nrf_802154_utils.h

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/*
 * Copyright (c) 2017, Nordic Semiconductor ASA
 * All rights reserved.
 *
 * SPDX-License-Identifier: BSD-3-Clause
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
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 * list of conditions and the following disclaimer.
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 * 2. Redistributions in binary form must reproduce the above copyright
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 * contributors may be used to endorse or promote products derived from this
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 *
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#ifndef NRF_802154_UTILS_H__
#define NRF_802154_UTILS_H__
 
#include "nrf_802154_assert.h"
#include <stdint.h>
#include <string.h>
#include "nrfx.h"
#include <soc/nrfx_coredep.h>
 
#define NRF_802154_RTC_FREQUENCY 32768UL
 
#define NRF_802154_US_PER_S 1000000ULL
 
#define NRF_802154_US_PER_TICK NRF_802154_RTC_TICKS_TO_US(1)
 
#define NRF_802154_FREQUENCY_US_PER_S_GCD_BITS 6
 
#define NRF_802154_DIVIDE_AND_CEIL(A, B) (((A) + (B)-1) / (B))
 
#define NRF_802154_RTC_TICKS_TO_US(ticks) \
 NRF_802154_DIVIDE_AND_CEIL( \
 (ticks) * (NRF_802154_US_PER_S >> NRF_802154_FREQUENCY_US_PER_S_GCD_BITS), \
 (NRF_802154_RTC_FREQUENCY >> NRF_802154_FREQUENCY_US_PER_S_GCD_BITS))
 
#define NUMELTS(X) (sizeof((X)) / sizeof(X[0]))
 
#define nrf_802154_delay_us(time_in_us) nrfx_coredep_delay_us(time_in_us)
 
typedef uint32_t nrf_802154_mcu_critical_state_t;
 
#define nrf_802154_mcu_critical_enter(mcu_critical_state) \
 do \
 { \
 (mcu_critical_state) = __get_PRIMASK(); \
 __disable_irq(); \
 } \
 while (0)
 
#define nrf_802154_mcu_critical_exit(mcu_critical_state) \
 do \
 { \
 __set_PRIMASK(mcu_critical_state); \
 } \
 while (0)
 
static inline uint64_t NRF_802154_US_TO_RTC_TICKS(uint64_t time)
{
 uint64_t t1, u1;
 uint64_t result;
 
 /* The required range for time is [0..315360000000000], and the calculation below is
 verified to work within a broader range [0...2^49 ~ 17 years].
 
 This first step in the calculation is to find out how many units
 of 15625 us there are in the input_us, because 512 RTC units
 correspond _exactly_ to 15625 us. The desired calculation is therefore
 t1 = time / 15625, but the division is slow and therefore let's calculate
 t1 = time * k instead. The constant k is 1/15625, shifted up by as many bits
 as possible without causing overflow during the calculation.
 
 49 bits are needed to store the maximum value that time can have, and the
 lowest 13 bits in that value can be shifted away because a minimum of 14 bits
 are needed to store the divisor.
 
 This means that the time can be reduced to 49 - 13 = 36 bits to make space
 for k.
 
 The most suitable number of shift for the value 1/15625 = 0.000064
 (binary 0.00000000000001000011000110111101111...) is 41, because that results
 in a 28-bit number that does not cause overflow in the multiplication.
 
 ((2^41)/15625) is equal to 0x8637bd0, and is written in a hexadecimal representation
 to show the bit width of the number. Shifting is limited to 41 bits because:
 1 The time uses up to 49 bits;
 2) The time can only be shifted down 13 bits to avoid shifting away
 a full unit of 15625 microseconds;
 3) The maximum value of the calculation would otherwise overflow (that is,
 (315360000000000 >> 13) * 0x8637bd0 = 0x4b300bfcd0aefde0 would no longer be less than
 0Xffffffffffffffff).
 
 There is a possible loss of precision, so that t1 will be up to 93*15625 _smaller_
 than the accurate number. This is taken into account in the next step.
 */
 
 t1 = ((time >> 13) * 0x8637bd0) >> 28; // ((time >> 13) * (2^41 / 15625)) >> (41 - 13)
 result = t1 * 512;
 t1 = time - t1 * 15625;
 
 /* This second step of the calculation is to find out how many RTC units there are
 still left in the remaining microseconds.
 
 ((2^56)/15625) is equal to 0x431bde82d7b, and is written in a hexadecimal representation
 to show the bit width of the number. Shifting 56 bits is determined by the worst
 case value of t1. The constant is selected by using the same methodology as in the
 first step of the calculation above.
 
 The possible loss of precision in the calculation above can make t1 93*15625 lower
 than it should have been here. The worst case found is that t1 can be 1453125, and
 therefore there is no overflow in the calculation
 1453125 * 0x431bde82d7b = 0x5cfffffffff76627 (that is, it is less than 0xffffffffffffffff).
 
 15625 below is the binary representation of 30.51757813 (11110.100001001)
 scaled up by 2^9, and the calculation below are therefore using that scale.
 
 Rounding up to the nearest RTC tick is done by adding the value of the least
 significant bits of the fraction (that is, adding the value of bits 1..47 of the scaled
 up timer unit size (2^47)) to the calculated value before scaling the final
 value down to RTC ticks.*/
 
 // ceil((time * (2^56 / 15625)) >> (56 - 9))
 NRF_802154_ASSERT(t1 <= 1453125);
 u1 = (t1 * 0x431bde82d7b); // (time * (2^56 / 15625))
 u1 += 0x7fffffffffff; // round up
 u1 >>= 47; // ceil(u1 >> (56 - 9))
 
 result += u1;
 
 return result;
}
 
#endif // NRF_802154_UTILS_H__