MPC555
/
MPC556
QUEUED ANALOG-TO-DIGITAL CONVERTER MODULE-64
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
13-27
NOTE
The guideline for selecting PSH and PSL is select is to maintain ap-
proximately 50% duty cycle. So for prescaler values less then 16, or
PSH ~= PSL. For prescaler values greater than 16 keep PSL as large
as possible.
shows that the prescaler is essentially a variable pulse width signal gen-
erator. A 5-bit down counter, clocked at the IMB clock rate, is used to create both the
high phase and the low phase of the QCLK signal. At the beginning of the high phase,
the 5-bit counter is loaded with the 5-bit PSH value. When the zero detector finds that
the high phase is finished, the QCLK is reset. A 3-bit comparator looks for a one’s com-
plement match with the 3-bit PSL value, which is the end of the low phase of the QCLK.
The PSA bit was maintained for software compatibility, but has no effect on QADC64.
The following equations define QCLK frequency:
High QCLK Time = (PSH + 1) ÷ F
SYS
Low QCLK Time = (PSL + 1) ÷ F
SYS
FQCLK= 1 ÷ (High QCLK Time + Low QCLK Time)
Where:
• PSH = 0 to 31, the prescaler QCLK high cycles in QACR0
• PSL = 0 to 7, the prescaler QCLK low cycles in QACR0
• F
SYS
= IMB clock frequency
• FQCLK = QCLK frequency
The following are equations for calculating the QCLK high/low phases in Example 1:
High QCLK Time = (11 + 1) ÷ 40 x 10
6
= 300 ns
Low QCLK Time = (7 + 1) ÷ 40 x 10
6
= 200 ns
FQCLK = 1/(300 + 200) = 2 MHz
The following are equations for calculating the QCLK high/low phases in Example 2:
High QCLK Time = (7 + 1) ÷ 32 x 10
6
= 250 ns
Low QCLK Time = (7 + 1) ÷ 32 x 10
6
= 250 ns
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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