11

second a conversion is automatically started on the next

channel selected in the CSR. This read-convert pattern can

be continued indefinitely.

When reading Data Registers in Mode 1, the user can be

certain that the contents of the CAR equal the channel

number which was just converted. Thus the Address/Control

Byte sent prior to the read will automatically match the CAR.

If a read from a Data Register, other than the one just

converted, is performed, the CAR must be set to the desired

register prior to sending the Address/Control Byte. Setting

CAR is done by writing the SAR with ENC = 0, SAE = 1, and

the CA2 - CA0 bits equal to the desired channel.

Mode 2 - Single Scan

In mode 2, when ENC is set in the SAR, conversions are

performed on all channels selected in the CSR. Conversions

begin on the channel specified by the CAR (this channel does

not have to be active in the CSR) and proceed in ascending

order until all channels selected in the CSR have been

converted. If the starting channel is not the lowest active

channel, when the highest active channel is done converting,

the CAR advances to the lowest active channel and continues

from that point until all channels have been converted once.

When ENC is set i n the SAR, the internal clock is activated

(if selected), the CIP flag is set in the SR, and conversions

begin. The CIP flag doesn’t remain high, as it momentarily

goes low between each channel conversion.

When all channels have been converted the INT and ACC

flags in the SR are set, the INT pin is driven low (if IE is true

in the MSR), the CIP flag is cleared, and, if active, the

internal oscillator is disabled.

Data Registers can safely be read after all channels have

been converted. If the starting channel was a channel active

in the CSR then the CAR will one again be pointing to that

channel (providing all channels had been read or CSR or

MSR written since the last set of conversions - see Note

below). IF a read from a Data Register, other than the one

first converted, is performed, the CAR must be set to the

desired register prior to sending the Address/Control Byte.

Setting CAR is done by writing the SAR with ENC = 0,

SAE = 1, and the CA2 - CA0 bits equal to the desired

channel.

NOTE: A write to the SAR does not clear the DV flag for each

channel. This implies that if: conversions are completed on all

registers selected in CSR; conversions stopped; an incomplete read

of the Data Registers is performed; and conversions reinitiated with

a write to the SAR - some DVs will still be set. In Mode 2, which

terminates when all DVs are true (ACC goes true), unread channels

may not be converted unless CSR is written to before setting ENC.

There are two ways to prematurely stop conversions in Mode

2. The first is to perform any “abort” action (see Abort Modes).

Performing an abort, may produce spurious conversion

values. The second, and preferred means to stop a Mode 2

conversion, is to clear the ENC bit by writing a $00 to the

SAR. Clearing ENC will synchronously stop conversions at

the end of the current conversion. When prematurely stopping

conversions, CIP is not valid. The CIP flag cannot be used to

determine when the current conversion is complete. Instead,

a time delay equal to one conversion time must be built into

the software. The appropriate delay will ensure the last

conversion is complete before Data Register reads begin.

Prematurely stopping the conversions leaves the CAR in an

unknown state. One remaining task, before Data Registers

are read, is to be certain the contents of the CAR match the

address sent in the Address/Control Byte. This is done by

jamming the CAR with a write to the SAR with ENC = 0,

SAE = 1, CA3 - C A2 - CA0 equal to the desired channel

address.

Mode 3 - Continuous Scan

In Mode 3, when ENC is set in the SAR, conversions are

performed on all channels selected in the CSR. COnversion

begin on the channel specified by the CAR (this channel

does not have to be active in the CSR) and proceed in

ascending order for all channels selected in the CSR. Each

time the highest active channel is done converting, the CAR

advances to the lowest active channel and continues from

that point.

When ENC is set in the SAR, the internal clock is activated

(if selected) and conversions begin.

When all channels have been converted one time the ACC

flag in the SR is set. This is the only valid status flag in Mode

3. The CIP flag is not valid in Mode 3. The INT flag and the

INT pin are both held in a disabled state during Mode 3.

Data Registers cannot be read until Mode 3 conversions

have been terminated. There are two ways to stop

conversions in Mode 3. The first is to perform any “abort”

action (see Abort Modes). Performing an abort, may produce

spurious conversion values. The second, and preferred

means to stop a Mode 3 conversion, is to clear the ENC bit

by writing a $00 to the SAR. Clearing ENC will

synchronously stop conversions at the end of the current

conversion. CIP is not valid following the clearing of ENC.

The CIP flag cannot be used to determine when the current

conversion is complete. Instead, a time delay equal to one

conversion time must be built into the software. The

appropriate delay will ensure the last conversion is complete

before Data Register reads begin.

The Data Registers can safely be read after ENC is cleared

and one conversion time has elapsed. One remaining task is

to be certain the contents of the CAR match the address

sent in the Address/Control Byte. This is done by jamming

the CAR with a write to the SAR with ENC = 0, SAE = 1, and

CA2 - CA0 equal to the desired channel address.

Abort Modes

Any active mode can be aborted by any one of the following

means:

CDP68HC68A2