Apple's heart rate monitoring example and byte

问题:

On the heart rate measurement characteristics:

http://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml

I want to make sure I'm reading it correctly. Does that actually says 5 fields? The mandatory, C1, C2, C3, and C4? And the mandatory is at the first byte, and C4 is at the last two bytes, C1 and C2 are 8-bit fields, and C3 to C4 are 16-bit each. That's a total of 8 bytes. Am I reading this document correctly?

EDIT:

I'm informed that the mandatory flag fields indicate something is 0, it means it's just not there. For example, if the first bit is 0, C1 is the next field, if 1, C2 follows instead.

END EDIT

In Apple's OSX heart rate monitor example:

- (void) updateWithHRMData:(NSData *)data 
{
    const uint8_t *reportData = [data bytes];
    uint16_t bpm = 0;

    if ((reportData[0] & 0x01) == 0) 
    {
        /* uint8 bpm */
        bpm = reportData[1];
    } 
    else 
    {
        /* uint16 bpm */
        bpm = CFSwapInt16LittleToHost(*(uint16_t *)(&reportData[1]));
    }

    ... // I ignore rest of the code for simplicity
}

It checks the first bit as zero, and if it isn't, it's changing the little endianness to whatever the host byte order is, by applying CFSwapInt16LittleToHost to reportData[1].

How does that bit checking work? I'm not entirely certain of endianess. Is it saying that whether it's little or big, the first byte is always the mandatory field, the second byte is the C1, etc? And since reportData is an 8-bit pointer (typedef to unsigned char), it's checking either bit 0 or bit 8 of the mandatory field.

If that bit is bit 8, the bit is reserved for future use, why is it reading in there?

If that bit is 0, it's little-endian and no transformation is required? But if it's little-endian, the first bit could be 1 according to the spec, 1 means "Heart Rate Value Format is set to UINT16. Units: beats per minute (bpm)", couldn't that be mis-read?

I don't understand how it does the checking.

EDIT: I kept on saying there was C5, that was a blunder. It's up to C4 only and I edited above.

回答1:

Am I reading this document correctly?

IMHO, you are reading it a little wrong.

C1 to C4 should be read as Conditional 1 to Conditional 4. And in the table for org.bluetooth.characteristic.heart_rate_measurement, if the lowest bit of the flag byte is 0, then C1 is met, otherwise, C2 is.

You can think it a run-time configurable union type in the C programming language(, which is determined by the flag. Beware this is not always true because the situation got complicated by C3 and C4).

// Note: this struct is only for you to better understand a simplified case.
// You should still stick to the profile documentations to implement.

typedef struct {
    uint8_t flag;
    union {
        uint8_t bpm1;
        uint16_t bpm2;
    }bpm;
} MEASUREMENT_CHAR; 

How does that bit checking work?

if ((reportData[0] & 0x01) == 0) effectively checks the bit with bitwise AND operator. Go and find a C/C++ programming intro book if any doubt.

The first byte is always the flag, in this case. The value of flag dynamically determines how should the rest of the bytes should be dealt with. C3 and C4 are both optional, and can be omitted if the corresponding bits in the flag were set zeroes. C1 and C2 are mutual exclusive.

There is no endianness ambiguity in the Bluetooth standard, as it has been well addressed that little-endian should be used all the time. You should always assume that those uint16_t fields are transferred as little endian. Apple's precaution is just to reassure the most portability of the code, since they would not guarantee the endianness of architectures used in their future products.

回答2:

I see how it goes. It's not testing for Endianness. Rather, it's testing for whether the field is 8 bit or 16 bit, and in the case of 16 bit, it'll convert from little endianness to host order. But I see that before conversion and after conversion it's the same number. So I guess the system is little endian to begin with so I don't know what's the point.