System Design Information
Table 84
summarizes the signal impedance targets. The driver impedances are targeted at minimum V
DD
,
nominal OV
DD
, 105°C.
Table 84. Impedance Characteristics
Impedance
R
N
R
P
Local Bus, Ethernet, DUART,
Control, Configuration, Power
Management
45 Target
45 Target
DDR DRAM
18 Target (full strength mode)
36 Target (half strength mode)
18 Target (full strength mode)
36 Target (half strength mode)
Symbol
Z
0
Z
0
Unit
Ω
Ω
Note:
Nominal supply voltages. See
Table 1,
T
j
= 105°C.
21.8
Configuration Pin Muxing
The MPC8572E provides the user with power-on configuration options which can be set through the use
of external pull-up or pull-down resistors of 4.7 kΩ on certain output pins (see customer visible
configuration pins). These pins are generally used as output only pins in normal operation.
While HRESET is asserted however, these pins are treated as inputs. The value presented on these pins
while HRESET is asserted, is latched when HRESET deasserts, at which time the input receiver is disabled
and the I/O circuit takes on its normal function. Most of these sampled configuration pins are equipped
with an on-chip gated resistor of approximately 20 kΩ. This value should permit the 4.7-kΩ resistor to pull
the configuration pin to a valid logic low level. The pull-up resistor is enabled only during HRESET (and
for platform /system clocks after HRESET deassertion to ensure capture of the reset value). When the input
receiver is disabled the pull-up is also, thus allowing functional operation of the pin as an output with
minimal signal quality or delay disruption. The default value for all configuration bits treated this way has
been encoded such that a high voltage level puts the device into the default state and external resistors are
needed only when non-default settings are required by the user.
Careful board layout with stubless connections to these pull-down resistors coupled with the large value
of the pull-down resistor should minimize the disruption of signal quality or speed for output pins thus
configured.
The platform PLL ratio, DDR complex PLL and e500 PLL ratio configuration pins are not equipped with
these default pull-up devices.
21.9
JTAG Configuration Signals
Correct operation of the JTAG interface requires configuration of a group of system control pins as
demonstrated in
Figure 66.
Care must be taken to ensure that these pins are maintained at a valid deasserted
state under normal operating conditions as most have asynchronous behavior and spurious assertion gives
unpredictable results.
Boundary-scan testing is enabled through the JTAG interface signals. The TRST signal is optional in the
IEEE Std 1149.1 specification, but it is provided on all processors built on Power Architecture technology.
The device requires TRST to be asserted during power-on reset flow to ensure that the JTAG boundary
logic does not interfere with normal chip operation. While the TAP controller can be forced to the reset
MPC8572E PowerQUICC III Integrated Processor Hardware Specifications, Rev. 4
Freescale Semiconductor
129
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
