Datasheet

MPC8250 Hardware Specifications, Rev. 2
Freescale Semiconductor 11
Electrical and Thermal Characteristics
where K is a constant pertaining to the particular part. K can be determined from equation (3) by measuring
P
D
(at equilibrium) for a known T
A
. Using this value of K, the values of P
D
and T
J
can be obtained by
solving equations (1) and (2) iteratively for any value of T
A
.
2.3.1 Layout Practices
Each V
CC
pin should be provided with a low-impedance path to the board’s power supply. Each ground
pin should likewise be provided with a low-impedance path to ground. The power supply pins drive
distinct groups of logic on chip. The V
CC
power supply should be bypassed to ground using at least four
0.1 µF by-pass capacitors located as close as possible to the four sides of the package. The capacitor leads
and associated printed circuit traces connecting to chip V
CC
and ground should be kept to less than half an
inch per capacitor lead. A four-layer board is recommended, employing two inner layers as V
CC
and GND
planes.
All output pins on the MPC8250 have fast rise and fall times. Printed circuit (PC) trace interconnection
length should be minimized in order to minimize overdamped conditions and reflections caused by these
fast output switching times. This recommendation particularly applies to the address and data buses.
Maximum PC trace lengths of six inches are recommended. Capacitance calculations should consider all
device loads as well as parasitic capacitances due to the PC traces. Attention to proper PCB layout and
bypassing becomes especially critical in systems with higher capacitive loads because these loads create
higher transient currents in the V
CC
and GND circuits. Pull up all unused inputs or signals that will be
inputs during reset. Special care should be taken to minimize the noise levels on the PLL supply pins.
Table 5 provides preliminary, estimated power dissipation for various configurations. Note that suitable
thermal management is required for conditions above P
D
= 3W (when the ambient temperature is 70° C or
greater) to ensure the junction temperature does not exceed the maximum specified value. Also note that
the I/O power should be included when determining whether to use a heat sink.
Table 5. Estimated Power Dissipation for Various Configurations
1
1
Test temperature = room temperature (25° C)
Bus
(MHz)
CPM
Multiplier
Core CPU
Multiplier
CPM
(MHz)
CPU
(MHz)
P
INT
(W)
2
2
P
INT
= I
DD
x V
DD
Watts
Vddl 1.8 Volts Vddl 2.0 Volts
Nominal Maximum Nominal Maximum
66.66 2 3 133 200 1.2 2 1.8 2.3
66.66 2.5 3 166 200 1.3 2.1 1.9 2.3
66.66 3 4 200 266 — — 2.3 2.9
66.66 3 4.5 200 300 — — 2.4 3.1
83.33 2 3 166 250 — — 2.2 2.8
83.33 2 3 166 250 — — 2.2 2.8
83.33 2.5 3.5 208 291 — — 2.4 3.1