wireless-wifi/turn-it-off-and-on-again/main.c

#include <avr/io.h>
#include <util/delay.h>
static inline void errBlinky(uint16_t repeat);

/*  Qualcomm AR9271 802.11n chip needs firmware from the manufacturer, but they
 *  are not capable of producing reliable software.  During normal use they
 *  require their chip to be reset for continued operation.
 *
 *  There is no documentation on when one might need to perform this reset.
 *  Based on some observation I found that the both chip and firmware support
 *  controlling a LED from the USB host.  When the manufacturer's firmware is
 *  requiring the host to perform the reset controlling the LEDs does not work
 *  as expected, indicating the request for reset.
 *
 *  Yes, I understand how dumb this is, but one must understand the situation
 *  that Qualcomm is in.  If they knew what they were doing things would be
 *  different, and that, might cause them some concern.
 *
 *  --
 *
 *  Monitor the LED output, the host should blink the LED once per second.  We
 *  will blink our LED in sync with their LED and blink rapidly if we are
 *  resetting the AR9271.
 */

int main(void) {
    // Set our led to output
    DDRB |= _BV(PB5);
    // Set input (pin labeled 12), connect this to the AR9271 LED output
    DDRB &= ~_BV(PB4);
  
    uint8_t last = 0;
    uint8_t count = 0;
    for(;;) {
        uint8_t status = 0;
        // host must keep the LED on for a minimum of one second per Nyquist-Shannon Sampling Theorem
        for(uint8_t i = 10; i--;) {
            // mirror our led to the AR9271 led, and remember it's status
            if(status = (PINB & _BV(PB4))) PORTB |= _BV(PB5); else PORTB &= ~_BV(PB5);
            // do this over about 500ms (one half second, we are in a fixed length loop)
            _delay_ms(50);
        }
        // compare the recorded state to the last state, it should alternate
        if(++count && last != status) count = 0;
        // stash status to compare next time...
        last = status;
        // if the counter is small we can skip reset
        if(3 >= count) continue; else count = 0;
        // power off USB device
        {   // switch outputs to high impedance rather than "turning off", same effect
            DDRB &= ~ (_BV(PB2) | _BV(PB3));
            // wait three seconds (50 thousandths of a minute)
            errBlinky(50);
        }
        // power on USB device, computer will load firmware into it
        {   // Set relay (pins labeled 10 and 11) to output.
            DDRB |= (_BV(PB2) | _BV(PB3));
            // [!] Use digital pins as power source, relay coil should be >200 Ohms
            PORTB |= _BV(PB2);
            PORTB &= ~_BV(PB3);
            // let the relay pull in, click, clack, and whatever
            _delay_ms(100);
            // drop power consumption (switch pin 11 to high impedance)
            DDRB &= ~_BV(PB3);
            // failure after power-up is unlikely, should consider computer boot time
            errBlinky(3000 /* thousandths of a minute is three minutes */);
        }
    }
    return 0;
}

static inline void errBlinky(uint16_t repeat) {
    // limit repeat to a maximum of about three minutes
    repeat = (3000 < repeat) ?3000 :repeat;
    // alternate our led every one thousandths of a minute to show error state
    for(;repeat--;) { PORTB ^= _BV(PB5); _delay_ms(60); }
}

/*  some numbers regarding the relay and our arduino pro mini
 *  we are using a 3V3 Pro Mini connected to a Omron G6DS-1A-H5
 *  the relay says it must switch at 70% of rated 5V, which is 3V5 so we
 *  are under powering the relay, cross your fingers
 *
 *  the relay datasheet claims an operate time of 10ms max, so when we switch
 *  it 100ms is plenty for debounce.  We will also pull back the current using
 *  a current limit resistor, the datasheet claims 5% of rated voltage for must
 *  release.  We use a 200 Ohm resistor and the coil is 208 Ohm.  We might push
 *  about 8mA current with our 3V3 digital pin, so that'll be interesting if
 *  that works.
 *
 *  [?] attempting to power the relay with a diode on PB3 and a resistor on
 *  PB2 was not enough, using a test PSU 3V0 is enough to energize relay, but
 *  it was right on the edge.  I ended up putting a PNP transistor on PB3 with
 *  it's emitter on the 5V rail, this is more than enough.  Code obviously needs
 *  to be adjusted because we want PB3 and PB2 to oppose each other.  The Zener
 *  is used to make sure that when PB3 floats it isn't pulled to 3V3 by the
 *  328p digital input pin proection diodes.  2V1 seemed to be enough, more is
 *  fine, but when PB3 is driven to ground the base of the PNP does need to come 
 *  down a bit less than the 5V so 4V2 might be the limit on the Zener or the relay
 *  may not energize.  I did test with two series silicon diodes with the opposite
 *  polarity hoping the 1V2 forward voltage was enough to let the base float to 5V
 *  but it did not work.
 *
 *
 *    Arduino 3V3 Pro Mini
 *   +---------------+
 *   |  o o o o o o  |               ___
 *   |o             o| <raw> -------|___> 5V
 *   |o   _     _   o| <gnd> -----+
 *   |o  | |   | |  o| <rst>      |
 *   |o  '-'   '-'  o| <3v3>      |
 *   |o      .      o|            V
 *   |o    .` `.    o|
 *   |o   <     >   o|
 *   |o    '. .'    o|
 *   |o      '      o| <led>    (PB5) error indicator
 *   |o     ___     o| <input>  (PB4) ----------------+--------+
 *   |o   -/ _ \-   o| <output> (PB3) ------+         |        |
 *   |o   -\___/-   o| <output> (PB2) --+   |        .-.      .-.
 *   +---------------+                  |  .-.       |=|      |R| pulldown
 *                                      |  |R|       |D|      '-'
 *                                     .-. '-'       '-'       |
 *                                     |R|  |         |        |
 *      +-------------+    +---+---+   '-' .-.        |        V
 *      | G6DS    c   |    |   |   |    |  |Z| 2V1    |___
 *      |  relay   o  +----+  .-.  +----+  |=| Zener  |___> AR9271_LED
 *      |           i |       |=|f      |  '-'
 *      |   sw       l+----+  |D|l      |   |      ___
 *      +--+---+------+    |  '-'y      |   |  E--|___> 5V
 *         |   |           |   | b d    |   +--B
 *         |   |___        |   | a i    |      C--+
 *         |   |___> 5V    +---+ c o    |         |
 *         |___                | k d    +---------+
 *         |___> AR9271_POWER  |   e
 *                             V
 */