HD-4702_06 Datasheet, PDF (4/8 Pages) Intersil Corporation – CMOS Programmable Bit Rate Generator

HD-4702
Application Information
Single Channel Bit Rate Generator
Figure 1 shows the simplest application of the HD-4702. This
circuit generates one of five possible bit rates as determined by
the setting of a single pole, 5-position switch. The Bit Rate Out-
put (Z) drives one standard TTL load or four low power Schot-
tky 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 ade-
quate.
1
SPST SWITCH
2
5
34
56pF
56pF
10M
†
2.4576 MHz
CRYSTAL
† See Table 1.
I_M S0 S1 S2 S3
C_P
E_CP
HD-4702
I_X
O_{X 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
CONFIGURATION 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 frequency signals. The 93L34 8-bit addressable
Latch, addressed by the same Scan Counter Outputs, re-con-
verts 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
I_M S0 S1 S2 S3
C_P
E_CP
HD-4702
I_X
O_{X 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 mul-
tiplexer, the HD-4702 can be used to generate this bit rate by
connecting 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.
I_M S0 S1 S2 S3
C_P
E_CP
I_X
HD-4702
O_{X CO Q0 Q1 Q2 Z}
OUTPUT
FIGURE 3. 19200 BAUD OPERATION
TABLE 1. CRYSTAL SPECIFICATIONS
PARAMETERS
Frequency
Series Resistance (Max)
Unwanted Modes
Type of Operation
Load Capacitance
TYPICAL CRYSTAL SPEC
2.4576MHz “AT” Cut
250
-6.0dB (Min)
Parallel
32pF +0.5
4
FN2954.2
August 24, 2006