Saturday, August 9, 2014

Global variables are good

It's a rather absolute statement, to the point of being ridiculous.  However many embedded systems "experts" say global variables are evil, and they're not saying it tongue-in-cheek.  In all seriousness though, I will explain how global variables are not only necessary in embedded systems, but also how they can be better than the alternatives.

Every embedded MCU I'm aware of, ARM, PIC, AVR, etc., uses globals for I/O.  Flashing a LED on PB5?  You're going to use PORTB, which is a global variable defining a specific I/O port address.  Even if you're using the Wiring API in Arduino, the code for digitalWrite ultimately refers to PORTB, and other global IO port variables as well.  Instead of avoiding global variables, I think a good programmer should localize their use when it can be done efficiently.

When using interrupt service routines, global variables are the only way to pass data.  An example of this is in my post Writing AVR interrupt service routines in assembler with avr-gcc.  The system seconds counter is stored in a global variable __system_time.  Access to the global can be encapsulated in a function:
uint32_t getSeconds()
    uint32_t long system_time;
    system_time = __system_time;
    return system_time;

On a platform such as the ARM where 32-bit memory reads are atomic, the function can simply return __system_time.

Global constants

Pin mappings in embedded systems are sometimes defined by global constants.  When working with nrf24l01 modules, I saw code that would define pin mappings with globals like:
uint8_t CE_PIN = 3;
uint8_t CSN_PIN = 4;

While gcc link-time optimization can eliminate the overhead of such code, LTO is not a commonly-used compiler option, and many people are still using old versions of gcc which don't support LTO.  While writing a bit-bang uart in assembler, I also wrote a version that could be used in an Arduino sketch.  The functions to transmit and receive a byte took a parameter which indicated the bit timing.  I wanted to avoid the overhead of parameter passing and use a compile-time global constant.

Compile-time global constants are something assemblers and linkers have supported for years.  In gnu assembler, the following directives will define a global constant:
.global answer
.equ answer 42

When compiled, the symbol table for the object file will contain an (A)bsolute symbol:
$ nm constants.o | grep answer
0000002a A answer

Another assembler file can refer to the external constant as follows:
.extern answer
 ldi, r0, answer

There's no construct in C to define absolute symbols, so for a while I didn't have a good solution.  Gcc supports inline assembler.  I find the syntax rather convoluted, but after reading the documentation over, and looking at some other inline assembler code, I found something that works:
// dummy function defines no code
// hack to define absolute linker symbols using C macro calculations
static void dummy() __attribute__ ((naked));
static void dummy() __attribute__ ((used));
static void dummy(){
asm (
    ".equ TXDELAY, %[txdcount]\n"
    ::[txdcount] "M" (TXDELAYCOUNT)
asm (
    ".equ RXSTART, %[rxscount]\n"
    ::[rxscount] "M" (RXSTARTCOUNT)
asm (
    ".equ RXDELAY, %[rxdcount]\n"
    ::[rxdcount] "M" (RXDELAYCOUNT)

The inline assembler I used does not work outside function definitions, so I had to put it inside a dummy function.  The naked attribute keeps the compiler from adding a return instruction at the end of the dummy function, and therefore no code is generated for the function.  The used attribute tells the compiler not to optimize away the function even though it is never called.

Build constants

The last type of constants I'll refer to are what I think are best defined as build constants.  One example would be conditionally compiled debug code, enabled by a compile flag such as -DDEBUG=1.  Serial baud rate is another thing I think is best defined as a build constant, such as how it is done in optiboot's Makefile.

Wednesday, August 6, 2014

Breaking out a QFP Attiny88 AVR

Several months ago I noticed the Attiny88.  It has several more I/O than the Atmega328, with an extra Port A  and PC7.  And unlike most of the other Attiny series, it has real SPI instead of USI, so libraries using SPI don't have to be re-written.  At just 86c for qty 1, it is the also the cheapest AVR with 8KB flash.  Since QFP-32 parts aren't easy to work with, I searched for breakout boards and found QFP32 to DIP32 boards that would allow me to use them in a small breadboard.

I had lots of experience soldering through-hole parts, but not surface-mount.  With the pin spacing of only 0.8mm, soldering individual pins with a standard soldering iron initially seemed like an impossibility.  After reading some guides and watching a couple youtube videos, I realized I should be able to solder the QFP-32 chips with my trusty old pencil-style soldering iron.

Besides the QFP Atiny, I figured I'd get some passive SMD parts as well.  I was surprised how cheap they are - 50c for 100 0.1uF ceramic capacitors and $3 for 1000 0805 resistors.  I got a little carried away and even ordered a full reel of 5000 15K 0603 resistors that were on special for $5.  Besides being more than I'll probably ever use, the 0603 size is almost too small for hand soldering.  Even the 0805 parts, at .08" or 2mm long are a bit tricky to handle.  The 0603 parts, at 1.6 by, are the size of a bread crumb.

After all the parts arrived, I started by tinning the pads on the breakout board.  That turned out to be a mistake since the leads from the tiny88 would slide off the solder bumps when I tried to solder the first lead.  A dab of flux on the bottom of the chip helped keep it in place, but for the second chip I did I only tinned the pads in to opposite corners.  I tack soldered one lead in one corner, adjusted it until it was straight, and then soldered the other corner.

Once the chip is held in place with two leads (double and triple-check it while it is easy to adjust), the rest of the leads can be soldered.  On the first chip I tried I used too much solder, which caused bridging between some of the leads.  So have some solder wick on hand.  When I soldered the second board, I only tinned the tip of my iron, which was enough solder for about 4 leads, and avoided bridging.  After the soldering is done check continuity between the leads and the DIP holes with a multimeter.  Also check for shorts by testing the adjacent dip holes.

By my second chip I had no shorts or lack of continuity between leads and the breakout pads.  What I did have was weak shorts - between 20 and 200K Ohms of resistance between some pins.  More flux and re-soldering didn't help.  The problem turned out to be the flux.  For the second chip I couldn't find my new flux, so I used an old can of flux.  Flux can be slightly conductive, but on old DIP parts with 1.5 to 2mm between leads, it's rarely an issue.  The space between the pads on the breakout boards is only 0.2-0.3mm, and along their 3mm length the conductivity of the flux residue can add up.  I was able to clean up the residue with acetone and an old toothbrush, and in the future I'll make sure to use low-conductivity flux designed for fine-pitch surface-mount parts.

On the side opposite the chip, the board has a ground plane and pads running along the breakout pins.  The pad spacing is perfect for 0805 parts, so I was able to solder a 0.1uF cap between Vcc and the ground plane.  Again I encountered a weak short, even though I hadn't used any flux.  At first I wondered if my cheap soldering iron may be too hot and could have damaged the MLCC.  This time the problem turned out to be a black residue on the the capacitor.  Surface tension can cause small parts to pull up when they are soldered, so I had used a toothpick to press the capacitor to the board while I soldered the ends.  The heat from the soldering iron charred the toothpick, leaving a black semi-conductive residue.  Getting out the acetone and toothbrush again cleaned it up, and a note to get some anti-static tweezers the next time I order parts.

Among the SMD parts I ordered were some 0603 yellow LEDs.  These were even worse to work with than the resistors.  First, reading the polarity marks is difficult with the naked eye (or at least with my middle-aged eyes).  Second they're much more fragile than resistors and capacitors.  While trying to solder one of them, my iron slipped and melted off the plastic covering the LED die.  On my first board I failed at soldering one of the surface-mount LEDs and a resistor between PB5 and Gnd.  For the second board I used a through-hole red LED.  I clipped the negative lead to go into the ground plane hole at one end of the board. I bent and clipped the positive lead so it could line up with a resistor on PB5.  To avoid a short to the ground plane pad adjacent to the PB5 pin, I insulated it with some nail polish.  Here's the finished board:

You might notice that the pin numbers don't seem to match up - AVcc is pin 18 on the tiny88, not 26.  I intentionally rotated the chip 90 degrees so the SPI pins and AVcc were all on the same side.  This way it's easy to use my breadboard programming cable.

Since I probably won't use all the breakout boards I have, I'm willing to sell some of the extras.  For $3 in bitcoin I'll send you 5 of the breakout boards, including postage to Canada/US.  I'll also throw in a few dozen of the 0603 resistors so you can see if you are any better at soldering the miniscule things than I am.  If you're interested email your shipping info to ralphdoncaster at gmail, and I'll email back with my bitcoin wallet ID.

Thursday, July 31, 2014

Busting up a breadboard

A few months ago I bought 10 mini breadboards for prototyping small electronics projects. I've noticed lots of other projects using these boards as well.  In the past couple weeks I've encountered strange problems with voltage drops and transient signal fluctuations, which I initially thought were problems with my circuits.  Eventually I started suspecting the breadboards.

One of the first things I did was measure resistance between pins that were connected by a 24AWG copper jumper wire.  The resistance of the jumper wire is no more than 0.1Ohms, but to my surprise I found the resistance from pin to pin was 6.4Ohms.  In case a bit of corrosion possibly reducing the conductance, I unplugged and plugged the jumper wire and pins a couple times.  I even tried putting some acid flux on the pins and jumper wire, but could not get a significant change in resistance.  Just from one end of a strip to the other (5 contact points) I was measuring as low as 0.4Ohms to as much as 2.1Ohms.

Most leads and connectors are made from copper, with tin or gold plating.  Copper conducts very well, but oxidizes easily so tin or gold is used to protect the copper from corrosion.  For the past number of years, the cost of copper has averaged over $3/lb, while stainless steel is about half the price.  While stainless steel resists corrosion, it's resistance is about 40 times higher than copper.  Since a breadboard with poor conductivity has limited usefulness, I decided to break apart one of the worst ones in the batch using a pair of wire cutters.

I pulled out one of the metal contact strips, and bent apart the fingers.  It certainly felt less malleable than copper.  I tried scraping the surface and snipped one of the fingers off, and the metal looked homogeneous.  Most kitchen cutlery is made of stainless steel, and if you or your friends have ever done hot knives, you probably noticed the discoloration caused by heating.  I took one of the metal strips outside and heated it with a propane torch.  Here's the result:

Another possibility besides stainless steel is nickel plated phosphor bronze, like these mini breadboards sold by dipmicro.  Phosphor bronze is close to copper color, and since the core of the metal looks the same as the outside, so I suspect the ones I received are not phosphor bronze.  It conducts about 3 times better than stainless steel so this may be one of those situations where paying a bit more is worth the money...

Monday, July 28, 2014

RK2928 wireless TV dongle

I recently purchased a wireless TV dongle for $18 (10% off the regular $20 price).  Now they're even selling for $16 on Aliexpress.  For power a microUSB-USB cable is included to plug it into a USB port on the TV or into a USB power supply.  If your TV supplies 5V power to the HDMI connector (most TVs don't), the dongle will draw power directly from the HDMI port.

It came with a single sheet double-sided "user guide".  There's no reference to the manufacturer, though after some searching I found it is functionally identical to the Mocreo MCast.  I found the setup somewhat confusing, as the dongle works in either miracast or DLNA/AirPlay mode.  Pressing the Fn button switches between modes.

The miracast mode is used to mirror your tablet or phone display to the TV.  Android 4.2 and above supports miracast.  In 4.4, it's a display option called "Cast screen".  The dongle appears as a wifi access point (with an SSID of Lollipop), and to use miracast you must connect to this access point.  This would be quite useful for presentations.  I used to do corporate training, and a dongle that can plug into the back of a projector avoids the problems associated with long VGA or HDMI cables.  Miracast is not fast enough for smooth video playback - for that you need UPnP/DLNA.

To setup DLNA, it is necessary to first connect to the Lollipop access point, and then browse to the IP address ( of the dongle.  The configuration page allows you to scan for your wifi router, and provide the password to connect.  When you are done, you'll have to switch your tablet connection to your wifi router.

At this point, if you don't have a UPnP/DLNA server and control point, you won't be able to do much with the dongle, since it's not Chromecast compatible.  XBMC is a popular DLNA server, and even Windows 7 includes a DLNA server.  Media players like the old Seagate Theatre+ will also work as a DLNA server if you have an attached hard drive.

Once you have a server, you'll also need a controller aka control point app for your android device.  The manual that came with my dongle recommended iMedia Share, but this app only supports sharing media that is already on your tablet.  Finding a decent app was rather frustrating, as the first couple free apps I tried, such as Allcast, are basically a teaser for the paid app.

After some searching I found Controldlna which does (mostly) work.  I was able to browse my DLNA server, and direct the dongle to stream video from the DLNA server.  The play/pause function in controldlna was flaky (frequently stopping the video rather than pause), so I had to use the dongle's web page controls.  Similar to the dongle's setup page, there's a page that has play/pause/stop buttons.

Playback of a 1mbps h.264 encoded HD video was very smooth.  There was a problem with the aspect ratio though.  The video was 2.25:1 aspect ratio, but the dongle displayed it at full screen 16:9, making the video look vertically stretched.

What is lacking is the ability to browse online videos (like youtube) and direct the dongle to play them.  The DLNA protocol supports arbitrary URLs, so the only barrier to playing online video is a control point app that allows selecting videos from the web.  If I can't find one, it may be time to see how my Java coding experience translates into writing Android apps.

The dongle has lots of potential, but the software is lacking at this point.  Although it's not something for your average person who wants to watch digital video on their TV, for the technical folks I think it's worth the money.

Tuesday, July 15, 2014

Testing 433Mhz RF antennas with RTL-SDR

A couple months ago I picked up a RTL2832U dongle to use with SDR#.  I've been testing 433Mhz RF modules, and wanted to figure out what kind of wire antenna works best.

Antenna theory is rather complicated, and designing an efficient antenna involves a number of factors including matching the output impedance of the transmitter.  Since I don't have detailed specs on the RF transmitter modules, I decided to try a couple different antenna designs, and use RTL-SDR to measure their performance.

I started with a ~16.5cm (6.5" for those who are stuck on imperial measurements) long piece of 24awg copper wire (from a spool of ethernet cable).  One-quarter wavelength would be 17.3cm (300m/433.9Mhz), however a resonant quarter-wave monopole antenna is supposed to be slightly shorter.  I started up SDR#, turned off AGC and set the gain fixed at 12.5dB.  The signal peaked at almost -10db:

The next thing I tried was coiling the antenna around a pen in order to make it a helical antenna.  This made the performance a lot (>10dB) worse:

I also tried a couple uncommon variations like a loop and bowtie antenna.  All were worse than the monopole.

The last thing I tried was a dipole, by adding another 16.5cm piece of wire soldered to the ground pin on the module.This gave the best performance of all, nearly 10dB better than the monopole.  An impedance-matched half-wave dipole is supposed to have about 3dB wrose gain than a quarter-wave monopole.  Given the improvement, I suspect the output impedance on the 433Mhz transmit modules is much closer to the ~70Ohm impedance of a half-wave dipole than it is to the ~35Ohm impedance of a quarter-wave monopole.

Have any other ideas on how to improve the antenna design?  Leave a comment.

Last minute update: I tried a 1/4-wave monopole wire antenna on the RTL dongle, and got 2-3dB better signal reception at 433Mhz than the stock antenna.  I tried a full-wave (69cm) wire antenna, and it performed better than the stock antenna, but slightly worse than the 1/4-wave monopole.

Controlling HD44780 displays

This post is a follow-on to my earlier post, What's up with HD44780 LCD displays?

A lesson in reading datasheets

A search for details on programming the HD44780 will result in many different ways of doing it.  I think one of the reasons is that datasheets are often ambiguous.  I'd say the HD44780U datasheet is not only ambiguous, it's not well organized.  When trying to understand the bus timing characteristics, you have to flip back and forth between the tables on pg. 52 and the diagrams on pg. After doing that too many times, I added the timing to the diagram:

When working with MCUs clocked up to 20Mhz, the minimum instruction time is 50ns.  Therefore if one instruction sets the R/W line low, and the next instruction sets E high, there will be at least 50ns between the two events.  Therefore when writing control code, it is safe to ignore tAS, tAH, and tH, which I've written in green.  If the next instruction after setting E high sets it back to low, the Pulse Width for E High (PW-EH) will be only 50ns.  To ensure the minimum pulse width is met, E should be kept high for at least 5 instruction times at 20Mhz or at least 4 instruction times at 16Mhz.

When analyzing the datasheet, it's helpful to engage in critical thinking, even if the author of the datasheet didn't!  The datasheet shows what timing is sufficient, however it's not completely clear on what timing is necessary.  For example, look at the RS line.  The timing diagram indicates it is sufficient to set the RS line 40ns before the E pulse, and hold it for 10ns after the E pulse.  Having a good idea of how the chip works based on the block diagram on pg 3, I'd say it's not necessary to assert RS until just before the falling edge of the E pulse.

One of the most frequent problems people seem to have controlling these devices is the initialization sequence.  In this matter the datasheet is not only unclear on what is necessary, it is even contradictory in places.  After reading about the problems people have encountered and experimenting with the devices myself, I believe I can condense what's sufficient to initialize the devices down to 6 steps:
  1. wait 15ms
  2. send command (1 E pulse) to set 8-bit interface mode
  3. wait 65us
  4. send command (1 E pulse) to set 8-bit interface mode
  5. wait 65us
  6. send command (1 E pulse) to set 4-bit interface mode
The first 15ms wait is from pg 23 of the datasheet referring to start-up initialization taking 10ms. This is probably dependent upon the internal oscillator frequency, which is typically 270kHz.  Page 55 of the datasheet shows how the frequency depends upon the voltage and the value of the external oscillation resistor Rf.
The modules have a 91k resistor on the back in the form of a small SMD part marked 913 (91 x 10^3).  With 5V power the minimum frequency would be 200kHz, or 35% slower than the typical timing listed in the datasheet.  I've put a red dot on the 3V graph to show what can happen if you try to a module at 3V that has 91k for Rf; the frequency could be as low as 150kHz, so commands could take almost twice as long as the typical values listed in the datasheet.  I bet this is one of the reasons people sometimes have problems using these displays - if the controlling code is based on the minimum frequency at 5V and the device is run at 3V, it may fail to work.

Peter Fleury's HD44780 library, and some others wait longer than the datasheet specified times to cover these differences.  For instructions the typical time required is 37us, so waiting 65us should be a safe value.  I based this on the 200kHz minimum frequency at 5V with 30% added for an extra margin of error.

The reason for steps 2-6 is because the device could be in 4 or 8-bit mode when it powers up.  The datasheet says, "If the electrical characteristics conditions listed under the table Power Supply Conditions Using Internal Reset Circuit are not met, the internal reset circuit will not operate normally and will fail to initialize the HD44780U."  All the devices I've seen do not initialize as described on pg. 23 of the datasheet.  I suspect they use cheaper clones of the HD44780U that didn't bother with the internal reset circuit.

If the device starts up in 4-bit mode, it will take two pulses on the E line to read 8 bits of an instruction.  The lower 4 bits of the instruction to set 4 or 8-bit mode do not matter.  By setting the high nibble (D4-D7) to the instruction to set 8-bit mode and then toggling E twice, it will either be processed as the same instruction twice in 8-bit mode, or one instruction in 4-bit mode.

Another HD44780 AVR library was written by Jeorg Wunsch.  The delays between instructions is 37us, so it is likely to have timing problems with displays that are running at less than the typical 270kHz frequency.  Both Peter's and Jeorg's code can write a nibble of data at a time.  Here's the code from Peter's library:
        dataBits = LCD_DATA0_PORT & 0xF0;
        LCD_DATA0_PORT = dataBits |((data>>4)&0x0F);

Corruption can occur if an ISR runs which changes the state of one of the high bits of LCD_DATA_PORT while that section of code executes.  In my LCD control code if LCD_ISR_SAFE is defined, interrupts are disabled while the nibble is written.  Another difference with my library code is it doesn't use the R/W (read/write) line.  Since the initialization code can't read the busy flag and has to used timed IO, there's almost no extra code to make all of the IO timed.  Overall the code is much smaller without reading the busy flag, and needs 6 instead of 7 IO pins to control the LCD.  Just short the RW line (pin 5 on the LCD module) to ground.  No power is wasted since there's no pull-up resistor on the RW line.

I did not use any explicit delay between turning on and off the E line.  Looking at the disassembled code, the duration of the E pulse is 7 CPU cycles, which would ensure the E pulse is at least 230ns even on an AVR overclocked to 30Mhz.   In addition to the control code, I've written a small test program.


Thursday, July 10, 2014

What's up with HD44780 LCD displays?

There's lots of projects and code online for using character LCD displays based on these controllers, particularly the ones with 2 rows of 16 characters (1602).  They're low power (~1mA @5V), and for only $2 each, they're the cheapest LCD modules I've found.  The controllers are over 20 years old, so as a mature technology you might think there's not much new to learn about them.  Well after experimenting with them for a few days, I've discovered a few things that I haven't seen other people discuss.

Before getting into software, the first thing you need to do after applying power is set the contrast voltage (pin3).  The amount of contrast is based on the difference between the supply voltage(VDD) and VE.  The modules have a ~10K pullup resistor on VE (pin3), so with nothing attached to it there is no display.  If VE is grounded when VDD is 5V, the contrast can be too high and you may only see black blocks.  With a simple 1N4148 diode between VE and ground, there's 0.6V on VE, and a good combination of contrast and viewing angle.

Like many other projects, I chose to use the display in 4-bit (nibble) mode, saving 4 pins on the Pro Mini.  There's also more software available to drive these displays in nibble mode than byte mode.  I like to keep wiring simple, so I spent some time figuring out the easiest way to connect the LCD module to my Pro Mini board.  After noticing I could line up D4-D7 on the module with pins 4-7 of the Pro Mini, here's what I came up with (1602 module on the left and the Pro Mini on the right):
It fits on a mini-breadboard and only requires 3 jumper wires - one for ground, one for power, and one for RS (connecting to pin 2 on the Pro Mini).  If you use the pro mini bootloader to program the chip, you may have to temporarily unplug the LCD since it connects to the UART lines.  If you use a breadboard programming cable to flash the AVR using SPI, then you can leave the module in.

These modules are also available with LED backlights powered from pin 15 and 16.  Those pins line up with pins 8 and 9 on the Pro Mini, which could be used to control the backlight.

Power Usage

A datasheet I found for a 1602 LCD module lists the power consumption as 1.1mA at 3V.  To measure the actual power usage, I put a 68-Ohm resistor in series with the 5V supply, and connected a 270 Ohm resistor between Gnd & VE.  The voltage drop on the power line was 45mV, and solving for I in V=IR means 0.66mA of current.  The voltage drop across the VE resistor was 120mV, so 2/3 of the power consumption is from the VE current, with an internal pullup resistance of 11.2K.  Most circuits I've seen for these modules recommend a 10K trimpot for controlling VE, which would add another 500uA (5V/10K) to the power consumption.

The 10K pullup resistors on the data and RS lines are another factor in power consumption.  If the AVR pins are left in output mode, four data lines and RS set low will draw a total of 2.5mA.  A good HD44780 library will set the AVR pins on those lines high (or to input mode) when not in use.  Speaking of software, it's a good point to finish this post and start on my next post about AVR software to control these displays.