
A-to-D Converter
A/D registers
MC68HC05E6 — Rev. 1.0
MOTOROLA
A-to-D Converter
69
5-adc
When the A/D RC oscillator is turned on, it takes a time t
ADRC to
stabilize (see
Table 23). During this time A/D conversion results may
be inaccurate.
When the internal RC oscillator is being used as the conversion clock,
the following limitations apply.
1. Due to the frequency tolerance of the RC oscillator and its
asynchronism with regard to the MCU bus clock, the conversion
complete flag (COCO) must be used to determine when a
conversion sequence has been completed.
2. The conversion process runs at the nominal 1.5MHz rate but the
conversion results must be transferred to the MCU result registers
synchronously with the MCU bus clock in order that conversion
time is limited to a maximum of one channel per bus clock cycle.
3. If the system clock is running faster than the RC oscillator, the RC
oscillator should be switched off and the system clock used as the
conversion clock.
ADON — A/D converter on
The ADON bit allows the user to enable/disable the A/D converter.
1 = A/D converter is switched on.
0 = A/D converter is switched off.
When the A/D converter is switched on, it takes a time t
ADON for the
current sources to stabilize (see
Table 23). During this time A/D
conversion results may be inaccurate.
Power-on or external reset will clear the ADON bit, thus disabling the
A/D converter.
Table 9 A/D clock selection
ADRC
ADON
RC
oscillator
A/D
converter
Comments
0
OFF
A/D switched off.
0
1
OFF
ON
A/D using CPU clock.
1
0
ON
OFF
Allows the RC oscillator to stabilize.
1
ON
A/D using RC oscillator clock.