166EMBEDDED CONTROLLER
Hardware Design
shared request lines and multiprocessor communication. The disadvantage to this type of I/O circuit is that it cannot source much current. The sink current is greater than the source current, but still less than other microcontrollers.
Output Current Limitations
The output low (sink) current for the 80C32 is limited to approximately15 milliamperes maximum. That is an absolute maximum specification value, meaning that output current in excess of this value can damage the device. Shorting a low output to the power supply would damage the device. In addition, the total sink current for an 8-bit port is limited to approximately 26 milliamperes. So if all the outputs of a port are low at the same time, they can only sink a little more than 3 milliamperes each.
On the other hand, the current source will not supply any more than about 50 microamperes under static conditions, so it cannot be destroyed by shorting an output to ground. The 80C32 current source also has an additional feature that improves input noise immunity. The current that must be sunk by an external device trying to pull the 80C32 pin low increases as it approaches ground during a one-to-zero transition. That means that weak low going noise pulses are less likely to cause an error.
Let’s examine a simple case, that of driving a LED which needs around 10 milliamperes to be clearly visible. In this case, we connect the LED and a resistor to limit the current between the power supply and the processor pin as shown in Figure 8-3.
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The LED will be off as long as the output pin is high. When the output pin goes low,
the output will sink current and the LED will turn on. LEDs have a relatively constant voltage (1.5 to 2 volts typical) across them when they are operating.
If the LED has 2 volts across it, then the resistor has the remaining 3 volts across it, then the current in the resistor and LED is 3 volts/330 ohms, or about 9 milliamperes. This will be enough current to light the LED, but it won’t be very bright. Also, the processor would only be capable of lighting a couple of LEDs. When more output current is required, other circuits can be used.
167CHAPTER EIGHT
Basic I/O Interfaces
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Figure 8-4: NPN transistor for greater load current.
Figure 8-4 shows how an NPN transistor can be used to amplify the current from the processor’s output. The processor’s output source current and transistor gain limit the potential load current. A special type of transistor, called a Darlington transistor, has a very high current gain, on the order of thousands. The CPU’s output high current is multiplied by the transistor’s gain, allowing much more current to flow in the load.
In this case, the 50 microampere source current is multiplied by the transistor gain, allowing more current to flow in the transistor collector, and hence the resistor and LED. When the output pin is high, the LED is on. For 8051 family parts, a current limiting resistor in series with the transistor base is not required, since the current source limits the base current. Other processor outputs will usually require the base resistor to limit the base current. The low source current and transistor gain is a limiting factor in this case, along with the higher saturation voltage on the collector-emitter output of the Darlington transistor compared to a regular transistor. Note that the output voltage switched by the transistor is separate from the processor supply, so this circuit can also be used to switch much higher voltages, limited only by the transistor’s maximum collector voltage specification. Yet another approach, using a PNP transistor may be a better solution for high current loads.
This approach is shown in Figure 8-5. Using a PNP transistor so that the processor’s output greater output low sink current to turn on the transistor, allows a standard transistor to be used in place of a Darlington device. It also allows the output switch to control a grounded load, which the previous versions could not. For an output low current of 1.6 milliamperes (one standard TTL load) and a
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Figure 8-5: PNP transistor output driver.
168EMBEDDED CONTROLLER
Hardware Design
modest transistor gain of 50, the transistor will be switched on with very little voltage across the transistor. Note that the LED will be on when the I/O pin is low. When the processor is reset, all the output pins are set high. This is good for loads that must star out without power when the device is first powered up.
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Figure 8-6: I/O pin voltage limits.
Because of the way the transistor is connected, this configuration does not allow the load to be connected to a supply voltage higher than that of the processor’s. By combining the NPN and PNP transistor circuits, it is possible to switch higher voltages. Higher voltages can cause problems on the input pins if not properly protected. The reasons for this are illustrated in Figure 8-6.
Looking at the absolute maximum ratings for a chip, you will observe that most device inputs must be kept within a diode’s forward voltage drop of the power supply and ground. When turned on, a silicon diode has about a 0.6 to 0.7 volt drop across it. There are parasitic diodes from the input pins to the power and ground signals, which are used to isolate the various internal circuits on the chip from one another on the chip’s substrate. The substrate is the foundation upon which all the transistors and other components are laid,
and is usually also the signal ground. The diodes can be turned on if the input goes above the power supply or below ground, causing large currents to flow in the chip. Even worse, these currents can cause a CMOS chip to “latch up,” damaging or destroying the chip. This occurs because CMOS chips have four layers, equivalent to a silicon-controlled rectifier (SCR), which shorts its outputs as long as power is applied, once it has been triggered. The net effect is that the CMOS chip will become a short between the power supply and ground, causing large currents to flow, quickly heating up and even burning out the entire chip. Generally this will occur in such a way as to burn out the most expensive chip on the board, thereby protecting the 10¢ power supply fuse from blowing out!
Voltages that exceed the chip’s allowable limits can be generated by overshoot on the signals due to unterminated transmission lines, electrostatic discharge (ESD) effects, or power transients. It can also be caused when an unpowered