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參數(shù)資料
| 型號： | LH79531 |
| 英文描述： | ARM RISC CORE ASSPs ARM7TDMI |
| 中文描述： | ARM的RISC核心專用標(biāo)準(zhǔn)的ARM7TDMI |
| 文件頁數(shù)： | 35/39頁 |
| 文件大?。?/td> | 296K |
| 代理商： | LH79531 |
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System-on-Chip
LH79520
Preliminary Data Sheet
8/21/02
35
Printed Circuit Board Layout Practices
LH79520 POWER SUPPLY DECOUPLING
The LH79520 has separate power and ground pins
for different internal circuitry sections. The VDD and
VSS pins supply power to I/O buffers, while VDDC and
VSSC supply power to the core logic.
Each of the VDD and VDDC pins must be provided
with a low impedance path to the corresponding board
power supply. Likewise, the VSS and VSSC pins must
be provided with a low impedance path to the board
ground.
Each power supply must be decoupled to ground
using at least one 0.1
μ
F high frequency capacitor
located as close as possible to a VDDx, VSSx pin pair
on each of the four sides of the chip. If room on the cir-
cuit board allows, add one 0.01
μ
F high frequency
capacitor near each VDDx, VSSx pair on the chip.
To be effective, the capacitor leads and associated
circuit board traces connecting to the chip VDDx, VSSx
pins must be kept to less than half an inch (12.7 mm)
per capacitor lead. There must be one bulk 10
μ
F
capacitor for each power supply placed near one side
of the chip.
REQUIRED LH79520 PLL, VDDA, VSSA FILTER
The VDDA pin supplies power to the chip PLL cir-
cuitry. VSSA is the ground return path for the PLL cir-
cuit. If the internal PLL circuit will be used, these pins
must have a low-pass filter attached as shown in
Figure 22.
The power pin VDDA path must be a single wire
from the IC package pin to the high frequency capaci-
tor, then to the low frequency capacitor, and finally
through the series resistor to the board power supply.
The distance from the IC pin to the high frequency
capacitor must be kept as short as possible.
Similarly, the VSSA path is from the IC pin to the
high frequency capacitor, then to the low frequency
capacitor, keeping the distance from the IC pin to the
high frequency cap as short as possible.
OTHER CIRCUIT BOARD LAYOUT PRACTICES
All output pins on the LH79520 have fast rise and fall
times. Printed circuit trace interconnection length must
therefore be reduced to minimize overshoot, under-
shoot and reflections caused by transmission line
effects of these fast output switching times. This rec-
ommendation particularly applies to the address and
data buses.
When considering capacitance, calculations must
consider all device loads and capacitances due to the
circuit board traces. Capacitance due to the traces will
depend upon a number of factors, including the trace
width, dielectric material the circuit board is made from
and proximity to ground and power planes.
Attention to power supply decoupling and printed cir-
cuit board layout becomes more critical in systems with
higher capacitive loads. As these capacitive loads
increase, transient currents in the power supply and
ground return paths also increase.
Add pull-up resistors to all unused inputs unless
an internal pull-down resistor has been specified;
see Table 2. Consider all signals that are Inputs at
Reset time.
Figure 22. VDDA, VSSA Filter Circuit
79520-64
VDDA
VDDC
VSSA
22 μF
100
VDDC
(SOURCE)
0.1 μF
PIN 92
+
PIN 91
LH79520
Note that the VSSA pin specifically does not have a connec-
tion to the circuit board ground. The LH79520 PLL circuit has
an internal DC ground connection to VSS (GND), so the ex-
ternal VSSA pin must NOT be connected to the circuit board
ground, but only to the filter components.
CAUTION
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