Motorola Axiom CMD11E1 Guía de usuario descargar pdf (Pagina 45)

3.10. HARDWARE INTERFACING 45

NOSTR FCC /NO/

FCB 4

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3.10 Hardware Interfacing

Working with random numbers is ﬁne, but we want to do something useful. In

HC11, there area special memory locations called PORTS. The special nature of

these locations allows us to have direct access to the individual bits using external

circuitry. Each bit in the port has an I/O line associated with it. This line

provides access to the bit. The bits in the port can be one of two types:

1. A bit in a port could be an input bit. If this is the case, then we can only

set or clear the bit using external electrical circuit connected to the I/O line

associated with the bit. This means that the code you wrote earlier to set or

clear a bit will have no eﬀect on the bit. You can however check the bit and

take appropriate action. To set the bit, you have to set the voltage of the

I/O line above 3 volts (without exceeding the supply voltage of 5 volts). To

clear the bit, you have to set the voltage of the bit to below 2 volts (without

going below zero).

2. A particular bit can be an output bit. If this is the case, we can set or clear

the bit in our program and the bit will control the I/O line associated with

the bit. If the bit get set, then the voltage on the line will go to 5 volts. If

the bit is cleared, the voltage on the line will go to zero. Make absolutely

sure that you do not connect any external device that can control

the voltage (such as a voltage source) to the line. The single biggest

reason why HC11 ports get burnt is when some external device tries to send

the voltage on the line to zero while your program tries to send it to 5 volts

or vice versa. If you are concerned about damaging the port. always

connect 2.2K or larger resistor in series with the port. This will

limit the port current to 1 mA or less.

3.10.1 PORTA at location $1000

In this experiment we shall work with PORTA which is at memory location $1000.

The bits of PORTA are designated as PA7, PA6, PA5, · · · PA0. If you are using

the

FOX11

board, the lines associated with these ports are clearly labelled. If you

are using CMD11E1 from Axiom, the lines are the ﬁrst 8 pins in the MCU CONNECTOR.

The bits PA0, PA1, PA2 are inputs. This means you can connect external circuits

to these pins. The bits PA3, PA4, PA5, PA6 are outputs. This means you can

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Motorola Axiom CMD11E1 Guía de usuario Pagina 45