FTDI FT232H communication with LM77 over I2C - c

Question

I am trying to get a temperature readout from a LM77 using I2C communication with and FTDI FT232H. I don't think it is a hardware problem since I have checked the connections along with multiple colleagues. The communication with the PC to the FT232H is okay, it is initializing and everything is good on that end. The FT232H sends a read setup byte and gets an ACK from the LM77. After that there is no more data. I expect the LM77 to send 10 bits of data for a temperature readout, but it is not. This is what the readout looks like on a logic probe.
I would expect to then see an additional two bytes come in after the ACK but am getting nothing. The code is pretty straightforward and I am using the libMPSSE I2C API. The address I am using 0x48 comes from the address given in the datasheet bit shifted right by 1. I do not understand why I am getting an ACK but no temperature readout after. The ftStatus for the read gives an error code FT_DEVICE_NOT_FOUND. I am not sure why it's giving this error code if there is an ACK.
#include <stdio.h>
#include <stdlib.h>
#include "libMPSSE_i2c.h"
#define DEVICE_ADDR 0x48
FT_STATUS ftStatus;
FT_HANDLE ftHandle;
int i2cInit(void)
{
int numChannels = 0;
FT_DEVICE_LIST_INFO_NODE chanInfo;
ChannelConfig chConfig = {.ClockRate = 100000,
.LatencyTimer = 255,
.Options = 0x0000};
ftStatus = I2C_GetNumChannels(&numChannels);
if (ftStatus != FT_OK)
{
return -1;
}
else
{
printf("Number of channels: %d\n", numChannels);
}
ftStatus = I2C_GetChannelInfo(0, &chanInfo);
if (ftStatus != FT_OK)
{
return -1;
}
else
{
printf("Channel info obtained\n");
}
ftStatus = I2C_OpenChannel(0, &ftHandle);
if (ftStatus != FT_OK)
{
return -1;
}
else
{
printf("Channel opened\n");
}
ftStatus = I2C_InitChannel(ftHandle, &chConfig);
if (ftStatus != FT_OK)
{
return -1;
}
else
{
printf("Channel initialized\n");
}
}
int lm77ReadTemp()
{
unsigned char writeBuffer[20] = {0};
unsigned char readBuffer[20] = {0};
unsigned char bytesTransferred = 0;
unsigned int bytesRead = 0;
ftStatus = I2C_DeviceRead(ftHandle, DEVICE_ADDR, 2, readBuffer, &bytesRead, I2C_TRANSFER_OPTIONS_START_BIT | I2C_TRANSFER_OPTIONS_STOP_BIT);
if (ftStatus != FT_OK)
{
printf("Read failed status code %d\n", ftStatus);
}
}
int main(void)
{
i2cInit();
lm77ReadTemp();
return 0;
}

Answer 1

The problem was on the hardware side. The MPSSE engine does not have bidirectional pins. On the FTDI chip side, you need a separate pin for SDA_out and SDA_in. There is a diagram in the FTDI documentation that shows it. If you don't have the pins connected like this you won't receive any data from the I2C slave.