How much time does writing to UART require?

[oxxyfx]

Hello

My project requires me to send 8-10 strings to the UART port of my PIC16F886 every 20ms.

Measuring the values requires about 10-12ms. Betwen 2 sets of values there is a sync period of 10ms which I am hoping is enough time to send out the 10 strings to the UART.

Here is the string which I need to send out:

[Ttelmah]

Depends on the baud rate.
The hardware buffer is just two characters.
For a larger buffer, use a software buffer (look at EX_SISR).

First two characters, basically instantaneous (into the hardware buffer), then the hardware waits to send the last character, before accepting the next.

If your UART is set to 9600bps, then each character (assuming 8 bits, no parity), takes: 1start bit, 8databits, 1stop bit =10/9600th second = 1.04166mSec.

Your string, is:
4 characters (255,)
2 characters (2,) - why not just include this in the fixed string?...).
1 to 5 characters (0 to 65535)
2 characters (LF, CR)

So 'worst case' 13 characters.
At 9600bps = 13.541666666mSec _per string_.
.1354 seconds for all ten strings

So. No (at 9600bps).
At anything over 130Kbaud, potentially yes (though getting hard - with transmission problems if the line length is significant....).


Best Wishes

[SherpaDoug]

Count how many bytes are in your string, including the \n\r. Each byte is sent as 10 bits. Each bit takes 1/baud rate time to send. For example 18 bytes at 9600 baud is 180 bits/9600 = 18.75ms. Always leave a little extra time for overhead. Shorter messages or higher baud rates will send faster.

On the PICs I know the hardware UART buffer is 2 bytes. The built in CCS software UART has no buffer. Using interrupts it is possible to create a software buffer for the hardware UART, but the basic transmission time is a simple product of message length and baud rate.
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