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ads131m0x: Optimized timer scheduling to attain higher sampling rates #7273

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ads131m0x: Optimized timer scheduling to attain higher sampling rates #7273

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103 changes: 72 additions & 31 deletions src/sensor_ads131m0x.c
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
// This file may be distributed under the terms of the GNU GPLv3 license.

#include <stdint.h>
#include <string.h> // memset
#include "basecmd.h" // oid_alloc
#include "board/gpio.h" // gpio_in_setup
#include "board/irq.h" // irq_disable
Expand All @@ -21,12 +22,22 @@ struct ads131m0x_adc {
uint32_t rest_ticks;
struct gpio_in data_ready;
struct spidev_s *spi;
uint8_t pending_flag, data_count, was_reset;
uint8_t frame_size, data_offset;
uint8_t flags, continuous_reads;
uint8_t channel, num_channels;
struct sensor_bulk sb;
struct trigger_analog *ta;
};

enum {
RESCHEDULE_TIMER = 1
};

enum {
FLAG_PENDING = 1<<0,
FLAG_RESET = 1<<1
};

// Internal errors transmitted to trigger_analog
enum {
SE_ERROR_CRC = 1,
Expand Down Expand Up @@ -71,14 +82,13 @@ ads131m0x_is_data_ready(struct ads131m0x_adc *ads131m0x) {

// CCITT CRC-16 calculation (polynomial 0x1021, init 0xFFFF, optimized version)
static uint16_t
ads131m0x_crc16_ccitt(const uint8_t *data, uint8_t len)
ads131m0x_crc16_ccitt(const uint8_t *data, uint_fast8_t len)
{
uint16_t crc = 0xFFFF;
while (len--) {
uint8_t x = (crc >> 8) ^ *data++;
uint16_t x = (crc >> 8) ^ *data++;
x ^= x >> 4;
crc = (crc << 8) ^ ((uint16_t)x << 12) ^ ((uint16_t)x << 5)
^ (uint16_t)x;
crc = (crc << 8) ^ (x << 12) ^ (x << 5) ^ x;
}
return crc;
}
Comment on lines -74 to 94

@KevinOConnor KevinOConnor May 25, 2026

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Just FYI, modern MCUs are really slow when doing math on uint8_t and uint16_t. The ARM mcus don't have 8bit or 16bit math operations (like add, subtract, shift, multiply). So, any math done on a uint16_t or uint8_t the resulting assembler has to scale the value to a uint32_t, perform the actual math, and then truncate the results back to 8bit (or 16bit).

So, in a nutshell, you almost assuredly want to make all these values uint32_t and change the last line to be return crc & 0xffff. Alternatively, if you care about performance on AVR (it's hard to imagine someone hooking one of these sensors up to an old 8-bit AVR cpu, but whatever) then you'll likely want to use uint_fast16_t and still change the last line to return crc & 0xffff.

Cheers,
-Kevin

Expand All @@ -92,20 +102,32 @@ ads131m0x_has_crc_error(uint8_t *msg, uint8_t frame_size)
return calc_crc != frame_crc;
}

// Helper code to reschedule the ads131m0x_event() timer
static void
ads131m0x_reschedule_timer(struct ads131m0x_adc *ads131m0x)
{
// The event handler will check the data availability
ads131m0x->flags &= ~FLAG_PENDING;
irq_disable();
uint32_t waketime = timer_read_time() + ads131m0x->rest_ticks;
if (timer_is_before(ads131m0x->timer.waketime, waketime))
ads131m0x->timer.waketime = waketime;
sched_add_timer(&ads131m0x->timer);
irq_enable();
}

// Event handler that periodically wakes the capture task
static uint_fast8_t
ads131m0x_event(struct timer *timer)
{
struct ads131m0x_adc *ads131m0x = container_of(
timer, struct ads131m0x_adc, timer);
uint32_t rest_ticks = ads131m0x->rest_ticks;
if (ads131m0x->pending_flag) {
ads131m0x->sb.possible_overflows++;
rest_ticks *= 4;
} else if (ads131m0x_is_data_ready(ads131m0x)) {
ads131m0x->pending_flag = 1;
if (ads131m0x_is_data_ready(ads131m0x)) {
ads131m0x->flags |= FLAG_PENDING;
sched_wake_task(&wake_ads131m0x);
rest_ticks *= 8;
ads131m0x->timer.waketime += rest_ticks << 3;
return SF_DONE;
}
ads131m0x->timer.waketime += rest_ticks;
return SF_RESCHEDULE;
Expand All @@ -131,36 +153,34 @@ ads131m0x_publish_error(struct ads131m0x_adc *ads131m0x, uint8_t oid
}

// Send a NULL command frame and read conversion data from all channels.
// Returns sample from the configured channel.
static void
// Appends a sample from the configured channel to the buffer.
// Returns RESCHEDULE_TIMER if a new timer even task should be rescheduled.
static uint_fast8_t
ads131m0x_read_adc(struct ads131m0x_adc *ads131m0x, uint8_t oid)
{
uint8_t frame_size = (ADS131M0X_STATUS_WORDS + ads131m0x->num_channels
+ ADS131M0X_CRC_WORDS) * ADS131M0X_WORD_SIZE;
uint8_t msg[ADS131M0X_MAX_FRAME_SIZE] = {0};
uint8_t frame_size = ads131m0x->frame_size;
// Frame layout: status(3), channel[0](3), channel[1](3), ..., crc(3)
// Data is 24-bit two's complement, MSB first
uint8_t msg[frame_size];
memset(msg, 0, frame_size);
spidev_transfer(ads131m0x->spi, 1, frame_size, msg);
ads131m0x->pending_flag = 0;
barrier();

// Validate CRC
if (ads131m0x_has_crc_error(msg, frame_size)) {
ads131m0x_publish_error(ads131m0x, oid, SE_ERROR_CRC);
return;
return RESCHEDULE_TIMER;
}

// Check for unexpected sensor reset
if (msg[0] & STATUS_RESET_BIT) {
ads131m0x->was_reset = 1;
ads131m0x->flags |= FLAG_RESET;
}
if (ads131m0x->was_reset) {
if (ads131m0x->flags & FLAG_RESET) {
ads131m0x_publish_error(ads131m0x, oid, SE_ERROR_RESET);
return;
return RESCHEDULE_TIMER;
}

// Frame layout: status(3), channel[0](3), channel[1](3), ..., crc(3)
// Data is 24-bit two's complement, MSB first
uint8_t data_offset = ADS131M0X_STATUS_WORDS * ADS131M0X_WORD_SIZE
+ ads131m0x->channel * ADS131M0X_WORD_SIZE;
uint_fast8_t data_offset = ads131m0x->data_offset;
uint32_t raw = ((uint32_t)msg[data_offset] << 16)
| ((uint32_t)msg[data_offset + 1] << 8)
| ((uint32_t)msg[data_offset + 2]);
Expand All @@ -172,6 +192,21 @@ ads131m0x_read_adc(struct ads131m0x_adc *ads131m0x, uint8_t oid)
trigger_analog_update(ads131m0x->ta, raw);
buffer_append_int32(&ads131m0x->sb, raw);
ads131m0x_flush_buffer(ads131m0x, oid);
if (unlikely(!ads131m0x_is_data_ready(ads131m0x))) {
ads131m0x->continuous_reads = 0;
return RESCHEDULE_TIMER;
}
if (unlikely(ads131m0x->continuous_reads >= 2)) {
// ADS131M0x FIFO can hold two readings
ads131m0x->sb.possible_overflows++;
} else {
ads131m0x->continuous_reads++;
}
// Delay the next timer event even further
ads131m0x->timer.waketime += ads131m0x->rest_ticks << 3;
// More data is available, wake up immediately
sched_wake_task(&wake_ads131m0x);
return 0;
}

// Create an ads131m0x sensor
Expand All @@ -181,7 +216,7 @@ command_config_ads131m0x(uint32_t *args)
struct ads131m0x_adc *ads131m0x = oid_alloc(args[0]
, command_config_ads131m0x, sizeof(*ads131m0x));
ads131m0x->timer.func = ads131m0x_event;
ads131m0x->pending_flag = 0;
ads131m0x->flags = 0;
ads131m0x->spi = spidev_oid_lookup(args[1]);
ads131m0x->channel = (uint8_t)(args[2] & 0xFF);
ads131m0x->num_channels = (uint8_t)(args[3] & 0xFF);
Expand All @@ -190,6 +225,10 @@ command_config_ads131m0x(uint32_t *args)
ads131m0x->channel >= ads131m0x->num_channels)
shutdown("Invalid ads131m0x channels configuration");
ads131m0x->data_ready = gpio_in_setup(args[4], 0);
ads131m0x->frame_size = (ADS131M0X_STATUS_WORDS + ads131m0x->num_channels
+ ADS131M0X_CRC_WORDS) * ADS131M0X_WORD_SIZE;
ads131m0x->data_offset =
(ADS131M0X_STATUS_WORDS + ads131m0x->channel) * ADS131M0X_WORD_SIZE;
}
DECL_COMMAND(command_config_ads131m0x, "config_ads131m0x oid=%c"
" spi_oid=%c channel=%c num_channels=%c data_ready_pin=%u");
Expand All @@ -212,14 +251,14 @@ command_query_ads131m0x(uint32_t *args)
uint8_t oid = args[0];
struct ads131m0x_adc *ads131m0x = oid_lookup(oid, command_config_ads131m0x);
sched_del_timer(&ads131m0x->timer);
ads131m0x->pending_flag = 0;
ads131m0x->flags = 0;
ads131m0x->rest_ticks = args[1];
if (!ads131m0x->rest_ticks) {
// End measurements
return;
}
// Start new measurements
ads131m0x->was_reset = 0;
ads131m0x->continuous_reads = 0;
sensor_bulk_reset(&ads131m0x->sb);
irq_disable();
ads131m0x->timer.waketime = timer_read_time() + ads131m0x->rest_ticks;
Expand Down Expand Up @@ -251,8 +290,10 @@ ads131m0x_capture_task(void)
uint8_t oid;
struct ads131m0x_adc *ads131m0x;
foreach_oid(oid, ads131m0x, command_config_ads131m0x) {
if (ads131m0x->pending_flag)
ads131m0x_read_adc(ads131m0x, oid);
uint_fast8_t flags = ads131m0x->flags;
if (flags & FLAG_PENDING)
if (ads131m0x_read_adc(ads131m0x, oid) == RESCHEDULE_TIMER)
ads131m0x_reschedule_timer(ads131m0x);
}
}
DECL_TASK(ads131m0x_capture_task);
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