---
title: "UDP mode - clearing receive buffer"
url: "https://maker.wiznet.io/forum/9997"
markdown_url: "https://maker.wiznet.io/forum/9997/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "rm69"
created: "2014-02-27T16:46:28+09:00"
last_activity: "2017-05-04T08:31:41+09:00"
language: "en"
views: 2403
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# UDP mode - clearing receive buffer

## Question

Asked by rm69 on 2014-02-27 in Ethernet Chips.

1. If I do not need the data in the receive buffer in UDP mode, can I just set Sn_CR to RECV without reading from RX_FIFOR? Is it necessary to copy data even if I do not need the data? Currently we are reading the data from RX_FIFOR - since there was some suggestion from some site that this is necessary to clear the buffer to make way for new data - and then setting RECV, but it is not required to read the data, is it not? When we set RECV, next data will simply overwrite the previous data, right?

2. If a wiznet UDP socket port is being bombarded with continuous data (may be junk / broadcast ), how does this affect the chip? Will it cause speed issues with other sockets? Can it cause the chip to reply to pings only once, or not reply at all? Can it cause the chip not to reply to arps?

3. What is the best way to deal with unwanted data sent to wiznet udp port? Can we check the source port and then simply set RECV and exit?

## Replies

### Reply 1 by rm69, 2014-02-28

Just a stupid question. A confusing piece of code. Data length appears after 14 bytes (6 times read of *S5_RX_FIFOR0, and 1. Wiznet is directly memory mapped. Definitions are shown at the top of the code.

uint8 xdata *S5_IR1 = (uint8 xdata *)0x2347;
uint8 xdata *S5_SSR1 = (uint8 xdata *)0x2349;
uint8 xdata *S5_RX_RSR3 = (uint8 xdata *)0x236B;
uint8 xdata *S5_RX_FIFOR0 = (uint8 xdata *)0x2370;
uint8 xdata *S5_RX_FIFOR1 = (uint8 xdata *)0x2371;

uint8 head0;
uint8 head1;
uint8 udpsizelsb;

if(*S5_SSR1 == 0X22) // check if the socket is in udp mode
{
/// get the length of the recv packet, LSB will indicate a minimum length. assuming it won’t be zero.
udpsizelsb = *S5_RX_RSR3;
if(*S5_IR1 & 0X04 || udpsizelsb > 0x08) // if minimum 8 bytes are recvd or if interrupt is set
{
*S5_IR1 = 0X04; // clear the recv interrupt bit of socket

```
			 head0   = *S5_RX_FIFOR0;                                     // read 2 bytes from wiznet recv memory and throw it. Need only data size.
		        head1  = *S5_RX_FIFOR1; 

                            head0   = *S5_RX_FIFOR0;                                     // read 2 more bytes from wiznet recv memory
		        head1  = *S5_RX_FIFOR1; 

                            head0   = *S5_RX_FIFOR0;                                     // read 2 more bytes from wiznet recv memory
		        head1  = *S5_RX_FIFOR1; 


              //completed - totally 6 bytes - next 2 bytes must be data size !!


                            head0   = *S5_RX_FIFOR0;                                     // read 2 more bytes from wiznet recv memory
		        head1  = *S5_RX_FIFOR1;
```

//totally 8 bytes

```
                            head0   = *S5_RX_FIFOR0;                                     // read 2 more bytes from wiznet recv memory
		        head1  = *S5_RX_FIFOR1; 

                            head0   = *S5_RX_FIFOR0;                  // read 2 more bytes from wiznet recv memory - 13/14 bytes
		        head1  = *S5_RX_FIFOR1;
```

//totally 14 bytes

// now, next 2 bytes indicate datalength. Actually, it should have been the last 2 bytes of the header (7 and 8), and not 15 and 16. But some how, 15/16 matches datalength. From 17nth onwards, it is data.

```
			 datalength = *S5_RX_FIFOR0;                                     // read the datasize now, from 15/16 bytes
		     datalength =(maclength << 8);
			 datalength = datalength + *S5_RX_FIFOR1;
```

for(index = 0 ; index &lt; datalength; index++) // copy the data contents of recv memory of he socket to user memory. variables are defined elsewhere.
{
mydata[index] = *S5_RX_FIFOR0;
index++;
mydata[index] = *S5_RX_FIFOR1;
}

Now, mydata has UDP data. Actually, I am reading from 17, 18 … of the buffer, instead of 9,10… So my data has to be incorrect, but it seems to be correct. Is this some datatype conversion issue?

Please comment. Code seems to be right, but something is wrong. (In the actual code, we are using a header array as per the sample code.). Code gives correct receive size o 15/16 reads. Some stupid interpretation, but if you can comment on this, we would appreciate it.

---

Source: https://maker.wiznet.io/forum/9997