2 The Smart “Things” in IoT
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Fig. 2.24 Configuration of
RS232
RX+
RXTX+
TXTX+
TXRX+
RXGND
2.5.3.9 RS232 and RS422
In microcontrollers, UARTs generally operate at 5 V TTL or 3 V levels. Transistortransistor logic (TTL) refers to a logic family created using bipolar junction
transistors. A TTL signal is “low” if it is between 0 V and 0.8 V, and the signal
is “high” if it is between 2 V and 5 V. RS232 fundamentally operates like a UART,
but with the addition of a new line driver to improve signal strength. Thanks to the
new line driver, RS232 is capable of functioning between +/− 15 V. In other words,
RS232 data is bipolar. Typically, “logic 1” is determined by +3 V to +12 V, whereas
“logic 0” is indicated by −3 V to −12 V. This feature enables the signal to transfer
data in a cable with a length of up to 10 meters. Note that like UART, RS232 also
has two data lines, namely, TXD and RXD (see Fig. 2.24).
The RS422 design also utilizes the same general UART but includes a different
line driver IC. The line driver generates a different signal from the original
signal. Therefore, there are four data lines in RS422: TXD+, TXD-, RXD+, and
RXD-. Utilizing a differential signal is beneficial because it gives the system greater
immunity to noise and allows for longer cables.
2.5.4 Clock Tree
A clock distribution network inside MCUs is known as a clock tree and includes
several circuitries and interconnects from the clock source to the destination. There
are two system clock (SYSCLK) sources available in the Cortex-M series, namely,
the external clock and the internal clock (see Fig. 2.25). The external clock is usually
created using one of the following ways [9]:
• High-Speed External (HSE) – Generated by an external resonator device, crystal,
or external clock signal.
• Low-Speed External (LSE) – Generated by an external crystal which then feeds
into the internal real-time clock (RTC). Note that RTC functions as a 24-hour
clock and/or a Julian calendar that monitors the current time.
83
Fig. 2.24 Configuration of
RS232
RX+
RXTX+
TXTX+
TXRX+
RXGND
2.5.3.9 RS232 and RS422
In microcontrollers, UARTs generally operate at 5 V TTL or 3 V levels. Transistortransistor logic (TTL) refers to a logic family created using bipolar junction
transistors. A TTL signal is “low” if it is between 0 V and 0.8 V, and the signal
is “high” if it is between 2 V and 5 V. RS232 fundamentally operates like a UART,
but with the addition of a new line driver to improve signal strength. Thanks to the
new line driver, RS232 is capable of functioning between +/− 15 V. In other words,
RS232 data is bipolar. Typically, “logic 1” is determined by +3 V to +12 V, whereas
“logic 0” is indicated by −3 V to −12 V. This feature enables the signal to transfer
data in a cable with a length of up to 10 meters. Note that like UART, RS232 also
has two data lines, namely, TXD and RXD (see Fig. 2.24).
The RS422 design also utilizes the same general UART but includes a different
line driver IC. The line driver generates a different signal from the original
signal. Therefore, there are four data lines in RS422: TXD+, TXD-, RXD+, and
RXD-. Utilizing a differential signal is beneficial because it gives the system greater
immunity to noise and allows for longer cables.
2.5.4 Clock Tree
A clock distribution network inside MCUs is known as a clock tree and includes
several circuitries and interconnects from the clock source to the destination. There
are two system clock (SYSCLK) sources available in the Cortex-M series, namely,
the external clock and the internal clock (see Fig. 2.25). The external clock is usually
created using one of the following ways [9]:
• High-Speed External (HSE) – Generated by an external resonator device, crystal,
or external clock signal.
• Low-Speed External (LSE) – Generated by an external crystal which then feeds
into the internal real-time clock (RTC). Note that RTC functions as a 24-hour
clock and/or a Julian calendar that monitors the current time.