Motorola ColdFire MCF5281 - Manual (Page 572)

Queued Analog-to-Digital Converter (QADC)
28-34 Freescale Semiconductor
28.7.3.6 Bias
The bias circuit is controlled by the STOP signal to power-up and power-down all the analog circuits.
28.7.3.7 Successive Approximation Register (SAR)
The input of the SAR is connected to the comparator output. The SAR sequentially receives the conversion
value one bit at a time, starting with the MSB. After accumulating the 10 bits of the conversion result, the
SAR data is transferred to the appropriate result location, where it may be read by user software.
28.7.3.8 State Machine
The state machine generates all timing to perform an A/D conversion. An internal start-conversion signal
indicates to the A/D converter that the desired channel has been sent to the MUX. CCW[IST[1:0]] denotes
the desired sample time. CCW[BYP] determines whether to bypass the sample amplifier. Once the end of
conversion has been reached a signal is sent to the queue control logic indicating that a result is available
for storage in the result RAM.
28.8 Digital Control Subsystem
The digital control subsystem includes the control logic to sequence the conversion activity, the system
clock and periodic/interval timer, control and status registers, the conversion command word table RAM,
and the result word table RAM.
The central element for control of QADC conversions is the 64-entry conversion command word (CCW)
table. Each CCW specifies the conversion of one input channel. Depending on the application, one or two
queues can be established in the CCW table. A queue is a scan sequence of one or more input channels.
By using a pause mechanism, subqueues can be created in the two queues. Each queue can be operated
using one of several different scan modes. The scan modes for queue 1 and queue 2 are programmed in
control registers QACR1 and QACR2. Once a queue has been started by a trigger event (any of the ways
to cause the QADC to begin executing the CCWs in a queue or subqueue), the QADC performs a sequence
of conversions and places the results in the result word table.
28.8.1 Queue Priority Timing Examples
This subsection describes the QADC priority scheme when trigger events on two queues overlap or
conflict.
28.8.1.1 Queue Priority
Queue 1 has priority over queue 2 execution. These cases show the conditions under which queue 1 asserts
its priority:
• When a queue is not active, a trigger event for queue 1 or queue 2 causes the corresponding queue
execution to begin.
• When queue 1 is active and a trigger event occurs for queue 2, queue 2 cannot begin execution until
queue 1 reaches completion or the paused state. The status register records the trigger event by
reporting the queue 2 status as trigger pending. Additional trigger events for queue 2, which occur
before execution can begin, are flagged as trigger overruns.
• When queue 2 is active and a trigger event occurs for queue 1, the current queue 2 conversion is
aborted. The status register reports the queue 2 status as suspended. Any trigger events occurring
for queue 2 while it is suspended are flagged as trigger overruns. Once queue 1 reaches the
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3