blob: 9c05dd803ea152a5792df2017f2113b24c394a16 [file] [log] [blame]
// Copyright 2022 The Pigweed Authors
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#include "pw_spin_delay/delay.h"
#include <cstddef>
#include <cstdint>
namespace pw::spin_delay {
// !!!WARNING!!!: This delay is not truly accurate! It's mostly just a rough
// estimate! Also, it only works in a baremetal context with no interrupts
// getting in the way or threads getting CPU time.
// TODO(amontanez): Replace this implementation with a loop checking a
// pw_chrono clock.
void WaitMillis(size_t delay_ms) {
// Default core clock. This is technically not a constant, but since Pigweed
// doesn't change the system clock a constant will suffice.
constexpr uint32_t kSystemCoreClock = 16000000;
constexpr uint32_t kCyclesPerMs = kSystemCoreClock / 1000;
// This is not totally accurate, but is close enough.
for (size_t i = 0; i < delay_ms; i++) {
// Do a 4 instruction loop enough times to be running for a millisecond.
// This is set up with assembly rather than a regular loop to make the
// instruction count predictable (no compiler variation).
uint32_t cycles = kCyclesPerMs;
asm volatile(
" mov r0, %[cycles] \n"
" mov r1, #0 \n"
"loop: \n"
" cmp r1, r0 \n"
" itt lt \n"
" addlt r1, r1, #4 \n"
" blt loop \n"
// clang-format off
: /*output=*/
: /*input=*/[cycles]"r"(cycles)
: /*clobbers=*/"r0", "r1"
// clang-format on
} // namespace pw::spin_delay