HD-4702 Ver la hoja de datos (PDF) - Renesas Electronics
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HD-4702 Datasheet PDF : 8 Pages
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HD-4702
Application Information
Single Channel Bit Rate Generator
Figure 1 shows the simplest application of the HD-4702. This cir-
cuit generates one of five possible bit rates as determined by the
setting of a single pole, 5-position switch. The Bit Rate Output (Z)
drives one standard TTL load or four low power Schottky loads
over the full temperature range. The possible output frequencies
correspond to 110, 150, 300, 1200, and 2400 Baud. For many low
cost terminals, these five bit rates are adequate.
1
SPST SWITCH
2
5
34
56pF
56pF
10M
†
2.4576 MHz
CRYSTAL
† See Table 1.
IM S0 S1 S2 S3
CP
ECP
HD-4702
IX
OX CO Q0 Q1 Q2 Z
OUTPUT
SWITCH POSITION
HD-4702 BIT RATE
1
110 Baud
2
150 Baud
3
300 Baud
4
1200 Baud
5
2400 Baud
FIGURE 1. SWITCH SELECTABLE BIT RATE GENERATOR CON-
FIGURATION PROVIDING FIVE BIT RATES
Simultaneous Generation of Several Bit Rates
Figure 2 shows a simple scheme that generates eight bit rates on
eight output lines, using one HD-4702 and one 93L34 Bit
Addressable Latch. This and the following applications take
advantage of the built-in scan counter (prescaler) outputs. As
shown in the block diagram, these outputs (Q0 to Q2) go through
a complete sequence of eight states for every half-period of the
highest output frequency (9600 Baud). Feeding these Scan
Counter Outputs back to the Select Inputs of the multiplexer
causes the HD-4702 to interrogate sequentially eight different fre-
quency signals. The 93L34 8-bit addressable Latch, addressed
by the same Scan Counter Outputs, re-converts the multiplexed
single Output (Z) of the HD-4702 into eight parallel output
frequency signals. In the simple scheme of Figure 2, input S3 is
left open (HIGH) and the following bit rates are generated:
Q0: 110 Baud
Q3: 1800 Baud
Q6: 300 Baud
Q1: 9600 Baud
Q4: 1200 Baud
Q7: 150 Baud
Q2: 4800 Baud
Q5: 2400 Baud
Other bit rate combinations can be generated by changing the
Scan Counter to Selector interconnection or by inserting logic
gates into this path.
56pF
56pF
10M
†
2.4576 MHz
CRYSTAL
IM S0 S1 S2 S3
CP
ECP
IX
HD-4702
OX CO Q0 Q1 Q2 Z
A0
A1
E
93L34
D
A2 CL Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7
† See Table 1.
FIGURE 2. BIT RATE GENERATOR CONFIGURATION WITH
EIGHT SIMULTANEOUS FREQUENCIES
19200 Baud Operation
Though a 19200 Baud signal is not internally routed to the multi-
plexer, the HD-4702 can be used to generate this bit rate by con-
necting the Q2 output to IM input and applying select code. An
additional 2-input NOR gate can be used to retain the “Zero Baud”
feature on select code 1 for the HD-4702 (See Figure 3).
56pF
56pF
10M
†
2.4576 MHz
CRYSTAL
† See Table 1.
IM S0 S1 S2 S3
CP
ECP
HD-4702
IX
OX CO Q0 Q1 Q2 Z
OUTPUT
FIGURE 3. 19200 BAUD OPERATION
TABLE 1. CRYSTAL SPECIFICATIONS
PARAMETERS
TYPICAL CRYSTAL SPEC
Frequency
Series Resistance (Max)
Unwanted Modes
Type of Operation
Load Capacitance
2.4576MHz “AT” Cut
250
-6.0dB (Min)
Parallel
32pF +0.5
FN2954 Rev 2.00
August 24, 2006
Page 4 of 8
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