Cortex-M0+ Delay Routine Inaccuracy Due to Instruction Cycle Miscalculation
The Cortex-M0+ is a popular choice for low-power, cost-sensitive embedded systems due to its simplicity and efficiency. However, its lack of advanced features like the Data Watchpoint and Trace (DWT) unit, which includes the cycle counter (CYCCNT), makes implementing precise delays more challenging. In this scenario, the goal is to create a microsecond-level delay routine using a simple loop with NOP instructions and cycle counting. The initial implementation involves a while loop that decrements a counter (CyclesToDelay) and uses NOP instructions to introduce delays. The total cycle count for the loop is calculated based on the assumption that each instruction takes a fixed number of cycles, as documented in the ARM Cortex-M0+ instruction set summary.
However, the observed delay does not match the expected values, leading to inaccuracies. This discrepancy arises from several factors, including incorrect cycle count assumptions, pipeline effects, and potential compiler optimizations. The Cortex-M0+ has a 2-stage pipeline (fetch and execute), which can introduce variability in instruction timing, especially when branches are involved. Additionally, the NOP instruction, while typically taking 1 cycle, may not always contribute to the delay as expected due to pipeline stalls or other microarchitectural behaviors.
To understand the issue fully, we must analyze the instruction sequence and its timing. The loop consists of the following instructions:
NOP: 1 cycle (assumed)LDR R1, [R0]: 2 cycles (load from memory)SUBS R1, R1, #1: 1 cycle (subtract with set flags)STR R1, [R0]: 2 cycles (store to memory)LDR R1, [R0]: 2 cycles (load from memory)CMP R1, #0: 1 cycle (compare)BNE 0x000273F8: 3 cycles (branch if not equal)
The total cycle count for one iteration of the loop is calculated as 12 cycles. However, this calculation assumes ideal conditions, which may not hold in practice. For example, memory access instructions like LDR and STR can take longer if the memory system is busy or if there are cache misses. Additionally, the branch instruction (BNE) can introduce pipeline stalls if the branch prediction fails, adding extra cycles to the loop.
Pipeline Effects and Memory Access Latency Impacting Delay Accuracy
The Cortex-M0+ pipeline consists of two stages: fetch and execute. While this simple pipeline reduces complexity and power consumption, it also introduces variability in instruction timing. When a branch instruction is encountered, the pipeline must flush and refetch instructions from the new address if the branch is taken. This process can add extra cycles to the loop, especially if the branch target is not aligned with the pipeline’s prefetch mechanism.
In the delay routine, the BNE instruction at the end of the loop is a conditional branch that depends on the result of the CMP instruction. If the branch is taken, the pipeline must refetch instructions from the start of the loop, introducing a stall. The number of additional cycles depends on the pipeline state and the alignment of the branch target. This variability makes it difficult to predict the exact cycle count for each loop iteration.
Memory access instructions (LDR and STR) also contribute to timing variability. The Cortex-M0+ uses a von Neumann architecture, where instructions and data share the same memory bus. If the memory system is busy servicing other requests, such as DMA transfers or interrupt handlers, the LDR and STR instructions may take longer to complete. Additionally, if the memory being accessed is located in a slower memory region (e.g., external flash or RAM), the access latency will increase, further impacting the delay accuracy.
Compiler optimizations can also affect the timing of the delay routine. For example, the compiler may reorder instructions or eliminate redundant operations to improve performance. While these optimizations are generally beneficial, they can introduce unintended side effects in timing-critical code. In this case, the __no_operation() macro (which typically expands to NOP) may be optimized away if the compiler determines that it has no effect on the program’s behavior. This optimization would reduce the number of cycles in the loop, leading to shorter delays than expected.
Implementing Precise Delays Using Instruction Timing and Pipeline Considerations
To achieve accurate microsecond delays on the Cortex-M0+, we must account for pipeline effects, memory access latency, and compiler optimizations. One approach is to use a combination of NOP instructions and carefully timed loops to create a predictable delay. However, this method requires a deep understanding of the processor’s microarchitecture and instruction timing.
First, we need to determine the exact number of cycles required for the desired delay. Given a core clock frequency of 48 MHz, one clock cycle corresponds to approximately 20.83 nanoseconds. A delay of 1 microsecond requires 48 clock cycles. To achieve this, we can unroll the loop and use a sequence of NOP instructions to fill the required number of cycles. For example:
MOV R0, #48 ; Load the number of cycles into R0
delay_loop:
NOP ; 1 cycle
SUBS R0, R0, #1 ; 1 cycle
BNE delay_loop ; 3 cycles if taken, 1 cycle if not
In this example, each iteration of the loop takes 5 cycles (1 for NOP, 1 for SUBS, and 3 for BNE if the branch is taken). To achieve a 1-microsecond delay, we need approximately 9.6 iterations of the loop. Since we cannot have a fractional iteration, we round up to 10 iterations, resulting in a total of 50 cycles (10 iterations * 5 cycles per iteration). This slight overestimation ensures that the delay meets or exceeds the required duration.
To further improve accuracy, we can adjust the number of NOP instructions in the loop. For example, adding an extra NOP instruction increases the cycle count per iteration to 6, reducing the number of iterations required for a 1-microsecond delay to 8 (48 cycles / 6 cycles per iteration). This adjustment reduces the impact of pipeline stalls and branch mispredictions, resulting in a more consistent delay.
Another approach is to use a hardware timer if available. While the Cortex-M0+ lacks the DWT unit found in higher-end Cortex-M processors, it typically includes a SysTick timer, which can be used to generate precise delays. The SysTick timer is a 24-bit down-counter that can be configured to generate interrupts at regular intervals. By configuring the SysTick timer to count down at the core clock frequency, we can create delays with microsecond precision. For example:
void delay_us(uint32_t microseconds) {
uint32_t start = SysTick->VAL; // Read the current SysTick value
uint32_t ticks = microseconds * (SystemCoreClock / 1000000); // Calculate the number of ticks
while ((start - SysTick->VAL) < ticks) {
// Wait for the specified number of ticks to elapse
}
}
In this example, the delay_us function uses the SysTick timer to create a delay of the specified number of microseconds. The SysTick->VAL register contains the current value of the SysTick timer, which decrements at the core clock frequency. By comparing the initial value of SysTick->VAL with its current value, we can determine when the specified number of ticks has elapsed.
In conclusion, implementing precise microsecond delays on the Cortex-M0+ requires careful consideration of instruction timing, pipeline effects, and memory access latency. By unrolling loops, adjusting the number of NOP instructions, and leveraging hardware timers like SysTick, we can achieve accurate and deterministic delays even in the absence of advanced features like the DWT unit.
