Echelon FT 3150 Smart Transceiver - Manual (Page 58)

Chapter 3 - Input/Output Interfaces
52 FT 3120 / FT 3150 Smart Transceiver Data Book
The FT Smart Transceiver may appear as two registers in the address space of the master; one of the registers being
the read/write data register, and the other being the read-only status register. Therefore, reads by the master to an odd
address access the status register for handshaking acknowledgments and all other reads or writes access the data
register for I/O transfers. The LSB of the control register, which is read through pin IO0, is the HS bit. The master
reads the HS bit after every master read or write. The D0/HS line should be pulled up (inactive) with a 10k
resistor to ensure proper resynch behavior after resets.
When acting as a slave to a different microprocessor, the FT Smart Transceiver slave B mode handles all handshaking
and token passing automatically. However, the master microprocessor must read the HS bit after each transaction and
must also internally track the token passing. This mode is designed for use with a master processor that uses memory-
mapped I/O, as the LSB of the address bus of the master is typically connected to the IO10 pin of the FT Smart
Transceiver. This is illustrated in Figures 3.19 and 3.20.
Token Passing
Virtual token passing is implemented to eliminate the possibility of data bus contention. The token is owned by the
master after synchronization and is passed between the master and slave devices. After each data transfer is
completed, the token owner writes an end of message (EOM) (0x00) to indicate that the transfer is complete. The
EOM is never read. Instead, “processing the EOM” indicates passing of the token. Token passing can be achieved by
executing either a data packet or a NULL transfer. Only the owner of the token can write to the bus. Therefore, when
the master performs two writes of data (1 – 255 bytes each) a dummy read cycle (NULL character = 0x00) must be
inserted between them in order to pass the token. Token passing is executed automatically in a FT Smart Transceiver-
to-FT Smart Transceiver interface. Refer to section , Data Transferring, for master/slave flow transactions.
Ω
Figure 3.19 Parallel I/O Master/Slave B (FT 3120/FT 3150 Smart Transceiver
as Memory-Mapped I/O Device)
R /W = 0
IO10 = 1 R /W = 0 OR 1
IO10 = 0
R/W = 1
IO10 = 1
READ ONLY
STATUS REGISTER
READ/WRITE
DATA REGISTER
D0/HS
IO10
IO9
IO8
IO0
IO1
IO2
IO3
IO4
IO5
IO6
IO7
A0
R/W
CS
HS
SLAVE B
D7
D6
D5
D4
D3
D2
D1
HS/D0 – D7
X
X
X
X
X
X
X
D0
D1
D2
D3
D4
D5
D6
D7
OR