How to combine 4 bytes and do math calculation in C using AVR

Question

I have an atMega1281 micro controller using C. I have a routine to pull 4 bytes off of a CAN bus to read an SPN. I am able to get the 4 bytes but I cannot print the 4 byte number because it is truncating the the first 2 bytes making it a 16 bit number. I have tried using unsigned long as the declaration with no success. What is the trick when using 32 bit numbers with an AVR?

unsigned long engine_hours_raw;
float engine_hours_uint;

engine_hours_raw =  (OneMessage.Msg.Data[3] << 24) | (OneMessage.Msg.Data[2] << 16) | (OneMessage.Msg.Data[1] << 8) | OneMessage.Msg.Data[0]);
engine_hours_uint = engine_hours_raw * 0.05;

ftoa(engine_hours_uint, engHours, 1);
UART1_Printf("Engine Hours: %s   ", engHours);

I actually convert the float to a string using ftoa and then print as a string. — Eddie, Aug 11 '17 at 19:53
@Eddie On the bare metal embedded systems it not needed to stick to the standard as we know that the pointer is an actual address in the memory :) — 0___________, Aug 11 '17 at 22:36
"unsigned long"? For goodness sake, do use `uint32_t` for an unsigned 32bit integer! — ndim, Aug 14 '17 at 20:02

score 2 · Accepted Answer · answered Aug 11 '17 at 21:19

(OneMessage.Msg.Data[3] << 24) will be 0 as the default size for an expression is an int. unless it is cast.

Instead, load the data into the long int and then perform the shift.

engine_hours_raw = OneMessage.Msg.Data[3];
engine_hours_raw <<= 8;
engine_hours_raw |= OneMessage.Msg.Data[2];
engine_hours_raw <<= 8;
engine_hours_raw |= OneMessage.Msg.Data[1];
engine_hours_raw <<= 8;
engine_hours_raw |= OneMessage.Msg.Data[0];

You could also cast the intermediate expressions as unsigned log or uint32_t, but it just as messy and may take longer.

engine_hours_raw =  ((unit32_t)(OneMessage.Msg.Data[3]) << 24) | ((unit32_t)(OneMessage.Msg.Data[2]) << 16) | ((unit32_t)(OneMessage.Msg.Data[1]) << 8) | (unit32_t)(OneMessage.Msg.Data[0]);

why to complicate it ? we are in the bare metal embedded world. pointers and addresses are well known :) — 0___________, Aug 11 '17 at 22:28
This solution worked. I found a document explaining more detail about declarations. Thanks UncleO! — Eddie, Aug 12 '17 at 03:06

score 0 · Answer 2 · answered Aug 11 '17 at 22:27

0

There is much easier and straightforward way :)

uint32_t *engine_hours_raw = (uint32_t *)OneMessage.Msg.Data;
float engine_hours_uint  = *engine_hours_raw * 0.05;

answered Aug 11 '17 at 22:27

0___________

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How to combine 4 bytes and do math calculation in C using AVR

2 Answers2