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
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.
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Details
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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