I. Sharing of Example Circuits for Level Conversion
(I) Diode Level Conversion
Typical application: Pull-up resistor plus diode solution
Application: For conversion circuits where the input signal level is higher than the output signal level.
Advantages: Low cost, fewer components required
Disadvantages: It can only transmit in one direction, and the input signal level is greater than the output signal level, which causes a large voltage drop in the diode.
When selecting diodes here, it is best to choose Schottky diodes with low voltage drop to ensure that signal transmission will not cause level reading errors due to excessive voltage drop of the diode.
Work process analysis:
When the 3.3V device outputs a high-level signal, the signal input device is pulled up to a 5V level due to the 5V pull-up effect.
When the 3.3V device outputs a low-level signal, the OUTPUT signal is pulled low, which in turn pulls the signal input device signal low.
(II) Transistor Level Conversion
The basic applications of transistor circuits are as follows:
Application: For conversion circuits where the input signal level is higher than the output signal level.
Advantages: Low cost, fewer components required. Unlike diodes, transistors can be driven by a smaller current at the signal input, which may be more advantageous in situations where the driving capability of certain peripherals is weak.
Disadvantages: It can only transmit in one direction, and the input signal level is higher than the output signal level.
Similar to diode circuits in terms of their operating range and advantages and disadvantages, transistors also generate a certain voltage drop due to the PN junction. Therefore, when selecting a transistor, you can choose one with a lower on-state voltage drop.
Work process analysis:
When the 3.3V device outputs a high-level signal, the signal input device is pulled up to a 5V level due to the 5V pull-up effect.
When the 3.3V device outputs a low-level signal, it causes the PNP diode to conduct, thereby pulling the signal input device low.
II. Summary of Level Conversion Methods
1. Voltage divider method
In the resistor divider method, a resistor is used to reduce the input voltage. A resistor divider then converts a high-level signal to a low-level signal. The resistor divider method is very simple because it only requires a small resistor to perform the level conversion. However, its disadvantages include signal amplitude loss, susceptibility to external interference, and generally unsuitability for long-distance transmission.
2. Bidirectional non-inverter
A bidirectional non-inverting inverter uses two inverters. One inverter converts a high level to a low level, while the other inverter converts a low level back to a high level. The main advantage of a bidirectional non-inverting inverter is that it can transmit signals without reducing the signal amplitude and there are no restrictions on the signal transmission path.
3. Transformer method
When resistor dividers and bidirectional non-inverting inverters fail to meet requirements, transformer methods are typically used. The transformer method involves first converting the digital signal to an analog signal, and then using a transformer to convert the voltage level. The advantage of the transformer method is that it can convert high-current signals without affecting signal quality. The disadvantage is that it is not suitable for digital signals.
4. TTL to CMOS conversion method
TTL to CMOS conversion is a method of converting between different signal levels. TTL and CMOS belong to different signal level standards; the high-level standard for TTL is 2.4V, while the high-level standard for CMOS is 5V. By using a level converter to convert TTL signals to CMOS signals, signal transmission can be facilitated.
