diff options
| -rw-r--r-- | fw/main.c | 44 | ||||
| -rw-r--r-- | hw/chibi/chibi_2024/rcalc.py | 127 |
2 files changed, 149 insertions, 22 deletions
@@ -74,32 +74,32 @@ int main(void) { SPI1->CR2 &= ~SPI_CR2_DS_Msk; SPI1->CR2 |= LL_SPI_DATAWIDTH_16BIT; /* FIXME maybe try w/o BIDI */ - SPI1->CR1 = SPI_CR1_BIDIMODE | SPI_CR1_BIDIOE | SPI_CR1_SSM | SPI_CR1_SSI | SPI_CR1_SPE | (1<<SPI_CR1_BR_Pos) | SPI_CR1_MSTR | SPI_CR1_CPOL | SPI_CR1_CPHA; + SPI1->CR1 = SPI_CR1_BIDIMODE | SPI_CR1_BIDIOE | SPI_CR1_SSM | SPI_CR1_SSI | SPI_CR1_SPE | (0<<SPI_CR1_BR_Pos) | SPI_CR1_MSTR | SPI_CR1_CPOL | SPI_CR1_CPHA; - int i = 0; - int val = 0x5555; + int val = 0xffff; GPIOA->BSRR = GPIO_BSRR_BR_6; + int j = 0; + int bval = 0x4000; while (42) { - if (i == 8) { - i = 0; - val = ~val; + for (int i=0; i<8; i++) { + spi_send(val); + spi_send(val); + strobe_leds(); + spi_send(0x0200 | bval | (0xff^(1<<i))); + strobe_aux(); + for(int i=0; i<10; i++) + tick(); + //j++; + if (j == 1000) { + j = 0; + if (bval == 0x4000) + bval = 0x8000; + else if (bval == 0x8000) + bval = 0x0000; + else + bval = 0x4000; + } } - spi_send((i&1 ? 0xfa00 : 0xf000) | (0xff^(1<<i))); - i++; // 1100'0100"0000'0000 - strobe_aux(); - - spi_send(val); - spi_send(val); - strobe_leds(); - /*if (i == 32) { - i = 0; - } - i++; - spi_send((i&16) ? 0 : (1<<i)); - spi_send((i&16) ? (1<<(i&15)) : 0); - strobe_leds(); - */ - LL_mDelay(200); } } diff --git a/hw/chibi/chibi_2024/rcalc.py b/hw/chibi/chibi_2024/rcalc.py new file mode 100644 index 0000000..3bba342 --- /dev/null +++ b/hw/chibi/chibi_2024/rcalc.py @@ -0,0 +1,127 @@ +#!/usr/bin/env python3 +""" + MBI5026 current set resistor calculations + + The MBI5026's output current is set by a current set via a single resistor + connected to its R_ext pin. + + To get a larger inter-frame dynamic range Megumin can switch between + four different current ranges. The ratio between one current range and the + next smaller one is r=1:8 (eq. -lg(r)=3 bit). This means at b=12bit BCM range we + get a minimum of + + bmin = b+lg(r) = 8bit @ r=1:16, b=12bit + + worst-case in the intermediate ranges using a static current setting. + Megumin uses BC847 small-signal NPN transistors to switch between three + current ranges: + + ┌─────────┐ + │ MBI5026 │ + │ │ + │ Rext─┼──┬──┤R1├───────────GND + │ │ │ + └─────────┘ ├──┤R2├──┤BC847├──GND + │ + ├──┤R3├──┤BC847├──GND + │ + └──┤R4├──┤BC847├──GND + + The transistors are used to select either or none of {R2, R3, R4}. This means + the R_ext pin sees either R1, R1||R2, R1||R3 or R1||R4. We don't do a full + R-2R or similar DAC configuration as we only have to maintain the ratio r + between ranges. + + Megumin's smallest BCM period is tb=250ns resulting in a base BCM rate of + 4MHz minus control overhead. This results in a BCM period and frame rate of + + Tm = tb*(2**b) = 1.024ms @ tb=250ns, b=12bit. + fm = 1/Tm ≈ 1kHz + + Now, if we want to modulate the display at a current range in between two + of the preset ranges, we can switch between both ranges with a ratio of + sqrt(r)=1:4 and still get a frame rate of + + f = fm/sqrt(r) = 250Hz @ fm=1kHz, r=1:16 + + Normalized to the larger of the two ranges (here r1=1) we get the following + equation for the ratio of the resulting modulated range: + + r_im1 = sqrt(r)*r1 = 0.25 @ r=1:16, r1=1 + r_im_tot = r_im1 + (1-sqrt(r))*r2 = 0.297 @ r2=r*r1 + + Including the 2 bit gained by inter-frame modulation this results in the + following basic ranges at framerate f=250Hz with a slight mid-range + discontinuity at the mixed ranges: + + Range max │ Total bits + ───────────┼────────────────────── + 1.000 │ 14 + 0.297 | 16 (14 at mid-range) + 0.250 | 14 + + The resistances of the resistors R1, R2, R3, R4 used are calculated in this + script. +""" + +prefixes = {' ': 1, 'k': 1e3, 'M': 1e6, 'm': 1e-3, 'μ': 1e-6, 'n': 1e-9} +def format_unit(val): + for prefix, magnitude in prefixes.items(): + if 1.0 <= val/magnitude < 1000.0: + return val/magnitude, prefix + else: + if val<1: + return val/10e-9, 'n' + else: + return val/10e6, 'M' + +def print_var(name, val, unit, **kwargs): + scaled, prefix = format_unit(val) + print('{} = {: >7.3f}{}{}'.format(name, scaled, prefix, unit), **kwargs) + +r = 1/16 +stages = 3 +mod_r = 1/8 +I_max_led = 0.01 +n_boards = 20 +n_digits_per_board = 8*4 +n_leds = n_boards*n_digits_per_board*8 +V_fw = 1.9 # V + +print('r = 1:{:.0f}'.format(1/r)) + +I_min_led = I_max_led*(r**(stages-1)) # A +I_max_mod = I_max_led/mod_r +I_min_mod = I_min_led/mod_r +print_var('I_max_led', I_max_led, 'A') +print_var('I_max_mod', I_max_mod, 'A') +print_var('I_min_mod', I_min_mod, 'A') +if (I_max_mod > 0.09): + print('\033[91mError: The MBI5026 has a maximum output current of 90mA!\033[0m') + +Vrext = 1.26 # V +# Iout = 15 * Vrext/Rext | acc. to MBI5026 datasheet +R1 = 15*Vrext/I_min_mod +Itot_1 = n_leds * mod_r * I_min_mod +Ptot_1 = Itot_1 * V_fw +print_var('R1', R1, 'Ω', end='\t') +print_var('I1', I_min_mod, 'A', end='\t') +print_var('Itot_1', Itot_1, 'A', end='\t') +print_var('Ptot_1', Ptot_1, 'W') + +for i in range(stages-2, -1, -1): + # Rpar = 15*Vrext/(I_max_mod*r) + # R1||R2 = 1/(1/R1 + 1/R2) =!= Rpar = 15*Vrext/(I_max_mod*r) + # ⇒ 1/R1 + 1/R2 = 1/(15*Vrext/(I_max_mod*r)) + # ⇒ 1/R2 = 1/(15*Vrext/(I_max_mod*r)) - 1/R1 + # ⇒ R2 = 1/((I_max_mod*r)/(15*Vrext) - 1/R1) + In = I_max_mod*(r**i) + Rn = 1/(In/(15*Vrext) - 1/R1) + Itot_n = n_leds * mod_r * In + Ptot_n = Itot_n * V_fw + scaled, prefix = format_unit(Rn) + print_var('R{}'.format(stages-i), Rn, 'Ω', end='\t') + print_var('I{}'.format(stages-i), In, 'A', end='\t') + print_var('Itot_{}'.format(stages-i), Itot_n, 'A', end='\t') + print_var('Ptot_{}'.format(stages-i), Ptot_n, 'W') + |