Motorola Axiom CMD11E1 Betriebsanweisung PDF herunterladen (Seite 95)

9.5. PROCESS OF TAKING A MEASUREMENT 95

pins PE0-PE3 or the pins PE4-PE7. If you chose the ﬁrst the group of pins then

the voltage in PE0 will be converted and stored in ADR1, the voltage in PE1 will

be converted and stored in ADR2, etc. If you chose the second group of pins then

the voltage in PE4 will be converted and stored in ADR1, the voltage in PE5 will be

converted and stored in ADR2, etc. Note that if you want to measure more than

4 pins then the only way to do it will be to make two sets measurements, one for

for PE0-PE3 and the other for PE4-PE7.

9.4.2 Scanning

To take a measurement, you have to initiate the conversion. Once it is initiated,

HC11 will take 128 E-clock ticks to complete all the four conversion and ﬁll ADR1,

ADR2,ADR3,and ADR4. In the non-scanning mode, the conversion process stops.

In the scanning mode, once the converted data is stored in ADR4, HC11 will

automatically initiate the next round of conversions to ﬁll ADR1, ADR2,ADR3,and

ADR4. In the scanning mode, ADR1, ADR2,ADR3,and ADR4 will always have the most

up to date value of the input voltage. However, scanning consumes more power,

and can be wasteful if you don’t need data at a fast rate.

9.5 Process of taking a measurement

9.5.1 Turning on (powering up) the convetor

Before you can take any measurements, you have to turn on the A/D convertor.

The convertor is normally oﬀ. You turn it on by setting bit #7 of OPTION which

is at location $1039. After power up you need to wait 100 microseconds before

you can us the convertor. 100 microseconds is 200 E-clock ticks. You can put

in a delay of 100 microseconds with this following do nothing counting loop (40

times around the loop with each pass taking 5 clock cycles):

LDAA #40

DECA

BNE LT

Note that you need to do this only once in your program.

9.5.2 Initiating a conversion

You initiate a conversion by writing to ADCTL at address $1030. What you write

to the location depends on what choices you make. Bit #5 is the SCAN bit. Set

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Motorola Axiom CMD11E1 Betriebsanweisung Seite 95