XPC850ZT50BU Datasheet, PDF (11/76 Pages) Motorola, Inc – Communications Controller Hardware Specifications

Layout Practices
P_D = P_INT + P_I/O
P_INT = I_DD x V_DD, watts—chip internal power
P_I/O = Power dissipation on input and output pins—user determined
For most applications P_I/O < 0.3 _• P_INT and can be neglected. If P_I/O is neglected, an
approximate relationship between P_D and T_J is:
P_D = K ÷ (T_J + 273°C)
(2)
Solving equations (1) and (2) for K gives:
K = P_D • (T_A + 273°C) + θ_JA • P_D²(3)
where K is a constant pertaining to the particular part. K can be determined from equation
(3) by measuring P (at equilibrium) for a known T . Using this value of K, the values of
D
A
P_D and T_J can be obtained by solving equations (1) and (2) iteratively for any value of T_A.
5.1 Layout Practices
Each V_CC pin on the MPC850 should be provided with a low-impedance path to the board’s
supply. Each GND 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 GND 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 MPC850 have fast rise and fall times. Printed circuit (PC) trace
interconnection length should be minimized in order to minimize undershoot and
reflections caused by these fast output switching times. This recommendation particularly
applies to the address and data busses. 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.
MOTOROLA
MPC850 (Rev. A/B/C) Hardware Specifications
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