Two I2C reads with no STOP in between

[sebastian-sms]

Hi,
I would like to use the MCP2221A to talk to a Texas Instruments TPS25751. Unfortunately, the TPS has variable-length registers. When reading a register, we are supposed to read the register length N first, then read N data bytes - all within a single I2C transaction:

To do this, I'd like to use two separate I2C reads (one for the length, one for the data) with no START or STOP conditions in between. As far as I can tell, the MCP2221 does not support this. Any ideas if there is a way to get this to work? Thanks!

[sebastian-sms]

I found a workaround: I can always read the maximum length that a register can be (64 Bytes), independently of the reported byte count N. I get a bunch of extra zero-bytes that can be thrown away after the read is complete. It is quite wasteful, as most registers are only 4...8 bytes, but it does seem to work.

Alternately, I could do two register reads: First, read only a single byte (the byte count). Then read the same register number again with the determined length. That should work unless the register length changes in the meantime, which should be very rare.

[sebastian-sms]

Code for the wasteful workaround (reading max. number of bytes):

i2c_addr = 0x20
reg_num = 3
max_bytes = 64
mcp.I2C_write(i2c_addr, bytes([reg_num]), kind='nonstop')
data = mcp.I2C_read(i2c_addr, max_bytes+1, kind='restart')
byte_count = data[0]
reg_data = data[1:byte_count+1]
return reg_data

[electronicayciencia]

Hello.

Reading a maximum number of bytes and discarding the ones that are not useful is what ‎SMBus.read_block_data does.

On the other hand, it is not so wasteful. I2C write/reads commands are issued every 2 ms or more due to USB polling interface. See Internal details. So reading 8 bytes and reading 60 takes almost the same time depending on the I2C bus speed. To read 64, you must issue two read commands to the MCP2221.

So it's a little wasteful, but really not much.

[sebastian-sms]

Thanks for the explanation, that makes me feel a little better about wasting precious bytes :-)