68HC705SP7 datasheet(106/230 Pages) FREESCALE

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Analog Subsystem

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Analog Subsystem

8.3 Analog Multiplex Register

The analog multiplex register (AMUX) controls the general

interconnection and operation. The control bits in the AMUX are shown

in Figure 8-2.

HOLD, DHOLD

These read/write bits control the source connection to the negative

input of voltage comparator 2 shown in Figure 8-3. This allows the

voltage on the internal temperature sensing diode, the channel

selection bus, or the divide-by-two channel selection bus to charge

the internal sample capacitor and to also be presented to comparator

2. The decoding of these sources is given in Table 8-1.

During the hold case when both the HOLD and DHOLD bits are clear

the VOFF bit in the Analog Status Register (ASR) can offset the VSS

reference on the sample capacitor by approximately 100 mV. This

offset source is bypassed whenever the sample capacitor is being

charged with either the HOLD or DHOLD bit set. The VOFF bit must

be enabled by the OPT bit in the COPR at location $1FF0.

During a reset the HOLD bit is set and the DHOLD bit is cleared,

which connects the internal sample capacitor to the channel selection

bus. And since a reset also clears the MUX1:4 bits then the channel

selection bus will be connected to VSS and the internal sample

capacitor will be discharged to VSS following the reset.

NOTE:

When sampling a voltage for later conversion the HOLD and DHOLD bit

should be cleared before making any changes in the MUX channel

selection. If the MUX channel and the HOLD/DHOLD are changed on

the same write cycle to the AMUX register, the sampled voltage may be

altered during the channel switching.

$0003

Bit 7

654321

Bit 0

Read:

HOLD

DHOLD

INV

VREF

MUX4

MUX3

MUX2

MUX1

Write:

Reset:

10000000

Figure 8-2. Analog Multiplex Register (AMUX)

Freescale Semiconductor, Inc.

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