Digital Converter - Microcontroller Systems - Lecture Slides, Slides of Microcontrollers

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MPC 555 QADC

Analog-to-Digital Converter

MPC555 Block Diagram

Internal Memory Map

A Simple MMR-Driven Device

Queued Analog-to-Digital Converter

• Each QADC module includes:
  • 16 basic channels (internal multiplexing)
  • Direct external multiplexing support for up to 41 channels
  • 10-bit A-to-D conversion with programmable input sample time
  • 2 Conversion Command Queues to control operation
    • Prioritize and “program” the conversions
  • Queue complete (or paused) interrupt generation
  • Conversions can be initiated by:
    • External trigger (off of a pin)
    • Time intervals
      • Includes an internal PIT-like timer
    • Software command
  • Results of conversions are cached until the next queue run-through is complete.
  • Ability to use the channel pins as digital I/O ports

QADC Block Diagram

The “Big Picture”

Conversion Example (single-scan software-triggered)

• Device: Thermocouple
  • A sensor which reports back temperature in the form of a voltage
  • Higher voltage  Higher temperature
• System:
  • A device which, upon prompting of the user, will report the temperature
• QADC Setup:
  • Single-scan mode
  • Connect the thermocouple to an input pin
  • Setup the command queue to read from the thermocouple’s pin
  • When the user asks, start the conversion by setting a register bit ( similar to PIT )

Conversion Example (continuous-scan timer-triggered)

• Device: Thermocouple
  • A sensor which reports back temperature in the form of a voltage
  • Higher voltage  Higher temperature
• System:
  • A device which samples the temperature every X seconds and performs a system shutdown if it becomes higher than a certain threshold (safety sensor!)
• QADC Setup:
  • Continuous-scan mode
  • Connect the thermocouple to an input pin
  • Setup the command queue to read from the thermocouple’s pin
  • Setup the QADC to generate an interrupt whenever a queue scan completes
  • Setup the QADC timer to start a scan every X seconds
  • Check the temperature in the Interrupt Handler

QADC Conversion Command Word Table (CCW)

Essentially an “Instruction Memory” for the converter

Each entry contains:

Using the CCW and Getting Results

The CCW is best accessed as a pointer/array, i.e.

unsigned short * CCW_A = (unsigned short *) 0x304A00; … CCW_A[0] = (unsigned short) 0x00F9;

CCW_A[1] = (unsigned short)0x003F;  “End of Queue”

On each scan pass-through, this queue will convert channel AN57 using

QCLK period x 16 sample time, then stop.

Using the CCW and Getting Results

You can also access the result queue as a pointer/array

The result of each scan in the CCW will show up in the corresponding entry of the result arrays. (parallel array structure)

From the example earlier: You can get the result of the analog scan at offset 0 from the result reg pointer

value = QADC_RJURR[0];