Copyright © 2009 Future Technology Devices International Limited

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Document No.: FT_000060

FT4232H QUAD HIGH SPEED USB TO MULTIPURPOSE UART/MPSSE IC

Datasheet Version 2.05

Clearance No.: FTDI#78

4.5

Synchronous and Asynchronous Bit-Bang Interface Mode Description

The FT4232H channel A,B,C or channel D can be configured as a bit-bang interface. There are two types of

bit-bang modes: synchronous and asynchronous.

Asynchronous Bit-Bang Mode

Asynchronous Bit-Bang mode is the same as BM-style Bit-Bang mode. On any channel configured in

asynchronous bit-bang mode. data written to the device in the normal manner will be self clocked onto the

parallel I/O data pins (those which have been configured as outputs). Each I/O pin can be independently set

as an input or an output. The rate that the data is clocked out at is controlled by the baud rate generator.

For the data to change there has to be new data written, and the baud rate clock has to tick. If no new data is

written to the channel, the pins will hold the last value written.

Synchronous Bit-Bang Mode

The synchronous Bit-Bang mode will only update the output parallel I/O port pins whenever data is sent from

the USB interface to the parallel interface. When this is done, data is read from the USB Rx FIFO buffer and

written out on the pins. Data can only be received from the parallel pins (to the USB Tx FIFO interface) when

the parallel interface has been written to.

With Synchronous Bit-Bang mode, data will only be sent out by the FT4232H if there is space in the

FT4232H USB TXFIFO for data to be read from the parallel interface pins. This Synchronous Bit-Bang mode

will read the data bus parallel I/O pins first, before it transmits data from the USB RxFIFO. It is therefore 1

byte behind the output, and so to read the inputs for the byte that you have just sent, another byte must be

sent.

For example :-

(1) Pins start at 0xFF

Send 0x55,0xAA

Pins go to 0x55 and then to 0xAA

Data read = 0xFF,0x55

(2) Pins start at 0xFF

Send 0x55,0xAA,0xAA

(repeat the last byte sent)

Pins go to 0x55 and then to 0xAA

Data read = 0xFF,0x55,0xAA

Synchronous Bit-Bang Mode differs from Asynchronous Bit-Bang mode in that the device parallel output is

only read when the parallel output is written to by the USB interface. This makes it easier for the controlling

program to measure the response to a USB output stimulus as the data returned to the USB interface is

synchronous to the output data.

Asynchronous Bit-Bang mode is enabled using Set Bit Bang Mode driver command. A hex value of 1 will

enable Asynchronous Bit-Bang mode.

Synchronous Bit-Bang mode is enabled using Set Bit Bang Mode driver command. A hex value of 4 will

enable Synchronous Bit-Bang mode.

See application note AN2232-02, “Bit Mode Functions for the FT2232” for more details and examples of

using the bit-bang modes.

An example of the synchronous bi-bang mode timing is shown in Figure 4.5 and Table 4.2.