Serial interfaces

These pages are about serial communication normally used between computers and microcontrollers.

It covers

bits and bytes
hardware based interface (UARTS)
software based interface (eg software impl of uart functionallity)

under construction see menu for first pages

Litterature and links

RS-232

Serial connections are often named as RS-232

RS232 (Recommended Standard no 232 - from telecomm world) from 1060
EIA-232 (Electronic Industries Alliance) sucesor of above from 1969

The signals have following characteristic

data line voltage levels [+3V to +15V] (logic 0)
data line voltage levels [-3V to -15V] (logic 1)
data line idle level same as logic 1

See https://en.wikipedia.org/wiki/RS-232

TTL serial

Same as RS-232 except for data signal levels

NOTE1: many call a TTL serial interfacs for RS-232 - this is pedantic not true
NOTE2: real RS-232 serial lines is very very rare. Its all TTL level serial

data line voltage levels 0V (logic 0)
data line voltage levels 3.3V or 5V or x.xV (logic 1)
data line idle level same as logic 1

Two TTL communicating devices must use same voltage for defining logical 1

If you connect a 3.3V device to a 5V device you might damage the 3.3V device if it is no 5V tolerant on input.

And the 5V device might have level problems so it cant recognice logical 1 (high) which is 3.3V less from the 3.3V device.

Arduino AVR series (uno,mega,…)	logic 1 is 5V
ESP32	logic 1 is 3.3V
many stm32	logic 1 is 3.3V
You have to find out

You can use a bidirectional level shifter between a 3.3V and 5V device to ensure proper connectivity.

Details

Schematics

LH High level side high level eg 5V
LV Low level side high level eg 3.3V
GND two of them one from hig level device and one from low level device
LV1,2,3,4 input/output on low level
HV …. on high level
you need one connection from each line

prices:

level shifter with four connections approx 40 dkk
one bss138 fet approx 2 dkk

The circuit works with devices from 1.8V to 5V approx