Optimize ADC temperature search
Use a binary search to find our target temperate in fewer comparisons. The search algorithm is ostensibly slower because it involves a division, but it's a div-by-two so should be optimized into a simple bit-shift. Fewer comparisons involves fewer pgm_read_words and should be faster overall, but the gain is arguably tiny. Note by Traumflug: According to @Wurstnase's performance measurements, gain is actually pretty huge. Temp conversions 0..1024 before this and the previous few commits: minimum: 1011.84 clock cycles. maximum: 1993.92 clock cycles. average: 1768.85 clock cycles. Now: minimum: 437.72 clock cycles. maximum: 494.76 clock cycles. average: 470.278 clock cycles. That's a speedup by factor 3.7 on average!
This commit is contained in:
Phil Hord
Markus Hitter
parent
65857b17dc
commit
fbe1832467
120
temp.c
120
temp.c
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@ -186,72 +186,72 @@ static uint16_t mcp3008_read(uint8_t channel) {
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*/
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#if defined TEMP_THERMISTOR || defined TEMP_MCP3008
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static uint16_t temp_table_lookup(uint16_t temp, uint8_t sensor) {
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uint8_t j;
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uint8_t lo, hi;
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uint8_t table_num = temp_sensors[sensor].additional;
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for (j = 1; j < NUMTEMPS; j++) {
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if (pgm_read_word(&(temptable[table_num][j][0])) > temp) {
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if (DEBUG_PID && (debug_flags & DEBUG_PID))
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sersendf_P(PSTR("pin:%d Raw ADC:%d table entry: %d"),
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temp_sensors[sensor].temp_pin, temp, j);
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if (sizeof(temptable[0][0]) == 2 * sizeof(uint16_t)) {
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/**
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This code handles temptables with value pairs and is deprecated.
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It's kept for compatibility with legacy, handcrafted tables, only.
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The new code expects tables with triples, nevertheless it's smaller
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and also faster. Configtool was already changed to create tables
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with triples, only.
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*/
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// Wikipedia's example linear interpolation formula.
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// y = ((x - x₀)y₁ + (x₁-x)y₀) / (x₁ - x₀)
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// y = temp
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// x = ADC reading
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// x₀= temptable[j-1][0]
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// x₁= temptable[j][0]
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// y₀= temptable[j-1][1]
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// y₁= temptable[j][1]
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temp = (
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// ((x - x₀)y₁
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((uint32_t)temp - pgm_read_word(&(temptable[table_num][j-1][0]))) *
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pgm_read_word(&(temptable[table_num][j][1]))
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// +
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+
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// (x₁-x)y₀)
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(pgm_read_word(&(temptable[table_num][j][0])) - (uint32_t)temp) *
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pgm_read_word(&(temptable[table_num][j - 1][1])))
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// /
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/
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// (x₁ - x₀)
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(pgm_read_word(&(temptable[table_num][j][0])) -
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pgm_read_word(&(temptable[table_num][j - 1][0])));
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} else
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if (sizeof(temptable[0][0]) == 3 * sizeof(uint16_t)) {
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// Linear interpolation using pre-computed slope.
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// y = y₁ - (x - x₁) * d₁
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#define X1 pgm_read_word(&(temptable[table_num][j][0]))
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#define Y1 pgm_read_word(&(temptable[table_num][j][1]))
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#define D1 pgm_read_word(&(temptable[table_num][j][2]))
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temp = Y1 - ((((int32_t)temp - X1) * D1 + (1 << 7)) >> 8);
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}
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if (DEBUG_PID && (debug_flags & DEBUG_PID))
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sersendf_P(PSTR(" temp:%d.%d"), temp / 4, (temp % 4) * 25);
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// Value found, no need to read the table further.
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break;
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}
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// Binary search for table value bigger than our target.
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//
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// lo = index of highest entry less than target.
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// hi = index of lowest entry greater than or equal to target.
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for (lo = 0, hi = NUMTEMPS - 1; hi - lo > 1; ) {
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uint8_t j = lo + (hi - lo) / 2 ;
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if (pgm_read_word(&(temptable[table_num][j][0])) >= temp)
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hi = j ;
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else
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lo = j ;
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}
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if (DEBUG_PID && (debug_flags & DEBUG_PID))
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sersendf_P(PSTR(" Sensor:%d\n"), sensor);
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sersendf_P(PSTR("pin:%d Raw ADC:%d table entry: %d"),
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temp_sensors[sensor].temp_pin, temp, hi);
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// Clamp for overflows.
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if (j == NUMTEMPS)
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temp = temptable[table_num][NUMTEMPS - 1][1];
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if (sizeof(temptable[0][0]) == 2 * sizeof(uint16_t)) {
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/**
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This code handles temptables with value pairs and is deprecated.
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It's kept for compatibility with legacy, handcrafted tables, only.
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The new code expects tables with triples, nevertheless it's smaller
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and also faster. Configtool was already changed to create tables
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with triples, only.
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*/
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// Wikipedia's example linear interpolation formula.
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// y = ((x - x₀)y₁ + (x₁-x)y₀) / (x₁ - x₀)
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// y = temp
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// x = ADC reading
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// x₀= temptable[lo][0]
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// x₁= temptable[hi][0]
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// y₀= temptable[lo][1]
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// y₁= temptable[hi][1]
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temp = (
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// ((x - x₀)y₁
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((uint32_t)temp - pgm_read_word(&(temptable[table_num][lo][0]))) *
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pgm_read_word(&(temptable[table_num][hi][1]))
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// +
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+
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// (x₁-x)y₀)
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(pgm_read_word(&(temptable[table_num][hi][0])) - (uint32_t)temp) *
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pgm_read_word(&(temptable[table_num][lo][1])))
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// /
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/
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// (x₁ - x₀)
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(pgm_read_word(&(temptable[table_num][hi][0])) -
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pgm_read_word(&(temptable[table_num][lo][0])));
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} else
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if (sizeof(temptable[0][0]) == 3 * sizeof(uint16_t)) {
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// Linear interpolation using pre-computed slope.
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// y = y₁ - (x - x₁) * d₁
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#define X1 pgm_read_word(&(temptable[table_num][hi][0]))
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#define Y1 pgm_read_word(&(temptable[table_num][hi][1]))
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#define D1 pgm_read_word(&(temptable[table_num][hi][2]))
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temp = Y1 - ((((int32_t)temp - X1) * D1 + (1 << 7)) >> 8);
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}
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if (DEBUG_PID && (debug_flags & DEBUG_PID))
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sersendf_P(PSTR(" temp:%d.%d"), temp / 4, (temp % 4) * 25);
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if (DEBUG_PID && (debug_flags & DEBUG_PID))
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sersendf_P(PSTR(" Sensor:%d\n"), sensor);
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return temp;
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}
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