Sending int data to BLE device

Question

I have 4 distinct int values that I need to send to a BLE device (connection established OK).

I'll call the int values A,B,C,D for clarity. A and B range between 0-100, C has a range of 0-2000 and D has a range of 0-10000. All values are determined by user input.

I need to send these four values to the BLE device in quick succession, and package each of them differently: A and B (8 bits), C (16 bits) and D (32 bits). I'm unsure as to how to package the values correctly.

Below are three methods I've tried with varying degrees of success.

Convert int to data and send, e.g. for A (8 bit) int:

 const unsigned char CHR = (float)A;
 float size = sizeof(CHR);
 NSData * aData = [NSData dataWithBytes:&CHR length:size];
 [p writeValue:aData forCharacteristic:aCHAR type:CBCharacteristicWriteWithResponse];

Convert to string first, e.g. for (16 bit) C:

NSString * cString = [NSString stringWithFormat:@"%i",C];
NSData * cData = [cString dataUsingEncoding:NSUTF16StringEncoding]; 
[p writeValue:cData forCharacteristic:cCHAR type:CBCharacteristicWriteWithResponse];

Use uint, e.g. for (32 bit) D int:

uint32_t val = D;
float size = sizeof(val);
NSData * dData = [NSData dataWithBytes:(void*)&val length:size];
[p writeValue:valData forCharacteristic:dCHAR type:CBCharacteristicWriteWithResponse];

What am I doing wrong in the above, and how best to convert and send an int value to the device, allowing for the 3 formats required?

Paulw11 · Accepted Answer · 2016-09-07 22:50:15Z

You need to know a little more information about the format your device expects:

Are the values signed or unsigned
Is the system little-endian or big-endian

Assuming that you want to use the little-endian format that iOS uses, you can just use dataWithBytes -

unsigned char a = 100
NSData *aData = [NSData dataWithBytes:&a length:sizeof(i)];

UInt16 c = 1000
NSData *cData = [NSData dataWithBytes:&c length:sizeof(c)];

Unit32 d = 10000
NSData *dData = [NSData dataWithBytes:&d length:sizeof(d)];

And then just write the NSData using writeValue:forCharacteristic:type:

If the device wants big-endian data then you will need to manipulate the bytes into the proper order. For this reason it is often easier just to send numeric values as ASCII strings and convert them back to numeric values on the receiving end, but this will depend on whether you have control over the format the device is expecting.

Can you update your answer for Swift also. It will be useful.

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