MC145572PB Freescale Semiconductor, MC145572PB Datasheet - Page 239

• Manufacturer Part Number: MC145572PB
• Manufacturer: Freescale Semiconductor

Specifications of MC145572PB

• Number Of Line Interfaces: 1
• Control Interface: HDLC
• Lead Free Status / Rohs Status: Not Compliant

MOTOROLA
H.1
H.2
H.3
HIGH IMPEDANCE DIGITAL OUTPUT MODE
The MC145572 U–interface transceiver has the capability of forcing all outputs (both analog and digital)
to the high impedance state. This feature, known as the Serial Control Port High Impedance Digital
Output Mode (SCP HIDOM), is provided to allow in–circuit testing of other circuits or devices resident
on the same PCB without requiring the removal of the MC145572.
The SCP HIDOM mode is entered by holding SCPEN low for a minimum of 33 consecutive rising
edges of SCPCLK while SCPRx is high. If SCPEN goes high, or if SCPRx goes low, the device will
exit the SCP HIDOM mode and return to normal operation.
CONTROL OF TRANSMIT SIGNALS
The MC145572 permits an external microcontroller to take control of the transmit Superframe Framer
by writing to control bits in Byte register 8. This is very useful for debugging prototypes, since the
MC145572 can be forced to transmit a variety of signals regardless of the presence or lack of pres-
ence, of a signal on the receive pins. Table 4–8 summarizes these signals and the control bits.
The MC145572 can be forced to transmit SL0, SL1, SL2, SL3, SN0, TN+ SN1, SN2, SN3, 10 kHz
and 40 kHz tones, and alternating quats. See the description of BR8 in Section 4.4.9 for more details.
CHARACTERIZATION OF THE PULLABLE CRYSTAL
The MC145572 makes it very easy to measure the free running frequency of oscillation of the
20.48 MHz crystal oscillator and to measure its frequency pullablility. This is done by using the
20.48 MHz square wave signal on the BUFXTAL out pin to drive a frequency counter. Make sure that
this output has not been turned off. Never probe the crystal pins, since the capacitance of the probe
introduces severe errors in the measurement.
Also, the measured frequency must be verified against the make tolerance of the crystal at 25 C.
Do not include the aging and temperature tolerances of the crystal when performing free running
frequency checks at room temperature. If it is desired to verify operation of the crystal oscillator over
temperature, then the crystal temperature tolerance should be included. The MC145572 has a typical
crystal load capacitance including board traces of about 24 pF. Note that individual board implemen-
tations may change this figure slightly.
The free running frequency of oscillation of the 20.48 MHz oscillator can be characterized when the
MC145572 is set to NT mode operation. In NT mode, the on–chip variable capacitance array is set
for the nominal center frequency point when the transceiver is deactivated. A frequency of 20.48 MHz
plus or minus the tolerance is measured at BUFXTAL out . It may be necessary to change the crystal
calibration load capacitance specification slightly in order to have the free running frequency of
oscillation meet the 20.48 MHz specification. The nominal crystal load capacitance is 24 pF.
The pullability of a crystal can be measured by putting the MC145572 into LT mode and changing
the frequency applied to the FREQREF input pin. Any external square wave clock source can be used
for this, but do not use a clock that is generated by, or derived from, the MC145572 on which the
test is being performed, since this may cause the on–chip PLL to force the on–chip capacitance array
to its mid–frequency point at all times.
To measure the pullability towards the low frequency direction, pull FREQREF to V DD or V SS . This
causes the on–chip PLL to attempt to pull the 20.48 MHz crystal towards dc. Since this is not possible,
the MC145572 pulls the crystal as low in frequency as possible by driving the on–chip capacitance
Freescale Semiconductor, Inc.
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MC145572
TEST AND DEBUG
H
H–1