Setting a Custom Default Latency Timer Value

The latency timer is a form of time-out mechanism for the read buffer of FTDI devices. When a FT_Read instruction is sent to the device, data will not be sent back to the host PC until the requested number of bytes has been read. If the requested number of bytes never comes, the device would not send data back.

The latency timer counts from the last time data was sent back to the PC. If the latency timer expires, the device will send what data it has available to the PC regardless of how many bytes it is waiting on. The latency timer will then reset and begin counting again.

The default value for the latency timer is 16ms. This value may be customised by adding or changing the following entries in the FTDIPORT.INF file of the driver before installation.

[FtdiPort232.NT.HW.AddReg]

HKR,,"LatencyTimer",0x00010001,50

This example will set the default latency timer value to 50ms. The valid range for the latency timer is 1ms - 255ms, although 1ms is not recommended as this is the same as the USB frame length.

The latency timer value is held in the registry under

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\FTDIBUS\{Device VID, PID and serial number}\0000\Device Parameters\LatencyTimer

Please see AN232B-04 Data Throughput, Latency and Handshaking for additional information on the latency timer.