我正在使用带有原型的 SAMD21 xPlained pro 来获取信号，使用配置为收集 5500 个样本的内部 ADC 和 DMAC。然后样本通过目标 USB 传输到 PC 应用程序。
我为此编写的固件正在工作，但我注意到 DMA 有时会陷入忙碌状态。为了调试，我取消了通过 USB 传输到 PC 的部分。
我注意到 DMA 工作正常并将样本传输到内存，但是如果我将 USB 电缆连接到 PC（不通过 USB 传输数据），DMA 会不时卡住；但是一旦我断开（目标）USB 电缆，DMA 就会连续工作而不会陷入忙碌状态。
我怀疑它与 USB 和 ADC 的中断和优先级有关，它们都使用 DMA。我想我应该将 ADC 采样设置为最高优先级，这样 USB 就不会导致 DMA 卡在忙碌状态，但我找不到如何在代码中配置它（我正在使用 ASF）。
知道为什么插入 USB 会导致 DMA 偶尔卡在忙碌状态吗？这个问题是否应该与我怀疑的优先级有关，知道如何降低 USB 中断优先级吗？
代码：
void GSWatchMainProcess(void)
{
static int i=0;
GSPacket PacketToStream;
switch (KnockKnockStateMachine)
{
case InitKnockKnock:
KnockKnockStateMachine = KnockKnockStandby;
break;
case KnockKnockStandby:
if (StreamADC && !RequestForAcknowledge) KnockKnockStateMachine = WakeupAphrodite;
KnockKnockStateMachine = WakeupAphrodite; //this line was added to skip waiting for a command from the PC
break;
case WakeupAphrodite:
if (dma_is_busy(&example_resource))
{
KnockKnockStateMachine = AbortKnockKnock;
}
else
{
port_pin_set_output_level(PIN_PB09, true);
port_pin_set_output_level(LED_0_PIN, false);
transfer_is_done = false;
if(dma_start_transfer_job(&example_resource))
{
KnockKnockStateMachine = AbortKnockKnock;
}
}
KnockKnockStateMachine = WaitForBurstToEnd;
i=200000; //this counter is used as work-around to reset the DMA when it get stuck after timeout
break;
case WaitForBurstToEnd:
if (!dma_is_busy(&example_resource))
{
KnockKnockStateMachine = ProcessBurst;
}
if(!--i) // work-around to reset DMA when it get stuck
{
KnockKnockStateMachine = AbortKnockKnock;
}
break;
case ProcessBurst:
PacketToStream.Type=Stream;
PacketToStream.ContentLength=0;
for (i = 0; i<(ADC_SAMPLES); i++)
{
PacketToStream.Content[PacketToStream.ContentLength++] = adc_result_buffer[i] / 256;
PacketToStream.Content[PacketToStream.ContentLength++] = adc_result_buffer[i] % 256;
if(PacketToStream.ContentLength>=PACKET_MAX_SIZE)
{
//SendViaGSWatchLink(PacketToStream);
PacketToStream.ContentLength=0;
}
}
//if(PacketToStream.ContentLength>0) SendViaGSWatchLink(PacketToStream);
RequestForAcknowledge = true;
KnockKnockStateMachine = KnockKnockStandby;
break;
case AbortKnockKnock:
dma_abort_job(&example_resource);
dma_free(&example_resource);
port_pin_set_output_level(PIN_PB09, false);
port_pin_set_output_level(LED_0_PIN, true);
transfer_is_done = true;
configure_dma_resource(&example_resource);
setup_transfer_descriptor(&DMA_ADC_descriptor);
dma_add_descriptor(&example_resource, &DMA_ADC_descriptor);
dma_register_callback(&example_resource, transfer_done, DMA_CALLBACK_TRANSFER_DONE);
dma_enable_callback(&example_resource, DMA_CALLBACK_TRANSFER_DONE);
system_interrupt_enable_global();
KnockKnockStateMachine = WakeupAphrodite;
break;
}
}
void transfer_done(struct dma_resource* const resource )
{
transfer_is_done = true;
port_pin_set_output_level(PIN_PB09, false);
port_pin_set_output_level(LED_0_PIN, true);
}
void setup_transfer_descriptor(DmacDescriptor *descriptor)
{
struct dma_descriptor_config descriptor_config;
dma_descriptor_get_config_defaults(&descriptor_config);
descriptor_config.beat_size = DMA_BEAT_SIZE_HWORD;
descriptor_config.block_transfer_count = ADC_SAMPLES;
descriptor_config.dst_increment_enable = true;
descriptor_config.src_increment_enable = false;
descriptor_config.source_address = (uint32_t)(&adc_instance.hw->RESULT.reg);
//descriptor_config.destination_address = (uint32_t)adc_result_buffer + 2 * ADC_SAMPLES;
descriptor_config.destination_address = (uint32_t)adc_result_buffer + sizeof(adc_result_buffer);
dma_descriptor_create(descriptor, &descriptor_config);
}
void configure_dma_resource(struct dma_resource *resource)
{
struct dma_resource_config config_dma;
dma_get_config_defaults(&config_dma);
config_dma.peripheral_trigger = ADC_DMAC_ID_RESRDY;
config_dma.trigger_action = DMA_TRIGGER_ACTON_BEAT;
config_dma.priority = DMA_PRIORITY_LEVEL_3;
dma_allocate(resource, &config_dma);
}
void configure_adc(void)
{
struct adc_config config_adc;
adc_get_config_defaults(&config_adc);
// config_adc.gain_factor = ADC_GAIN_FACTOR_DIV2; //TODO: check if we need this feature
config_adc.clock_prescaler = ADC_CLOCK_PRESCALER_DIV32; //TODO: check whether it possible to work with 8
config_adc.reference = ADC_REFERENCE_INTVCC0; //ADC_REFERENCE_INT1V; //ADC_REFERENCE_INTVCC1;
config_adc.positive_input = ADC_POSITIVE_INPUT_PIN8;
config_adc.negative_input = ADC_NEGATIVE_INPUT_GND;
config_adc.resolution = ADC_RESOLUTION_12BIT;
config_adc.sample_length = 0; //4
config_adc.differential_mode = false;
config_adc.freerunning = true;
adc_init(&adc_instance, ADC, &config_adc);
adc_enable(&adc_instance);
}