Where is the definition of system_adc_read() in the esp8266 Arduino code?

Question

Where is the definition of uint16 system_adc_read(void) in the esp8266 Arduino code?

I can see it being declared in Arduino/tools/sdk/include/user_interface.h, however there seems to be no definition of this. system_adc_read() gets called in analogRead() in Arduino/cores/esp8266/core_esp8266_wiring_analog.c

I suspect system_adc_read() calls system_adc_read_fast() to read the ADC, but can't actually find it in the code! Could somebody please point me to this? I've been struggling for quite some time.

Purpose: To manipulate the system_adc_read() in ESP8266 to get higher ADC sampling rates. Right now ADC sampling rate is 10ksps. I want it to be more than 30-40ksps.

• Juraj: esp8266 sdk is not open source
• Ashish Ranjan: Oh my god! Thanks for telling me this. So, what are my alternatives here? I've searched for ways to improve the sampling rate, but couldn't get to the desired (30-40 ksps) level.
• Juraj: try ESP8266 Non-OS SDK API Reference here espressif.com/en/support/download/…

Answer 1

The "definition" of uint16 system_adc_read(void) is inside the compiled archive/object file libmain.a inside one of the NONOSDK folders which is located in /tools/sdk/lib/ . The version is selected in platform.txt. For example in my platform.txt:

# default SDK for all boards
# (generic board overrides this variable)
build.sdk=NONOSDK22x_190703
#build.sdk=NONOSDK22x_191024
#build.sdk=NONOSDK22x_191105

Running the xtensa-lx106-elf-objdump.exe -d on this file I can find this section inside:

00000258 <system_adc_read>:
 258: 020c movi.n a2, 0
 25a: f0c112 addi a1, a1, -16
 25d: 0109 s32i.n a0, a1, 0
 25f: fffc01 l32r a0, 250 <system_pp_recycle_rx_pkt+0x18>
 262: 0000c0 callx0 a0
 265: fff901 l32r a0, 24c <system_pp_recycle_rx_pkt+0x14>
 268: f42020 extui a2, a2, 0, 16
 26b: 1b1207 beq a2, a0, 28a <system_adc_read+0x32>
 26e: b30c movi.n a3, 11
 270: 902220 addx2 a2, a2, a2
 273: 1122e0 slli a2, a2, 2
 276: fff701 l32r a0, 254 <system_pp_recycle_rx_pkt+0x1c>
 279: 0000c0 callx0 a0
 27c: f42020 extui a2, a2, 0, 16
 27f: 00a402 movi a0, 0x400
 282: 013027 bltu a0, a2, 287 <system_adc_read+0x2f>
 285: 020d mov.n a0, a2
 287: f42000 extui a2, a0, 0, 16
 28a: 0108 l32i.n a0, a1, 0
 28c: 10c112 addi a1, a1, 16
 28f: f00d ret.n
 291: 00 .byte 00
 292: 00 .byte 00
 293: 00 .byte 00
 294: ff ff 00 00 
 ...
000002a0 <system_adc_read_fast>:
 2a0: e0c112 addi a1, a1, -32
 2a3: 11c9 s32i.n a12, a1, 4
 2a5: 21d9 s32i.n a13, a1, 8
 2a7: 31e9 s32i.n a14, a1, 12
 2a9: 5129 s32i.n a2, a1, 20
 2ab: 41f9 s32i.n a15, a1, 16
 2ad: 0109 s32i.n a0, a1, 0
 2af: 03fd mov.n a15, a3
 2b1: fff901 l32r a0, 298 <system_adc_read+0x40>
 2b4: 0000c0 callx0 a0
 2b7: 4fbc beqz.n a15, 2ef <system_adc_read_fast+0x4f>
 2b9: fff6e1 l32r a14, 294 <system_adc_read+0x3c>
 2bc: 0c0c movi.n a12, 0
 2be: 51d8 l32i.n a13, a1, 20
 2c0: 90dcd0 addx2 a13, a12, a13
 2c3: 001d32 l16ui a3, a13, 0
 2c6: 1a13e7 beq a3, a14, 2e4 <system_adc_read_fast+0x44>
 2c9: 902330 addx2 a2, a3, a3
 2cc: 1122e0 slli a2, a2, 2
 2cf: b30c movi.n a3, 11
 2d1: fff201 l32r a0, 29c <system_adc_read+0x44>
 2d4: 0000c0 callx0 a0
 2d7: f44020 extui a4, a2, 0, 16
 2da: 00a402 movi a0, 0x400
 2dd: 003d mov.n a3, a0
 2df: 013047 bltu a0, a4, 2e4 <system_adc_read_fast+0x44>
 2e2: 043d mov.n a3, a4
 2e4: 005d32 s16i a3, a13, 0
 2e7: cc1b addi.n a12, a12, 1
 2e9: f4c0c0 extui a12, a12, 0, 16
 2ec: ce9fc7 bne a15, a12, 2be <system_adc_read_fast+0x1e>
 2ef: 11c8 l32i.n a12, a1, 4
 2f1: 21d8 l32i.n a13, a1, 8
 2f3: 31e8 l32i.n a14, a1, 12
 2f5: 41f8 l32i.n a15, a1, 16
 2f7: 0108 l32i.n a0, a1, 0
 2f9: 20c112 addi a1, a1, 32
 2fc: f00d ret.n
 2fe: 00 .byte 00
 2ff: 00 .byte 00
 300: ff ff 00 00 
 ...

• Ashish Ranjan: Cool detective work. Thanks! I was able to achieve my use case using this: arduino.stackexchange.com/a/52854/25931
• S. Imp: @loadingnow can anyone clarify whether system_adc_read returns 0-1023 or whether it returns 1-1024?
• loadingnow: @S.Imp I am not sure, but you should ask a separate question to clarify. It would be much more helpful for others who have the same question.