Setup:

• M5Stack UnitV = I2C Slave
• ESP8266 = I2C Master + Wifi

Setting up the UnitV Stick as a I2C slave is failing. I'm connecting a ESP8266 with ports 34,35 of the UnitV and a 4,7kO Pull-Up resistor.

The scan from the ESP8266 master is successful, but any read or write operation fails with a timeout message. The connection to other I2C devices is successful, when the UnitV is removed from the bus.
UPDATE: Any write operation - before a read - is successful. After the first read, the bus is blocked.

import time
from machine import I2C

I2C_SLAVE_ADDR = 0x3a
count=0

def i2c_on_receive (data):
    print ("on_receive:", data)

def i2c_on_transmit ():
    count = count + 1
    print ("on_transmit, send:", count)
    return count

def i2c_on_event (event):
    print ("on_event:", event)

print ("starting i2c")
i2c = I2C(I2C.I2C1, mode = I2C.MODE_SLAVE, scl=34, sda=35,
            freq = 400000, addr = I2C_SLAVE_ADDR,
            addr_size = 7,
            on_receive = i2c_on_receive,
            on_transmit = i2c_on_transmit,
            on_event = i2c_on_event)
while True:
    time.sleep(20)

ESP8266 Micropython Code:

from machine import Pin, I2C
i2c = I2C(scl=Pin(12), sda=Pin(13), freq=400000)
i2c.scan()  # returns [58]
i2c.writeto(0x3a,'test') # success, write 4 bytes to slave with address 0x3a
result = i2c.readfrom(0x3a, 4)  # fails with Timeout
print(q)

UPDATE 2020-06-26:

• Code Cleanup (remove ws2812 reference), add esp8266 code