Файл: Embedded Controller Hardware Design (Ken Arnold, 2001).pdf

163CHAPTER EIGHT

Basic I/O Interfaces

to another bit, such as the carry bit, which is very useful when sending and receiving information by a serial bit sequence. This is a useful way to transfer data and addresses between the processor and serial I/O and memory devices.

For example, to output eight bits to Port 1 the following instructions can be used:

It is also possible to output a single bit, as shown below:

Likewise, eight bits can be input into the accumulator, using:

Single bit input can be accomplished from Port one bit 1 to the carry bit:

An input bit can also be used to control program flow:

Monitor commands can also be used to access the I/O pins on the SDK:

#SB 92 allows observing and set/clr of P1.2 bit

Port 1 can be accessed one bit at a time in the bit addressable address space from 90h to 97h, which correspond to each of the eight bits of port 1. The MSB (P1.7) can be accessed at bit address location 97h. The entire port can be reset to zero by moving the value zero to location 90h executing the instruction:

MOV 90h,#0

The MSB (P1.7) could be set to logic one by setting bit number 97h executing the following instruction:

SETB 97h

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.

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.

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 cur­ rent 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.