is brought low to poll the status, it will bring CE low as well, which will cause the module to drop out of Rx mode.  It only takes 130uS to return to Rx mode once CE goes high, so this may not be a concern for you.


Using a red LED in series with Vcc as my post on nrf24l01 control with 3 ATtiny85 pins makes it possible to quickly see how much power the module is using.  When in power down mode with CSN high, no light is visible from the LED due to the very low power use.  When powered up in Rx or Tx mode, the LED glows brightly, with 10-15mA of current.  By watching the power use I was able to tell that after the Rx fifo is full, the module stops receiving, causing the power consumption to drop.  The diagram in section 7.5.2 of the datasheet indicates that will happen if CE is low, but it still happens even with CE high.  Once a single packet is read out from the Rx fifo, it starts listening for packets again.

I also found connecting a LED to the IRQ pin (#8) helpful to see when the Rx or Tx IRQ fired.

Lastly, when testing connectivity, start with enhanced shockburst disabled (EN_AA = 0), and CRC off.  Then once you've confirmed connectivity, enable the features you want.  If you decide to use CRC, go with 2-byte CRC (EN_CRC and CRC0 in the CONFIG register) since a 1-byte CRC will miss 1 in every 256 bit errors vs 1 in 65,536 for a 2-byte CRC.

Undocumented registers

Register 6 (RF_SETUP) seems to be a 9-bit register.  Attempting to write 2 bytes to the register of all 1s (ff ff) followed by a read results in the following response:
0e ff 01
For other registers documented as being a single byte, attempts to read multiple bytes results in the same byte being repeated after the status byte.

There's also an undocumented multi-byte register at address 1e.  I found some code online that refers to this register as AGC_CONFIG, which implies it is for automatic gain control.  I could not find any documentation on how to use this register.  With my modules, the first three bytes of this register defaulted to 6d 66 05.  Reading them back after writing all 1s resulted in fd ff 07, so some bits are fixed at 0.