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* Reduce the IRAM (and heap) usage of I2C code The I2C code takes a large chunk of IRAM space. Attempt to reduce the size of the routines without impacting functionality. First, remove the `static` classifier on the sda/scl variables in the event handlers. The first instructions in the routines overwrite the last value stored in them, anyway, and their addresses are never taken. * Make most variables ints, not uint8_ts Where it doesn't make a functional difference, make global variables ints and not unit8_t. Bytewide updates and extracts require multiple instructions and hence increase IRAM usage as well as runtime. * Make local flag vars int Sketch uses 270855 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27940 bytes (34%) of dynamic memory, leaving 53980 bytes for local variables. Maximum is 81920 bytes. ./xtensa-lx106-elf/bin/xtensa-lx106-elf-objdump -t -j .text1 /tmp/arduino_build_9615/*elf | sort -k1 | head -20 401000cc l F .text1 00000014 twi_delay 401000ec l F .text1 00000020 twi_reply$part$1 4010010c g F .text1 00000035 twi_reply 4010014c g F .text1 00000052 twi_stop 401001a0 g F .text1 0000003b twi_releaseBus 40100204 g F .text1 000001e6 twi_onTwipEvent 40100404 l F .text1 000001f7 onSdaChange 40100608 l F .text1 000002fd onSclChange 40100908 l F .text1 0000003b onTimer * Factor out !scl in onSdaChange If SCL is low then all branches of the case are no-ops, so factor that portion outo to remove some redundant logic in each case. Sketch uses 270843 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27944 bytes (34%) of dynamic memory, leaving 53976 bytes for local variables. Maximum is 81920 bytes. 401000cc l F .text1 00000014 twi_delay 401000ec l F .text1 00000020 twi_reply$part$1 4010010c g F .text1 00000035 twi_reply 4010014c g F .text1 00000052 twi_stop 401001a0 g F .text1 0000003b twi_releaseBus 40100204 g F .text1 000001e6 twi_onTwipEvent 40100404 l F .text1 000001e7 onSdaChange 401005f8 l F .text1 000002fd onSclChange 401008f8 l F .text1 0000003b onTimer 0x0000000040107468 _text_end = ABSOLUTE (.) * Make tiny twi_reply inline twi_reply is a chunk of code that can be inlined and actually save IRAM space because certain conditions acan be statically evaluated by gcc. Sketch uses 270823 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27944 bytes (34%) of dynamic memory, leaving 53976 bytes for local variables. Maximum is 81920 bytes. 401000cc l F .text1 00000014 twi_delay 401000f4 g F .text1 00000052 twi_stop 40100148 g F .text1 0000003b twi_releaseBus 401001b0 g F .text1 00000206 twi_onTwipEvent 401003d0 l F .text1 000001e7 onSdaChange 401005c4 l F .text1 000002fd onSclChange 401008c4 l F .text1 0000003b onTimer 40100918 g F .text1 00000085 millis 401009a0 g F .text1 0000000f micros 401009b0 g F .text1 00000022 micros64 401009d8 g F .text1 00000013 delayMicroseconds 401009f0 g F .text1 00000034 __digitalRead 401009f0 w F .text1 00000034 digitalRead 40100a3c g F .text1 000000e4 interrupt_handler 40100b20 g F .text1 0000000f vPortFree 0x0000000040107434 _text_end = ABSOLUTE (.) * Inline additional twi_** helper functions Sketch uses 270799 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27944 bytes (34%) of dynamic memory, leaving 53976 bytes for local variables. Maximum is 81920 bytes. 401000cc l F .text1 00000014 twi_delay 401000f4 w F .text1 0000003b twi_releaseBus 4010015c g F .text1 00000246 twi_onTwipEvent 401003bc l F .text1 000001e7 onSdaChange 401005b0 l F .text1 000002f9 onSclChange 401008ac l F .text1 0000003b onTimer 0x000000004010741c _text_end = ABSOLUTE (.) * Convert state machine to 1-hot for faster lookup GCC won't use a lookup table for the TWI state machine, so it ends up using a series of straight line compare-jump, compare-jumps to figure out which branch of code to execute for each state. For branches that have multiple states that call them, this can expand to a lot of code. Short-circuit the whole thing by converting the FSM to a 1-hot encoding while executing it, and then just and-ing the 1-hot state with the bitmask of states with the same code. Sketch uses 270719 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27944 bytes (34%) of dynamic memory, leaving 53976 bytes for local variables. Maximum is 81920 bytes. 401000cc l F .text1 00000014 twi_delay 401000f4 w F .text1 0000003b twi_releaseBus 4010015c g F .text1 00000246 twi_onTwipEvent 401003c0 l F .text1 000001b1 onSdaChange 40100580 l F .text1 000002da onSclChange 4010085c l F .text1 0000003b onTimer 0x00000000401073cc _text_end = ABSOLUTE (.) Saves 228 bytes of IRAM vs. master, uses 32 additional bytes of heap. * Factor out twi_status setting twi_status is set immediately before an event handler is called, resulting in lots of duplicated code. Set the twi_status flag inside the handler itself. Saves an add'l ~100 bytes of IRAM from prior changes, for a total of ~340 bytes. earle@server:~/Arduino/hardware/esp8266com/esp8266/tools$ ./xtensa-lx106-elf/bin/xtensa-lx106-elf-objdump -t -j .text1 /tmp/arduino_build_849115/*elf | sort -k1 | head -20 401000cc l F .text1 00000014 twi_delay 401000f4 w F .text1 0000003b twi_releaseBus 40100160 g F .text1 0000024e twi_onTwipEvent 401003c8 l F .text1 00000181 onSdaChange 40100558 l F .text1 00000297 onSclChange * Use a struct to hold globals for TWI Thanks to the suggestion from @mhightower83, move all global objects into a struct. This lets a single base pointer register to be used in place of constantly reloading the address of each individual variable. This might be better expressed by moving this to a real C++ implementaion based on a class object (the twi.xxxx would go back to the old xxx-only naming for vars), but there would then need to be API wrappers since the functionality is exposed through a plain C API. Saves 168 additional code bytes, for a grand total of 550 bytes IRAM. earle@server:~/Arduino/hardware/esp8266com/esp8266/tools$ ./xtensa-lx106-elf/bin/xtensa-lx106-elf-objdump -t -j .text1 /tmp/arduino_build_849115/*elf | sort -k1 | head -20 401000cc l F .text1 00000014 twi_delay 401000e8 w F .text1 00000032 twi_releaseBus 40100128 g F .text1 00000217 twi_onTwipEvent 4010034c l F .text1 00000149 onSdaChange 4010049c l F .text1 00000267 onSclChange 40100704 l F .text1 00000028 onTimer * Use enums for states, move one more var to twi struct Make the TWI states enums and not #defines, in the hope that it will allow GCC to more easily flag problems and general good code organization. 401000cc l F .text1 00000014 twi_delay 401000e8 w F .text1 00000032 twi_releaseBus 40100128 g F .text1 00000217 twi_onTwipEvent 4010034c l F .text1 00000149 onSdaChange 4010049c l F .text1 00000257 onSclChange 401006f4 l F .text1 00000028 onTimer Looks like another 16 bytes IRAM saved from the prior push. Sketch uses 267079 bytes (25%) of program storage space. Maximum is 1044464 bytes. Global variables use 27696 bytes (33%) of dynamic memory, leaving 54224 bytes for local variables. Maximum is 81920 bytes. * Save 4 heap bytes by reprdering struct * Convert to C++ class, clean up code Convert the entire file into a C++ class (with C wrappers to preserve the ABI). This allows for setting individual values of the global struct(class) in-situ instead of a cryptic list at the end of the struct definition. It also removes a lot of redundant `twi.`s from most class members. Clean up the code by converting from `#defines` to inline functions, get rid of ternarys-as-ifs, use real enums, etc. For slave_receiver.ino, the numbers are: GIT Master IRAM: 0x723c This push IRAM: 0x6fc0 For a savings of 636 total IRAM bytes (note, there may be a slight flash text increase, but we have 1MB of flash to work with and only 32K of IRAM so the tradeoff makes sense. * Run astyle core.conf, clean up space/tab/etc. Since the C++ version has significant text differences anyway, now is a good time to clean up the mess of spaces, tabs, and differing cuddles. * Add enum use comment, rename twi::delay, fix SDA/SCL_READ bool usage Per review comments * Replace clock stretch repeated code w/inline loop There were multiple places where the code was waiting for a slave to finish stretching the clock. Factor them out to an *inline* function to reduce code smell. * Remove slave code when not using slave mode Add a new twi_setSlaveMode call which actually attached the interrupts to the slave pin change code onSdaChenge/onSclChange. Don't attach interrupts in the main twi_begin. Because slave mode is only useful should a onoReceive or onRequest callback, call twi_setSlaveMode and attach interrupts on the Wire setters. This allows GCC to not link in slave code unless slave mode is used, saving over 1,000 bytes of IRAM in the common, master-only case.