Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
7.2 EEPROM Configuration
Figure 7.3 EEPROM Configuration
Figure 7.3 illustrates how to connect the FT232B to the 93C46 (93C56 or 93C66) EEPROM. EECS (pin 32)
is directly connected to the chip select (CS) pin of the EEPROM. EESK (pin 1) is directly connected to the
clock (SK) pin of the EEPROM. EEDATA (pin 2) is directly connected to the Data In (Din) pin of the
EEPROM. There is a potential condition whereby both the Data Output (Dout) of the EEPROM can drive out
at the same time as the EEDATA pin of the FT232B. To prevent potential data clash in this situation, the
Dout of the EEPROM is connected to EEDATA of the FT232B via a 2.2K resistor.
Following a power-on reset or a USB reset, the FT232B will scan the EEPROM to find out (a) if an EEPROM
is attached to the Device and (b) if the data in the device is valid. If both conditions are true, then the
FT232B will use the data in the EEPROM, otherwise it will use its built-in default values. When a valid
command is issued to the EEPROM from the FT232B, the EEPROM will acknowledge the command by
pulling its Dout pin low. In order to check for this condition, it is necessary to pull Dout high using a 10K
resistor. If the command acknowledge doesn’t happen then EEDATA will be pulled high by the 10K resistor
during this part of the cycle and the device will detect an invalid command or no EEPROM present.
There are two varieties of these EEPROM’s from many sources such as Microchip, STMicro, ISSI etc. – one
is configured as being 16 bits wide, the other is configured as being 8 bits wide. The FT232B requires
EEPROMs with a 16-bit wide configuration such as the Microchip 93LC46B device. The EEPROM must be
capable of reading data at a 1Mb clock rate at a supply voltage of 4.35V to 5.25V. Most
available parts are capable of this.
Check the manufacturers’ data sheet to find out how to connect pins 6 and 7 of the EEPROM. Some
devices specify these as no-connect, others use them for selecting 8 / 16 bit mode or for test functions.
Some other parts have their pin out rotated by 90˚ . It is recommended to select the required part and its
options carefully.
It is possible to “share” the EEPROM between the FT232B and another external device such as an MCU.
However, this can only be done when the FT232B is held in the reset condition as it tri-states its EEPROM
interface at that time. A typical configuration would use four bit’s of an MCU IO Port. One bit would be
used to hold the FT232B reset (using RESET#) on power-up, the other three would connect to the EECS,
EESK and EEDATA pins of the FT232B in order to read / write data to the EEPROM at this time. Once the
MCU has read / written the EEPROM, it would take RESET# high to allow the FT232B to configure itself and
enumerate over USB.
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