Motorola Axiom CMD11E1 Betriebsanweisung PDF herunterladen (Seite 73)

Chapter 6

Timing using Polling

6.1 Objective

Introduces polling timer overﬂow ﬂag to create a simple clock and to generate

ON-OFF output signals.

6.2 Getting started

Make sure you have made the external connections show in ﬁgures 3.1 and 3.2

before proceeding further.

6.3 Timing

In this part of the lab, we will monitor the TOF ﬂag. This ﬂag is controlled by the

free running counter. The free running counter is a 16 bit counter that counts the

clock ticks. It is set to $0000 on power up. It counts up to $FFFFand then rolls

over to $0000 and the process is repeated until you power oﬀ the HC11. Every

time the counter rolls over, it sets the TOF ﬂag. This ﬂag is not automatically

reset and it is your responsibility to reset it in your code depending on your need.

To reset the ﬂag you have to write a 1 to it

Your HC11 most likely uses a 8 MHz crystal which means that the processor

speed is 2 MHz, or you get 2 × 10

clock ticks every second. The free running

counter rolls over every 2

ticks, or you get

2 × 10

= 30.52 overﬂows every second

This is true of all HC11 ﬂags. You reset a ﬂag, i.e. make it go to zero, by writing a 1 to it!

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Motorola Axiom CMD11E1 Betriebsanweisung Seite 73