Software SPI ===========
Software SPI is likely to be of use only for ESP8266, but is enabled for all architectures.
The ESP8266 can manage a minimum of about 70ns between bit edges, with a maximum of about 1.8 MBit/s at normal CPU clock frequency or 2.5 MHz if CPU clock set to fast.
SPISOFT_DELAY_VARIABLE
default: 0 (disabled)
This setting must be enabled in order to use :cppSPISoft::setDelay. The base clock speed (i.e. delay=0) is reduced to about 1.4 MBit/s (2.1 MBit/s for fast CPU).
The appropriate delay factor can be provided in the :cppSPISoft constructor, or by calling :cppSPISoft::setDelay.
With the ESP8266, then the appropriate delay factor is calculated automatically from the speed passed in :cppSPISettings. This will override any previously set delay factor. Use a speed of 0 to use the manually configured delay value.
SPISOFT_DELAY_FIXED
default: 0 (disabled) maximum: 10
Adds the requested number of 'NOP' CPU instructions to every clock transition.
Has less impact than enabling variable delays.
Use variable delays if 10 is insufficient.
Will be ignored if variable delays are enabled.
The following information has been obtained by measurement. The test application was built using:
make SPISOFT_CALIBRATE=1 SPISOFT_DELAY_VARIABLE=1 CPU_FAST=0then flashed to an ESP-12F. An oscilliscope was hooked up to the SCK output and the maximum frequency at each delay setting noted. (Not too awkward, just note down the frequency each time the figure changes.)
Entering these into the spisoft.ods spreadsheet allows the values to be charted and co-efficients calculated.
Curiously values from 8 upward form a straight line so we can calculate those. Smaller values are non-linear so a small lookup table is used.
Repeat for CPU_FAST=1.
Note that these figures are concerned with maximum (i.e. burst) frequencies at each delay setting, since that is what devices are sensitive to.
| . | Frequency (kHz) |
|---|---|
| Delay | Variable Fixed |
| ======= | ========== ======= |
| 0 | 1357 2288 |
| 1 | 1315 1869 |
| 2 | 1215 1824 |
| 3 | 1129 1780 |
| 4 | 1055 1744 |
| 5 | 990 1709 |
| 6 | 932 1639 |
| 7 | 881 1575 |
| 8 | 819 1511 |
| 9 | 743 1462 |
| 10 | 680 1408 |
| 11 | 626 1338 |
| 12 | 578 1294 |
| 20 | 367 |
| 30 | 251 |
| 40 | 192 |
| 50 | 155 |
| 60 | 129 |
| 70 | 112 |
| 80 | 98 |
| 90 | 87 |
| 100 | 79 |
| . | Frequency (kHz) |
|---|---|
| Delay | Variable Fixed |
| ======= | ========== ======= |
| 0 | 2133 2857 |
| 1 | 2126 2510 |
| 2 | 2106 2500 |
| 3 | 1958 2439 |
| 4 | 1861 2429 |
| 5 | 1745 2372 |
| 6 | 1668 2353 |
| 7 | 1573 2294 |
| 8 | 1482 2283 |
| 9 | 1357 2242 |
| 10 | 1250 2222 |
| 11 | 1160 2174 |
| 12 | 1081 2165 |
| 20 | 702 |
| 30 | 488 |
| 40 | 374 |
| 50 | 303 |
| 60 | 255 |
| 70 | 220 |
| 80 | 193 |
| 90 | 172 |
| 100 | 156 |
soft_spi