FT121T-R FTDI, FT121T-R Datasheet - Page 11

• Manufacturer Part Number: FT121T-R
• Description: USB Interface IC USB CONTROLLER W/SPI SLAVE IC
• Manufacturer: FTDI

Specifications of FT121T-R

• Rohs: yes
• Product: USB 2.0
• Interface Type: SPI, USB
• Operating Supply Voltage: 4 V to 5.5 V
• Operating Supply Current: 3.9 mA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: TSSOP-16
• Minimum Operating Temperature: - 40 C

Document No.: FT_000648
FT121 ENHANCED USB DEVICE CONTROLLER WITH SPI SLAVE IC
Datasheet Version 1.0
312
Clearance No.: FTDI#
Figure 4-3 4-wire read operation
4.3.3 3-wire write operation
When the SPI master performs data write operation, it drives the SS_n line low, send the 8-bit command
byte on MOSI line, followed by sending the data bytes. The length of the data byte ranges from 0 to
maximum 506, depending on the command type. The master will signal the end of the data write
operation by drive the SS_n line high.
Figure 4-4 3-wire write operation
4.3.4 3-wire read operation
When the SPI master performs data read operation, it drives the SS_n line low, send the 8-bit command
byte on MOSI line. The master will stop driving the MOSI line. The FT121 will output data bytes on the
MOSI line. The length of the data byte ranges from 0 to maximum 506, depending on the command type.
The master will signal the end of the data read operation by drive the SS_n line high.
Figure 4-5 3-wire read operation
It is important that bus contention should be avoided during the bus turnaround period – i.e. moving
from the command byte to a read byte. In this instance the master will stop driving at the same clock
edge (rising edge) as the slave starts to drive. To avoid bus contention the FT121 will implement delays
so that it only start to drive the MOSI line at least 5ns after the SCLK driving edge – this will give the
master a minimum of 5ns to stop driving MOSI, which should ensure no bus contentions. At this bus
turnaround point it’s also more natural for a clash not to occur, as the master is the source of the clock,
so will drive the clock and MOSI off at similar times. For the slave to drive MOSI it has to pass the clock
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