Innovative Baud Rate Generation
In another section of this edition, an RC oscillator is employed as a baud rate generator. Achieving precise calibration of its frequency, typically within a few percent, using a frequency meter ensures optimal performance. However, over time, this circuit might experience some drift, leading to potential issues. As a solution, we introduce a compact crystal-controlled oscillator in this context.
Generating Baud Rates Using Crystal Frequency
Starting with a crystal frequency of 2.45765 MHz and dividing it by multiples of 2, you can efficiently obtain popular baud rates like 9600, 4800, 2400, 600, 300, 150, and 75. It’s worth noting that the series lacks 1200 baud due to the absence of a Q10 output in the 4060 divider. This omission isn’t an issue if 1200 baud isn’t a requirement.
Addressing Missing Baud Rates
Given that 1200 baud is more commonly used than 600 baud in practical applications, a divide-by-two stage has been integrated into the circuit after the 4060, utilizing a 74HC74 flip-flop. This modification results in a similar series of baud rates, excluding 600 baud. A trimmer is included for calibration purposes; typically, a 33 pF capacitor offers adequate accuracy. Notably, the circuit’s current consumption remains minimal (around 1 mA) due to the use of CMOS components.
Understanding 74HC74 and 74HCT74 Flip-Flops
The 74HC74 and 74HCT74 are dual positive edge-triggered D-type flip-flops. They feature individual data (nD), clock (nCP), set (nSD), and reset (nRD) inputs, along with complementary nQ and nQ outputs. Each device comprises two D flip-flops with individual Set, Reset, and Clock inputs. Data at a D-input transfers to the corresponding Q output on the subsequent positive-going edge of the clock input. Both Q and Q outputs are available for each flip-flop. The Set and Reset inputs operate asynchronously, providing versatile functionality.
- Output Drive Capability: 10 LSTTL Loads
- Outputs Directly Interface to CMOS, NMOS, and TTL
- Operating Voltage Range: 2.0 to 6.0 V
- Low Input Current: 1.0 A
- High Noise Immunity Characteristics of CMOS Devices
- In Compliance with JEDEC Standard No. 7A Requirements
- ESD Performance: HBM 2000 V; Machine Model 200 V
- Chip Complexity: 128 FETs or 32 Equivalent Gates
- Pb−Free Packages are Available