Non-Inverting Drivers

The software solution to driving a servo via a simple inverting buffer is elegant if slightly messy when it comes to computing the duty cycle needed.

The traditional solution is to use two transistors to create a non-inverting buffer:

 

There is a way to use a single transistor as a non-inverting buffer using a common base configuration: 

In this variation on a common base mode the transistor's base is connected to the 3.3V line  and its collector to the 5V supply. Note that the two power supplies have to share a common earth.

If the GPIO output is low then R6 sets the  base emitter voltage to 0.6V and the transistor is hard on, pulling the output to the servo low.

If the GPIO output is high the base emitter voltage is zero and the transistor is cut off, making the output to the servo high. 

You can see that this is non-inverting, but the problem is that the current that flows through R7 is also the emitter current, which is the current the GPIO line has to sink. What this means is that the current in R7 is limited to around 1mA and this circuit provides no amplification. Of course you could add another transistor to provide current amplification but in this case you would be better off going back to the standard 2-transistor arrangement. 

This circuit does, however, work with most servos so in this role it is useful. The current in the GPIO line is 1mA and this is well within the drive power of a standard GPIO line.

Here is a "non-inverted" version of the test program given earlier. The range of duty cycle is limited to  make sure it works with all servos. Change the range or center value to make it work with your servo:

  if (!bcm2835_init())
        return 1;
    bcm2835_gpio_fsel(18, BCM2835_GPIO_FSEL_ALT5);
    bcm2835_pwm_set_clock(375);
    bcm2835_pwm_set_mode(0, 1, 1);
    bcm2835_pwm_set_range(0, 1024);

    for(;;){
     bcm2835_pwm_set_data(0,30);
     bcm2835_delayMicroseconds(2000000);
      bcm2835_pwm_set_data(0, 70);
      bcm2835_delayMicroseconds(2000000);
      bcm2835_pwm_set_data(0, 110);
      bcm2835_delayMicroseconds(2000000);
    }

  

What Else Can You Use PWM For?

PWM lines are incredibly versatile and it is always worth asking the question "could I use PWM?" when you are considering almost any problem. 

The LED example shows how you can use PWM as s power controller. You can extend this idea to a computer controlled switch mode power supply. All you need is a capacitor to smooth out the voltage and perhaps a transformer to change the voltage.

You can also use PWM to control the speed of a DC motor and if you add a simple bridge circuit you can control its direction and speed. 

Finally, you can use a PWM signal as a modulated carrier for data communications. For example, most infrared controller make use of a 38KHz carrier, which is roughly a 26 microseconds. This is switched on and off for 1ms and this is well within the range that the PWM can manage. So all you have to do is replace the red LED in the previous circuit with an infrared LED and you have the start of a remote control or data transmission link.

Some Hardware Details

Advanced Topic

The PWM hardware is more complicated than what is required just to produced a simple PWM signal. It also has a serialize mode which can clock a 32 bit word out of a register to create a pulse train with the characteristic you require. To make this even more useful it also supports the use of DMA to keep the serialization queue full. These features are beyond the scope of this book but if you understand how things work they are not difficult to use. 

The PWM has a large number of registers associated with it.

A control register and status register for both channels and a pair of data and range registers for each channel, plus a DMA and a FIFO register for serialization. 

The offsets from bcm2835_pwm as word addresses are:

BCM2835_PWM_CONTROL 0
BCM2835_PWM_STATUS  1
BCM2835_PWM_DMAC    2
BCM2835_PWM0_RANGE  4
BCM2835_PWM0_DATA   5
BCM2835_PWM_FIF1    6
BCM2835_PWM1_RANGE  8
BCM2835_PWM1_DATA   9

There is also a single clock which has the same registers as the GP clocks described in the previous chapter with word offset from bcm2835_clk given by:

BCM2835_PWMCLK_CNTL     40
BCM2835_PWMCLK_DIV      41

You can look up the details of the control and status registers and write your own functions to make use of the addition modes of operation. 

One easy to implement and useful addition is to modify the PWM clock source and rate. The function given at the end of the previous chapter is easy to modify to work with the PWM clock:

void bcm2835_pwm_set_clock_source(
          uint32_t source, 
          uint32_t divisorI,
          uint32_t divisorF) {
if (bcm2835_clk == MAP_FAILED
      || bcm2835_pwm == MAP_FAILED)
  return;
divisorI &= 0xfff;
divisorF &= 0xfff;
source &= 0xf;
uint8_t mask = bcm2835_peri_read(
  bcm2835_clk + BCM2835_PWMCLK_CNTL) & 0xffffffef;
bcm2835_peri_write(bcm2835_clk + BCM2835_PWMCLK_CNTL, 
   BCM2835_PWM_PASSWRD | mask);
while ((bcm2835_peri_read(bcm2835_clk + BCM2835_PWMCLK_CNTL) & 0x80) != 0) {};
bcm2835_peri_write(bcm2835_clk + BCM2835_PWMCLK_DIV, BCM2835_PWM_PASSWRD | (divisorI << 12) | divisorF);
bcm2835_peri_write(bcm2835_clk + BCM2835_PWMCLK_CNTL, BCM2835_PWM_PASSWRD | source | 0x200);
bcm2835_peri_write(bcm2835_clk + BCM2835_PWMCLK_CNTL, BCM2835_PWM_PASSWRD | 0x210 | source);
}

This works in the same way as the previous function. If first disables the clock, then sets the new divisor and source and then enables the clock again.

If you use PLLD then the clock can be as high as 500MHz. This allows you to set a range of 256 and get an effective repeat frequency of 1.9MHz which means you can get a PWM sample every half a microsecond. 

Repeating the previous triangle wave generator given earlier but at this new frequency pushes the upper limit on audio generation using this method to around 20KHz. 

  bcm2835_gpio_fsel(18, BCM2835_GPIO_FSEL_ALT5);
  bcm2835_pwm_set_clock_source(6, 2, 0);
  bcm2835_pwm_set_mode(0, 1, 1);
  bcm2835_pwm_set_range(0, 256);
  int i;
   for (;;) {
    for (i = 0; i < 256; i = i + 10) {
       bcm2835_pwm_set_data(0, i);
       bcm2835_delayMicroseconds(2);
    }
   }

Using the same filter circuit given earlier the output looks a better when you take into account the frequency is now 20KHz:

 

There are more sophisticated coding methods that can create higher quality audio than this simple approach. However this is at least gives you the basic tools to do the job.