[Fixing FTDI USB driver latency timer causing slow industrial sensor packet reception]

by

Root Cause Diagnosis

FTDI USB-to-serial chips (e.g., FT232R, FT232H) buffer incoming serial data until the hardware internal buffer fills (typically 62 bytes) or the driver’s latency timer expires. By default, FTDI drivers assign a latency timer of 16 milliseconds to optimize USB bus throughput and reduce CPU polling overhead. When industrial sensors stream small, periodic telemetry packets (e.g., 8–16 bytes every few milliseconds), data remains trapped in the FTDI chip’s internal FIFO buffer until the 16 ms timer elapses, introducing artificial latency and packet delivery stutter into real-time polling loops.

Quick Fix

Reduce the FTDI driver latency timer from the 16 ms default down to 1 ms using OS-specific driver overrides:

  • On Linux: “`bash echo 1 | sudo tee /sys/bus/usb-serial/devices/ttyUSB0/latency_timer


* **On Windows (PowerShell as Admin):**
```powershell
Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Enum\FTDIBUS\*\0000\Device Parameters' -Name "LatencyTimer" -Value 1

Step-by-Step Resolution

1. Reduce Latency Timer in Linux (sysfs / Udev)

Transient changes via /sys reset when the device is unplugged or the host reboots. Enforce persistent 1 ms latency across all FTDI serial devices using a custom Udev rule.

  1. Create a Udev rule file: “`bash sudo nano /etc/udev/rules.d/99-ftdi-latency.rules


2. Add the rule to set `latency_timer` to `1` automatically upon FTDI device binding:
```udev
ACTION=="add", SUBSYSTEM=="usb-serial", DRIVER=="ftdi_sio", ATTR{latency_timer}="1"
  1. Reload and trigger Udev rules: “`bash sudo udevadm control –reload-rules && sudo udevadm trigger


4. Verify active latency setting for your interface (e.g., `ttyUSB0`):
```bash
cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer

Expected output: `1`

2. Reduce Latency Timer in Windows (Device Manager / Registry)

  • via Device Manager GUI:
  • Open Device Manager (devmgmt.msc).
  • Expand Ports (COM & LPT), right-click your FTDI device, and select Properties.
  • Navigate to Port Settings tab > click Advanced….
  • Under BM Options, change Latency Timer (msec) from 16 to 1.
  • Click OK and reconnect the USB device.
  • via Windows Registry (System-wide FTDI Driver Defaults): Modify the FTDI driver service key (FTDIBUS.sys) to force a 1 ms default for all newly enumerated interfaces: “`cmd reg add “HKLM\SYSTEM\CurrentControlSet\Services\FTDIBUS\Parameters” /v LatencyTimer /t REG_DWORD /d 1 /f



#### 3. Programmatic Override in Custom Applications (D2XX API)

If your polling application interacts directly with FTDI chips using the `FTD2XX` library instead of VCP (Virtual COM Port) drivers, set the latency timer inside your application initialization sequence.

* **C / C++ (FTD2XX API):**
```c
#include "ftd2xx.h"

FT_HANDLE ftHandle;
FT_STATUS ftStatus;

ftStatus = FT_Open(0, &ftHandle);
if (ftStatus == FT_OK) {
    // Set latency timer to 1 millisecond
    FT_SetLatencyTimer(ftHandle, 1);
}
  • Python (pyftdi / pyserial): “`python import serial
See also  Thermal printer skipping every alternate blank label: Gap sensor calibration

When using PySerial on Linux, write directly to the sysfs path

with open(‘/sys/bus/usb-serial/devices/ttyUSB0/latency_timer’, ‘w’) as f: f.write(‘1’)

ser = serial.Serial(‘/dev/ttyUSB0’, baudrate=115200, timeout=0.1)

“`

Why This Fails for Some Users

  • USB Host Controller Frame Polling Caps: On older USB 2.0 EHCI host controllers or unpowered hubs, the hardware microframe polling rate is capped at 1 ms. Setting latency_timer below 2 on high-traffic USB buses can cause frame drops if the host controller cannot clear the interrupt endpoint fast enough.
  • FTDI Chip Revision Limits: Legacy FTDI microcontrollers (such as older FT8U232AM chips) do not support 1 ms latency settings. The minimum hardware latency timer supported on first-generation FTDI silicons is hardcoded to 2 ms or 10 ms.
  • Windows Driver Overwrites on Windows Update: Major Windows Feature Updates regularly overwrite customized registry parameters under HKLM\SYSTEM\CurrentControlSet\Enum\ by reinstalling default WHQL FTDIBUS.sys drivers, resetting the latency timer back to 16 ms.
  • Virtual Machine Passthrough Overhead: When passing FTDI devices into virtual machines (e.g., VMware, VirtualBox, or KVM/QEMU), hypervisor USB emulation layers introduce their own interrupt batching delays (typically 2 ms to 8 ms). Reducing the guest OS latency timer to 1 ms will not bypass host-to-guest USB packet buffering bottlenecks.

Leave a Comment