Akinori Hashimoto

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10 years ago.

Rx buffer of serial is 16 Byte (chars)?

I wrote bottom code to check the length of serial rx buffer. I can not get the Over 16 chars.

#include "mbed.h"
DigitalOut led[]= {LED1, LED2, LED3, LED4};
Serial serial(p9, p10);
int main()
{
    serial.baud(115200);
    char chr[100];
    int idx;
    while(true) {
        led[0]= !led[0];
        wait(10);
        for(idx= 0; serial.readable(); idx++)
            chr[idx]= serial.getc();
        chr[++idx]= '\0';
        for(idx= 0; chr[idx] != '\0'; idx++)
            serial.putc(chr[idx]);
    }
}
// eof

I have 2 questions.

  • 1. Internal rx buffer of serial is 16Byte, really?
  • 2. Over 16 chars is erased? (Rx buffer overflow?)

I wont to receive 40 chars per line without rx irq attach.

In using mbed official SERIAL, Please teach me how to simple way of receiving 40 chars.

16 bytes as a fairly common, that's what the 16550 UART has and that part is the de facto standard for UARTs.

posted by Andy A 27 Oct 2015

Dear Andy A,

I understand that 16 Bytes of rx buffer in UART is de-facto standard and fairly-common pattern.

Thanks for your answer.

posted by Akinori Hashimoto 28 Oct 2015

1 Answer

Erik -
10 years ago.

I assume you are using an LPC1768? Yes that has a 16-byte buffer for its RX: I don't think any other MCU at mbed comes even close to that (except some other LPCs with the same UART).

If you want more you will need a software buffer, for example using MODSERIAL or BufferedSerial libraries. But these do use RX interrupts in the background.

Accepted Answer

Dear Erik Olieman,

I thank for your replay.

Yes, I use an LPC1768. I understand that UART rx buf of other MCU is equal to 16 Byte or under 16 Byte, and I need the feature of software buffer.

I decide to code copying rx buffer to private string (ex. rxStored) with rxIrq attach. Public read() returns rxStored.

I appreciate you teach me.

posted by Akinori Hashimoto 28 Oct 2015