Why does CORDIC give not accurate results?

[yonatan]

Hi all.

I have this piece of code:

	__HAL_RCC_CORDIC_CLK_ENABLE();

	CORDIC->CSR |= CORDIC_CSR_NARGS;
	CORDIC->CSR |= (CORDIC_CSR_PRECISION_3 | CORDIC_CSR_PRECISION_2 | CORDIC_CSR_PRECISION_1 | CORDIC_CSR_PRECISION_0);
	CORDIC->CSR |= CORDIC_CSR_FUNC_1; // Phase function (atan2)

	int32_t a = 1000;
	int32_t b = 1000;

	CORDIC->WDATA = a;
	CORDIC->WDATA = b;

	volatile double result_from_hw = (double)CORDIC->RDATA / 2147483648.0 * 180.0;
	volatile double result_from_sw = ((atan2((double)a, (double)b)) / M_PI) * (180.0);

	__NOP();
	__NOP();
	__NOP();

The result from SW is 45 (as expected).

However the result from HW is 43.2.

WHY??

[AScha.3]

Maybe its just the way you let it calculate: first divide, then multiply = bad for precision;

try 

(double)(CORDIC->RDATA  * 180.0 )  / 2147483648.0 ;

[yonatan]

I tried but it does not work :(

Furthermore, I checked the register value:

		volatile uint32_t register_value = CORDIC->RDATA;
		volatile double result_from_hw = (double)(register_value  * 180.0 )  / 2147483648.0;
		volatile double result_from_sw = ((atan2((double)a, (double)b)) / M_PI) * (180.0);

And I got that register_value = 0x1ec0e600.

However I was expecting to get 0x200000 (As 45 is quarter of 180 so 0.25 * 2 ^ 31 = 2 ^29)