How to receive stream of bytes using PIC USART

[unifoxz]

Hi,

I would like to know how does PIC read the stream of bytes from serial port? I was trying to read in stream of bytes at 4800 baud rate but the data crashed. The received data is not the expected data.

I welcome any idea and thank you in advance.

[mackrackit]

Welcome to the forum.

With out seeing your code it is hard to help you figure out where the problem is.

With out seeing your code I will have to point you to the PBP manual and the STR option. STR lets you read in a string to an array where it can be used for something.

[Ioannis]

Are you reading a string from GPS?

Ioannis

[unifoxz]

No, I am reading data from a SpO2 module. The module sends out 5 bytes in sequence continuously. Among the bytes sent out from module, I only need to extract two of them which provide the information of the SpO2 and pulse rate. I used software UART to communicate with the SpO2 module while hardware UART to communicate with PC UART. When I connect the SpO2 directly to the PC, the data is correct. However, when I connect the SpO2 module and PC through PIC16F877, the data all mess up. I think it's probably because of the insufficient buffer in the PIC. I never handle such problem and I failed to solve the problem after trying for the whole day. What can I do to solve this problem?

Here is the PIC code written in PICBasic Pro:

INCLUDE "modedefs.bas"
DEFINE LOADER_USED 1        
DEFINE OSC 20 
B0 VAR BYTE
B1 VAR BYTE
B2 VAR BYTE
B3 VAR BYTE
B4 VAR BYTE
B5 VAR BYTE
B6 VAR BYTE
B7 VAR BYTE
B8 VAR BYTE
PR VAR BYTE
SPO2 VAR BYTE
ID_2 VAR BYTE
ID_1 VAR BYTE
ID_0 VAR BYTE

'PB.7 = Rx
TRISB = %10000000 

'data acquisition ID
ID_2 = "0"
ID_1 = "1"
ID_0 = "1"

'request from PC, 9600 baud rate
standby:
SerIn PORTC.7,6,["P",ID_2, ID_1, ID_0]

loop:
'4800 baud rate, 
SerIn2 PORTB.7,16572,[B0, B1, B2, B3, B4, B5, B6, B7, B8]

' detect synchorous bit (the header byte in 5 bytes data)
IF B0 > 127 Then  
GoTo b0_true
EndIF

IF B1 > 127 Then 
GoTo b1_true
EndIF

IF B2 > 127 Then 
GoTo b2_true
EndIF

IF B3 > 127 Then 
GoTo b3_true
EndIF

IF B4 > 127 Then 
GoTo b4_true
EndIF

GoTo loop

' extract PR and SpO2 data
b0_true:
IF B2 > 63 Then 
    B2=64
Else
    B2=0
EndIF
B3=B3+B2
PR=B3
SPO2=B4
GoTo send_data

b1_true:
IF B3 > 63 Then 
    B3=64
Else
    B3=0
EndIF
B4=B4+B3
PR=B4
SPO2=B5
GoTo send_data

b2_true:
IF B4 > 63 Then 
    B4=64
Else
    B4=0
EndIF
B5=B5+B4
PR=B5
SPO2=B6
GoTo send_data

b3_true:
IF B5 > 63 Then 
    B5=64
Else
    B5=0
EndIF
B6=B6+B5
PR=B6
SPO2=B7
GoTo send_data

b4_true:
IF B6 > 63 Then 
    B6=64
Else
    B6=0
EndIF
B7=B7+B6
PR=B7
SPO2=B8
GoTo send_data

'send to PC, 9600 baud rate
send_data:
SerOut PORTC.6,6,[ID_2, ID_1, ID_0, "^P^", #PR, "^", #SPO2]

GoTo standby

End

[Ioannis]

I think something is not clear here.

You say that the following piece of code is requesting from PC,but I see that it is just waiting fo the string of characters to arrive.

'request from PC, 9600 baud rate
standby:
SerIn PORTC.7,6,["P",ID_2, ID_1, ID_0]

After that the next lines are executed, but are you sure you catch the beginning of the transmission? How can you be sure about that?

It is better to wait for the device to send a preample or a start character and the store the array of the 8 bytes.

instead of this:

loop:
'4800 baud rate, 
SerIn2 PORTB.7,16572,[B0, B1, B2, B3, B4, B5, B6, B7, B8]

do something like this:

my_array var byte[8]

loop:
'4800 baud rate, 
SerIn2 PORTB.7,16572,[wait("abc"), str my_array\8]

Ioannis

[mackrackit]

Another variation that might be handy in your case

SERIN2 PORTB.1,16572,[WAIT("ABC"),DEC1 VAR1, SKIP1, DEC1 VAR2, SKIP1, DEC1 VAR3]

The above will save every other character to a variable. Mix and match as needed.
Many ways to play/parse in coming strings.