Electronic – arduino – Why do some Arduino-compatible boards provide a crystal for the CH340?

Transferring to PCB

See How do I create a permanent application from something prototyped with Arduino? for general guidance. Doesn't cover serial comms specifically.

Serial comms

For serial comms you can connect a suitable USB-to-serial cable such as an FTDI cable. You'd connect it to header pins on your PCB that you linked to the appropriate pins on the ATmega.

Or you can add the USB interface to your PCB design using the same FTDI FT232RL chip and support components used on the older Arduinos. See page 27 of the data sheet. I believe you can omit the level convertor (as you are connecting the ATmega IC not a +/- 12V serial device). See also SparkFun's breakout as an example.

See also "ATmega328 hardcore"

The problem lies in the fact that the data stream as produced and consumed by a UART does not have an official name.

In the 'good old days' the UART data was always send through line drivers^* before they came out of the equipment. The connector and the signal levels outside the equipment where formalized in the "RS232" standard.

Nowadays we use a lot of micro controllers en single board computers and the signals out of the "UART" interface are used directly. Which raises problem: how do you call that type of interface?

You will find that the interface is often referred to as "R232-TTL". With which the mean the RS232 data format but with TTL signal levels.

Therefore there is a high probability that the equipment has TTL level signals. But you can only find out for certain after you bought one or talked to the vendor.

To connect such an interface to a standard PC there are USB-RS232 converters/adapters. Yes, they are called as such although they are not conform to the RS232 standard in neither the signal levels nor the connector pin-out. Which proves the point I made above.

^*_{The line drivers also inverted the UART signals which gave raise to more confusion: is the idle state high or low?}