Freescale Semiconductor MC68HC908MR32 - Manual (Page 48)

Analog-to-Digital Converter (ADC)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
48 Freescale Semiconductor
3.3.3 Conversion Time
Conversion starts after a write to the ADSCR. A conversion is between 16 and 17 ADC clock cycles,
therefore:
The ADC conversion time is determined by the clock source chosen and the divide ratio selected. The
clock source is either the bus clock or CGMXCLK and is selectable by ADICLK located in the ADC clock
register. For example, if CGMXCLK is 4 MHz and is selected as the ADC input clock source, the ADC
input clock divide-by-4 prescale is selected and the CPU bus frequency is 8 MHz:
NOTE
The ADC frequency must be between f
ADIC
minimum and f
ADIC
maximum
to meet A/D specifications. See 19.13 Analog-to-Digital Converter (ADC)
Characteristics .
Since an ADC cycle may be comprised of several bus cycles (eight, 136 minus 128, in the previous
example) and the start of a conversion is initiated by a bus cycle write to the ADSCR, from zero to eight
additional bus cycles may occur before the start of the initial ADC cycle. This results in a fractional ADC
cycle and is represented as the 17th cycle.
3.3.4 Continuous Conversion
In continuous conversion mode, the ADC data registers ADRH and ADRL will be filled with new data after
each conversion. Data from the previous conversion will be overwritten whether that data has been read
or not. Conversions will continue until the ADCO bit is cleared. The COCO bit is set after each conversion
and will stay set until the next read of the ADC data register.
When a conversion is in process and the ADSCR is written, the current conversion data should be
discarded to prevent an incorrect reading.
3.3.5 Result Justification
The conversion result may be formatted in four different ways:
1. Left justified
2. Right justified
3. Left Justified sign data mode
4. 8-bit truncation mode
All four of these modes are controlled using MODE0 and MODE1 bits located in the ADC clock register
(ADCR).
Left justification will place the eight most significant bits (MSB) in the corresponding ADC data register
high, ADRH. This may be useful if the result is to be treated as an 8-bit result where the two least
16 to17 ADC Cycles
Conversion time =
ADC Frequency
Number of Bus Cycles = Conversion Time x CPU Bus Frequency
16 to 17 ADC Cycles
Conversion Time =
4 MHz/4
Number of bus cycles = 16
µs x 8 MHz = 128 to 136 cycles
= 16 to 17
µs