Selecting true crystal settings for 328P [7.3728 mHz]

Using Arduino General Electronics
#1

I want to make a new board working with 115200 (gsm modem) UART.

so I want minimum baud error rate. I look at

https://cache.amobbs.com/bbs_upload782111/files_22/ourdev_508497.html

table and decide to work with ATmega328P 7.3728 Mhz / 3v3

The second step is choosing the right XTAL capacitance (Ce)

Cl = 18 pF (Load Capacitance of External Crystal) [ECS-073-18-20BM-JEN-TR]
Cs = 5 pF (Stray Capacitance; in additional sources says you can take it 2-5 pF)
Ci = 18 pF (ATmega328P datasheet; XTAL/TSOC capacitance)

This formula is given in ATmega datasheet
Ce = 2Cl - Cs - Ci

so

Ce = 2.18 - 5 - 18

External Capacitance is

Ce = 13 pF
CRYSTAL ESR = 70 ohm

is that a true calculation? and true settings for my clock setting?

Fuse Settings :
Low Byte Fuse : 0xFD
High Byte Fuse : 0xDA
Extended Byte Fuse : 0xFD

#2

Hi Akkoyun,

Did you ever get anywhere with this, if so did you have to change the boards.txt entry for your Arduino board?

#3

You will indeed need to make the change in the boards.txt file, because other things - namely, the uart settings and the millis() function - are based on that value. I think it's called something like "f_cpu".

To answer the OP's question: It's often not necessary to make calculations for the crystal capacitors. For 7MHz, 22pF caps will be fine. Once you get up to around 20MHz you want to drop the value to around 15pF, as you can damage things with too much capacitance.

#4