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Synchronous Bit Bang Mode
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Synchronous Bit Bang mode is the same as the FT2232 Synchronous Bit Bang mode. With Synchronous Bit Bang mode, data will only be sent out if there is space in the device for data to be read from the pins. This Synchronous Bit Bang mode will read the data bus pins first, before it sends out the byte that has just been transmitted. 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:

·Pins start at 0xFF  
·Send 0x55, 0xAA  
·Pins go to 0x55 and then to 0xAA  
·Data read = 0xFF, 0x55  

·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  


The timing for Synchronous Bit bang is described in this diagram and the table below:


synchronousbitbangtiming


Time
Description
t1
Current pin state is read
t2
RD# is set inactive
t3
RD# is set active again, and any pins that are output will change to the new data.
t4
Clock state for data setup
t5
WR# goes active
t6
WR# goes inactive




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 device, the pins will hold the last value written.


When Synchronous Bit Bang mode is enabled the IO signal lines are configured as follows:

FT232RL/ FT245RL Pin Number
FT232RQ/ FT245RQ Pin Number


Signal



Type



Description
1
30
D0
Input/Output
Bit Bang Data Bus Bit 0
5
2
D1
Input/Output
Bit Bang Data Bus Bit 1
3
32
D2
Input/Output
Bit Bang Data Bus Bit 2
11
8
D3
Input/Output
Bit Bang Data Bus Bit 3
2
31
D4
Input/Output
Bit Bang Data Bus Bit 4
9
6
D5
Input/Output
Bit Bang Data Bus Bit 5
10
7
D6
Input/Output
Bit Bang Data Bus Bit 6
6
3
D7
Input/Output
Bit Bang Data Bus Bit 7



In the case of the FT232R, CBUS0/CBUS1/CBUS2/CBUS3 can be configured to bring out the internal RD# strobe and CBUS0/CBUS1 can be configured to bring out the internal WR# strobe when the device is in Synchronous Bit Bang mode. Alternatively, the CBUS pins can be used to provide clock signals by setting the appropriate values in the EEPROM. The CBUS must be configured in the FT232R EEPROM. This option is not available for the FT245R.

FT232RL Pin Number
FT232RQ Pin Number

Signal


Type


Description

23
22
C0
Input/Output
Configurable function
22
21
C1
Input/Output
Configurable function
13
10
C2
Input/Output
Configurable function
14
11
C3
Input/Output
Configurable function



A number of D2XX driver commands are needed to use Synchronous Bit Bang mode. The commands of interest are listed below:

D2XX Function
Description
FT_SetBitMode
Synchronous Bit Bang mode is enabled using the FT_SetBitMode command. A value of 0x04 will enable it and a value of 0x00 will reset the device mode.

FT_SetBaudRate
The rate of data transfer can be controlled by using the FT_SetBaudRate command. The maximum Baud rate is 3MBaud, but to allow time for the data to be setup and held around the WR# strobe the Baud rate should be less than 1MBaud.
The clock for the Synchronous Bit Bang mode is actually 16 times the Baud rate. A value of 9600 Baud would transfer the data at (9600x16) = 153600 bytes per second, or 1 every 6.5
mS.

FT_Write
Data can be written to the device in Synchronous Bit Bang mode using the FT_Write command. If multiple bytes are written to the device the values on the pins will change at the rate set by FT_SetBaudRate.

FT_GetBitMode
FT_GetBitMode returns the instantaneous value of the pins. A single byte will be returned containing the current values of the pins, both those which are inputs and those which are outputs.

FT_Read
FT_Read will return a buffer of values which have been sampled from the pins at the rate set by FT_SetBaudRate. If the read buffers have filled, data willl be lost. Each byte returned contains the values of the pins, both those which are inputs and those which are outputs.




Full descriptions of these functions are available in the
D2XX Programmer's Guide.

Code examples can be downloaded from
http://www.ftdichip.com/Projects/CodeExamples.htm.