HPC46100 Datasheet, PDF (27/32 Pages) National Semiconductor (TI) – HPC46100 High-Performance microController with DSP Capability

Design Considerations
Designs using the HPC family of 16-bit high speed CMOS
microcontrollers need to follow some general guidelines on
usage and board layout
Floating inputs are a frequently overlooked problem CMOS
inputs have extremely high impedance and if left open can
float to any voltage You should thus tie unused inputs to
V_CC or ground either through a resistor or directly Unlike
the inputs unused outputs should be left floating to allow
the output to switch without drawing any DC current
To reduce voltage transients keep the supply line’s parasit-
ic inductances as low as possible by reducing trace lengths
using wide traces ground planes and by decoupling the
supply with bypass capacitors In order to prevent additional
voltage spiking this local bypass capacitor must exhibit low
inductive reactance You should therefore use high frequen-
cy ceramic capacitors and place them very near the IC to
minimize wiring inductance
 Keep V_CC bus routing short When using double sided or
multilayer circuit boards use ground plane techniques
 Keep ground lines short and on PC boards make them
as wide as possible even if trace width varies Use sepa-
rate ground traces to supply high current devices such as
relay and transmission line drivers
 In systems mixing linear and logic functions and where
supply noise is critical to the analog components’ per-
formance provide separate supply buses or even sepa-
rate supplies
 If you use local regulators bypass their inputs with a tan-
talum capacitor of at least 1 mF and bypass their outputs
with a 10 mF to 50 mF tantalum or aluminum electrolytic
capacitor
 If the system uses a centralized regulated power supply
use a 10 mF to 20 mF tantalum electrolytic capacitor or a
50 mF to 100 mF aluminum electrolytic capacitor to de-
couple the V_CC bus connected to the circuit board
 Provide localized decoupling For random logic a rule of
thumb dictates approximately 10 nF (spaced within
12 cm) per every two to five packages and 100 nF for
every 10 packages You can group these capacitances
but it’s more effective to distribute them among the ICs If
the design has a fair amount of synchronous logic with
outputs that tend to switch simultaneously additional de-
coupling might be advisable Octal flip-flop and buffers in
bus-oriented circuits might also require more decoupling
Note that wire-wrapped circuits can require more decou-
pling than ground plane or multilayer PC boards
TL DD 11289 – 32
FIGURE 30 Recommended
Fundamental Crystal Circuit
A recommended crystal oscillator circuit to be used with the
HPC is shown in Figure 30 See table for recommended
component values The recommended values given in the
table have yielded consistent results and are made to match
a crystal with a 18 pF load capacitance with some small
allowance for layout capacitance
For frequencies between 26 MHz and 40 MHz a third over-
tone frequency ‘‘AT’’ cut crystal or fundamental frequency
‘‘BT’’ cut crystal may be used Tite ‘‘AT’’ crystal has a tight-
er frequency tolerance over temperature than the ‘‘BT’’ cut
The ‘‘BT’’ crystal network is easier to design due to its fun-
damental nature For the ‘‘BT’’ crystal
R_F e 1 – 2 MX R₁ e 0X – 100X
C₁ e 22 pF – 27 pF C₂ e 27 pF – 53 pF
The ‘‘AT’’ crystal can be configured in one of two ways One
circuit is shown in Figure 31
where
R_F e 1 MX – 2 MX R₁ e 0X – 100X
C_I e 22 pF – 27 pF C₂ e 27 pF – 33 pF
C₃ e 15 pF – 30 pF and L₁ is determined by the equation
1
fe
2q0L₁ C₃
f is the frequency which is the crystal frequency from
this the value of L₁ can be calculated The second circuit is
similar to Figure 30 where R_F e 2 kX R₁ e 0X C₁ e
12 pF – 15 pF C₂ e 15 pF – 22 pF The lower C₁ and C₂
values allow a greater influence to stray capacitance and
EMI The lower R_F resistance decreases gain while increas-
ing bandwidth The oscillator networks and component val-
ues are supplied for reference only Actual networks and
component values should be obtained from the crystal man-
ufacturer
XTAL
Frequency
(MHz)
R1 (X)
s₂
1500
4
1200
6
910
8
750
10
600
12
470
14
390
16
300
18
220
20
180
22
150
24
120
R_F e 3 3 MX C1 e 27 pF C2 e 33 pF
XTAL Specifications The crystal used was an M-TRON Industries MP-1 Se-
ries crystal Fundamental frequency ‘‘AT’’ cut parallel resonant with a load-
ing capacitance C_L e 18 pF and a series resistance of 25X 25 MHz 40X
10 MHz or 600X 2 MHz
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