/* * Copyright © 2011 Keith Packard * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; version 2 of the License. * * This program is distributed in the hope that it will be useful, but * WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * General Public License for more details. * * You should have received a copy of the GNU General Public License along * with this program; if not, write to the Free Software Foundation, Inc., * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA. */ #ifndef AO_FLIGHT_TEST #include "ao.h" #endif /* * Current sensor values */ __pdata uint16_t ao_sample_tick; /* time of last data */ __pdata int16_t ao_sample_pres; __pdata int16_t ao_sample_alt; __pdata int16_t ao_sample_height; #if HAS_ACCEL __pdata int16_t ao_sample_accel; #endif __data uint8_t ao_sample_adc; /* * Sensor calibration values */ __pdata int16_t ao_ground_pres; /* startup pressure */ __pdata int16_t ao_ground_height; /* MSL of ao_ground_pres */ #if HAS_ACCEL __pdata int16_t ao_ground_accel; /* startup acceleration */ __pdata int16_t ao_accel_2g; /* factory accel calibration */ __pdata int32_t ao_accel_scale; /* sensor to m/s² conversion */ #endif static __pdata uint8_t ao_preflight; /* in preflight mode */ static __pdata uint16_t nsamples; __pdata int32_t ao_sample_pres_sum; #if HAS_ACCEL __pdata int32_t ao_sample_accel_sum; #endif static void ao_sample_preflight(void) { /* startup state: * * Collect 512 samples of acceleration and pressure * data and average them to find the resting values */ if (nsamples < 512) { #if HAS_ACCEL ao_sample_accel_sum += ao_sample_accel; #endif ao_sample_pres_sum += ao_sample_pres; ++nsamples; } else { ao_config_get(); #if HAS_ACCEL ao_ground_accel = ao_sample_accel_sum >> 9; ao_accel_2g = ao_config.accel_minus_g - ao_config.accel_plus_g; ao_accel_scale = to_fix32(GRAVITY * 2 * 16) / ao_accel_2g; #endif ao_ground_pres = ao_sample_pres_sum >> 9; ao_ground_height = ao_pres_to_altitude(ao_ground_pres); ao_preflight = FALSE; } } uint8_t ao_sample(void) { ao_wakeup(DATA_TO_XDATA(&ao_sample_adc)); ao_sleep(DATA_TO_XDATA(&ao_adc_head)); while (ao_sample_adc != ao_adc_head) { __xdata struct ao_adc *ao_adc; /* Capture a sample */ ao_adc = &ao_adc_ring[ao_sample_adc]; ao_sample_tick = ao_adc->tick; ao_sample_pres = ao_adc->pres; ao_sample_alt = ao_pres_to_altitude(ao_sample_pres); ao_sample_height = ao_sample_alt - ao_ground_height; #if HAS_ACCEL ao_sample_accel = ao_adc->accel; #if HAS_ACCEL_REF /* * Ok, the math here is a bit tricky. * * ao_sample_accel: ADC output for acceleration * ao_accel_ref: ADC output for the 5V reference. * ao_cook_accel: Corrected acceleration value * Vcc: 3.3V supply to the CC1111 * Vac: 5V supply to the accelerometer * accel: input voltage to accelerometer ADC pin * ref: input voltage to 5V reference ADC pin * * * Measured acceleration is ratiometric to Vcc: * * ao_sample_accel accel * ------------ = ----- * 32767 Vcc * * Measured 5v reference is also ratiometric to Vcc: * * ao_accel_ref ref * ------------ = ----- * 32767 Vcc * * * ao_accel_ref = 32767 * (ref / Vcc) * * Acceleration is measured ratiometric to the 5V supply, * so what we want is: * * ao_cook_accel accel * ------------- = ----- * 32767 ref * * * accel Vcc * = ----- * --- * Vcc ref * * ao_sample_accel 32767 * = ------------ * ------------ * 32767 ao_accel_ref * * Multiply through by 32767: * * ao_sample_accel * 32767 * ao_cook_accel = -------------------- * ao_accel_ref * * Now, the tricky part. Getting this to compile efficiently * and keeping all of the values in-range. * * First off, we need to use a shift of 16 instead of * 32767 as SDCC * does the obvious optimizations for byte-granularity shifts: * * ao_cook_accel = (ao_sample_accel << 16) / ao_accel_ref * * Next, lets check our input ranges: * * 0 <= ao_sample_accel <= 0x7fff (singled ended ADC conversion) * 0x7000 <= ao_accel_ref <= 0x7fff (the 5V ref value is close to 0x7fff) * * Plugging in our input ranges, we get an output range of 0 - 0x12490, * which is 17 bits. That won't work. If we take the accel ref and shift * by a bit, we'll change its range: * * 0xe000 <= ao_accel_ref<<1 <= 0xfffe * * ao_cook_accel = (ao_sample_accel << 16) / (ao_accel_ref << 1) * * Now the output range is 0 - 0x9248, which nicely fits in 16 bits. It * is, however, one bit too large for our signed computations. So, we * take the result and shift that by a bit: * * ao_cook_accel = ((ao_sample_accel << 16) / (ao_accel_ref << 1)) >> 1 * * This finally creates an output range of 0 - 0x4924. As the ADC only * provides 11 bits of data, we haven't actually lost any precision, * just dropped a bit of noise off the low end. */ ao_sample_accel = (uint16_t) ((((uint32_t) ao_sample_accel << 16) / (ao_accel_ref[ao_sample_adc] << 1))) >> 1; if (ao_config.pad_orientation != AO_PAD_ORIENTATION_ANTENNA_UP) ao_sample_accel = 0x7fff - ao_sample_accel; ao_adc->accel = ao_sample_accel; #endif #endif if (ao_preflight) ao_sample_preflight(); else ao_kalman(); ao_sample_adc = ao_adc_ring_next(ao_sample_adc); } return !ao_preflight; } void ao_sample_init(void) { nsamples = 0; ao_sample_pres_sum = 0; ao_sample_pres = 0; #if HAS_ACCEL ao_sample_accel_sum = 0; ao_sample_accel = 0; #endif ao_sample_adc = ao_adc_head; ao_preflight = TRUE; }