Motorola MCF5281, MCF5282 user manual Digital Control Subsystem, Comparator, Bias, State Machine

Digital Control Subsystem

27.7.3.5Comparator

The comparator output feeds into the SAR, which accumulates the A/D conversion result sequentially, beginning with the MSB.

27.7.3.6Bias

The bias circuit is controlled by the STOP signal to power-up and power-down all the analog circuits.

27.7.3.7Successive 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.

27.7.3.8State 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.

27.8 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.

MOTOROLA Chapter 27. Queued Analog-to-Digital Converter (QADC) 27-37