I2CTarget: strange sequential read

[insekt]

I encountered some strange behavior with I2CTarget on Pico / RP2040.
Test setup: RPi - I2C - Pico
Linux rpi 6.6.51+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.6.51-1+rpt3 (2024-10-08) aarch64 GNU/Linux
MicroPython v1.26.0 on 2025-08-09; Raspberry Pi Pico with RP2040

On Pico I emulate EEPROM:

from machine import I2CTarget, Pin

mem = "0304070000000212"
mem = bytearray(bytes.fromhex(mem))
i2c0 = I2CTarget(0, addr=0x50, mem=mem, mem_addrsize=8, sda=Pin(0), scl=Pin(1))

Reading from RPi:

pi@rpi:~ $ i2cget -y 1 0x50 0x0
0x03
pi@rpi:~ $ i2cget -y 1 0x50
0x07
pi@rpi:~ $ i2cget -y 1 0x50
0x00
pi@rpi:~ $ i2cget -y 1 0x50
0x02

When responding, one byte is skipped, i.e. each reading will shift not by 1 byte, but by 2.

If I replace Pico to EEPROM chip.
Test setup: RPi - I2C - EEPROM IC

pi@rpi:~ $ i2cget -y 1 0x50 0x0
0x03
pi@rpi:~ $ i2cget -y 1 0x50
0x04
pi@rpi:~ $ i2cget -y 1 0x50
0x07
pi@rpi:~ $ i2cget -y 1 0x50
0x00
pi@rpi:~ $ i2cget -y 1 0x50
0x00
pi@rpi:~ $ i2cget -y 1 0x50
0x00
pi@rpi:~ $ i2cget -y 1 0x50
0x02
pi@rpi:~ $ i2cget -y 1 0x50
0x12

Reading works fine.

Any ideas what is the cause?

[robert-hh]

I do not know what i2cget is sending to to target, but the baseline I2CTarget expects the sequence issued by i2c.readfrom_mem(),which writes the address as first byte. When using i2c.readfrom(), the first bytes will be skipped. You can use a callback for the IRQ_READ_REQ to support the i2c.readfrom() sequence. See the documentation for I2CTarget.

[insekt]

@robert-hh I see.
The strange thing is that I2CTarget internally increments address by 2, after each read, not by 1.

[robert-hh]

The function machine_i2c_target_data_stop() in extmod/machine_i2c_target.c is changed to. Code below. It rolls back the buffer pointer in case of READ and when a buffer is used, which was supplied via the mem= argument of the constructor.

static inline void machine_i2c_target_data_stop(machine_i2c_target_data_t *data) {
    // Upon stop while reading, roll back the buffer pointer
    if (data->mem_buf != NULL && data->state == STATE_READING) {
        if (data->mem_addr == 0) {
            data->mem_addr = data->mem_len - 1;
        } else {
            data->mem_addr--;
        }
    }
    machine_i2c_target_data_reset_helper(data);
}

[insekt]

@robert-hh how did you test this code? It doesn't work in my setup: RPi - I2C - Pico

[robert-hh]

ESP32 as Controller, SAMD51 as target. The target has to set up a memory object first.
Then read with i2c.readfrom(addr, count), like i2c.readfrom(118, 1). But generally there is no benefit in doing so. You could as well use i2c.readfrom_mem().

[insekt]

I add some debug prints and see that function machine_i2c_target_data_stop() is never called. Instead function machine_i2c_target_data_restart_or_stop() is called all times. So I add you fix code and now my setup: RPi - I2C - Pico works fine.
Do you have understanding of the reason for that?

[robert-hh]

The hardware layer RPi for I2cTarget is different.