Copyright © 2015 Future Technology Devices International Limited

11

FT234XD USB TO BASIC UART IC

Version 1.2

Document No.: FT_000753 Clearance No.: FTDI#324

4 Function Description

The FT234XD is a compact USB to a basic serial UART interface device which simplifies USB

implementations in a small optimised package. The device reduces external component count by fully

integrating an MTP memory, and an integrated clock circuit which requires no external crystal. It has

been designed to operate efficiently with USB host controllers by using as little bandwidth as possible

when compared to the total USB bandwidth available.

4.1 Key Features

Functional Integration. Fully integrated MTP memory, clock generation, AVCC filtering, Power-On-

Reset (POR) and LDO regulators.

Configurable CBUS I/O Pin Options. The fully integrated MTP memory allows configuration of the

Control Bus (CBUS) functionality and drive strength selection. There is one CBUS I/O pin CBUS0. The

configurable options of CBUS0 are detailed in section 3.2

The CBUS line can be configured with any one of these output options by setting bits in the internal MTP

memory. The device is shipped with the most commonly used pin definitions pre-programmed - see

Section 8 for details.

Asynchronous Bit Bang Mode with RD# and WR# Strobes. The FT234XD supports FTDI’s previous

chip generation bit-bang mode. In bit-bang mode, the four UART lines can be switched from the regular

interface mode to a 4-bit general purpose I/O port. Data packets can be sent to the device and they will

be sequentially sent to the interface at a rate controlled by an internal timer (equivalent to the baud rate

pre-scalar). Either the internal RD# or the internal WR# strobe signal can mapped to the CBUS0 pin

(only one CBUS pin available) which can be used to allow external logic to be clocked by access to the

bit-bang I/O bus. This option will be described more fully in a separate application note available from

FTDI website (www.ftdichip.com).

Synchronous Bit Bang Mode. The FT234XD supports synchronous bit bang mode. This mode differs

from asynchronous bit bang mode in that the interface pins are only read when the device is written to.

This makes it easier for the controlling program to measure the response to an output stimulus as the

data returned is synchronous to the output data. An application note, AN232R-01, available from FTDI

website (www.ftdichip.com) describes this feature.

Source Power and Power Consumption. The FT234XD is capable of operating at a voltage supply

between +3.3V and +5.25V with a nominal operational mode current of 8mA and a nominal USB suspend

mode current of 125µA. This allows greater margin for peripheral designs to meet the USB suspend mode

current limit of 2.5mA. An integrated level converter within the UART interface allows the FT234XD to

interface to UART logic running at +1.8V to +3.3V (5V tolerant).

4.2 Functional Block Descriptions

The following paragraphs detail each function within the FT234XD. Please refer to the block diagram

shown in Figure 2.1

Internal MTP Memory. The internal MTP memory in the FT234XD is used to store USB Vendor ID (VID),

Product ID (PID), device serial number, product description string and various other USB configuration

descriptors. The internal MTP memory is also used to configure the CBUS pin functions. The FT234XD is

supplied with the internal MTP memory pre-programmed as described in Section 8. A user area of the

internal MTP memory is available to system designers to allow storing additional data from the user

application over USB. The internal MTP memory descriptors can be programmed in circuit, over USB

without any additional voltage requirement. The descriptors can be programmed using the FTDI utility

software called FT_PROG, which can be downloaded from FTDI Utilities on the FTDI website

(www.ftdichip.com).

+3.3V LDO Regulator. The +3.3V LDO regulator generates the +3.3V reference voltage for driving the

USB transceiver cell output buffers. It requires an external decoupling capacitor to be attached to the

3V3OUT regulator output pin. It also provides +3.3V power to the 1.5kΩ internal pull up resistor on

USBDP. The main function of the LDO is to power the USB Transceiver and the Reset Generator Cells

rather than to power external logic. However, it can be used to supply external circuitry requiring a

+3.3V nominal supply with a maximum current of 50mA.

+1.8V LDO Regulator. The +1.8V LDO regulator generates the +1.8V reference voltage for internal use

driving the IC core functions of the serial interface engine and USB protocol engine.