AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 59

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Manual A/D Conversions
Software Loop
Timing
Timing Via Timer
Reads
AN1740
MOTOROLA
Mode 0 or mode 1 conversions are completely under the control of the
software making all events predictable and occurring in a known
sequence.
The main limitation of mode 0 and mode 1 is that the timing will be slower
either due to software timing loops or slower accesses to the 16-bit timer
or multifunction timer. Therefore, mode 0 or mode 1 is usually limited to
8-bit conversions unless extremely long conversion times can be
tolerated in the application.
Timing the A/D conversion in software timing loops requires careful
attention to loop times and cycle times between the events that start
charging ramp and test the output of comparator 2. Also, all interrupts
must be suspended while the conversion is in progress, since preserving
accurate software loop times is important. In applications which require
rapid interrupt, servicing of events may not work well with mode 0 or
mode 1 if software timing loops are used.
Timing of the conversion can be done by reading the state of the 16-bit
timer or the multifunction timer. The software must account for the
delays in reading these registers and the asychronous clocking of these
SOFTWARE DELAYS
Freescale Semiconductor, Inc.
For More Information On This Product,
TOF/OCF
CMP2
CFR2
CHG
PB0
V
ICF
IN
Go to: www.freescale.com
Figure 26. Conversion Masking A/D Fault
FAULT
V
SS
VALID
Manual A/D Conversions
Application Note
59