
28-4
MPC8260 PowerQUICC II User’s Manual
MOTOROLA
Part IV.  Communications Processor Module
Table 28-1 describes GFMR fields. 
Table 28-1. GFMR Register Field Descriptions 
Field
Name 
Description
0–1
DIAG
Diagnostic mode. 
00 Normal operation—Receive data enters through RXD, and transmit data is shifted out through 
TXD. The FCC uses the modem signals (CD and CTS) to automatically enable and disable 
transmission and reception. Timings are shown in Section 28.11, “FCC Timing Control.”
01 Local loopback mode—Transmitter output is connected internally to the receiver input, while the 
receiver and the transmitter operate normally. RXD is ignored. Data can be programmed to 
appear on TXD, or TXD can remain high by programming the appropriate parallel port register. 
RTS can be disabled in the appropriate parallel I/O register. The transmitter and receiver must 
use the same clock source, but separate CLKxpins can be used if connected to the same 
external clock source.
If external loopback is preferred, program DIAG for normal operation and externally connect 
TXD and RXD. Then, physically connect the control signals (RTS connected to CD, and CTS 
grounded) or set the parallel I/O registers so CD and CTS are permanently asserted to the FCC 
by configuring the associated CTS and CD pins as general-purpose I/O.; see Chapter 35, 
“Parallel I/O Ports.”
10 Automatic echo mode—The channel automatically retransmits received data, using the receive 
clock provided. The receiver operates normally and receives data if CD is asserted. The 
transmitter simply transmits received data. In this mode, CTS is ignored. The echo function can 
also be accomplished in software by receiving buffers from an FCC, linking them to TxBDs, and 
transmitting them back out of that FCC.
11 Loopback and echo mode—Loopback and echo operation occur simultaneously. CD and CTS 
are ignored. Refer to the loopback bit description for clocking requirements.
For TDM operation, the diagnostic mode is selected by SIxMR[SDMx; see Section 14.5.2, “SI 
Mode Registers (SIxMR).”
2
TCI
Transmit clock invert
0 Normal operation.
1 The FCC inverts the internal transmit clock.
3
TRX
Transparent receiver. The MPC8260 FCCs offer totally transparent operation. However, to increase 
flexibility, totally transparent operation is configured with the TTX and TRX bits instead of the 
MODE bits. This lets the user implement unique applications such as an FCC transmitter 
configured to HDLC and a receiver configured to totally transparent operation. To do this, program 
MODE = HDLC, TTX = 0, and TRX = 1.
0 Normal operation
1 The receiver operates in totally transparent mode, regardless of the protocol selected for the 
transmitter in the MODE bits.
Note that full-duplex, totally transparent operation for an FCC is obtained by setting both TTX and 
TRX. Attempting to operate an FCC with Ethernet or ATM on its transmitter and transparent 
operation on its receiver causes erratic behavior. In other words, if the MODE = Ethernet or ATM, 
TTX must equal TRX.
4
TTX
Transparent transmitter. The MPC8260 FCCs offer totally transparent operation. However, to 
increase flexibility, totally transparent operation is configured with the TTX and TRX bits instead of 
the MODE bits. This lets the user implement unique applications, such as configuring an FCC 
receiver to HDLC and a transmitter to totally transparent operation. To do this, program MODE = 
HDLC, TTX = 1, and TRX = 0.
0 Normal operation.
1 The transmitter operates in totally transparent mode, regardless of the receiver protocol selected 
in the MODE bits.
Note that full-duplex totally transparent operation for an FCC is obtained by setting both TTX and 
TRX. Attempting to operate an FCC with Ethernet or ATM on its receiver and transparent operation 
on its transmitter causes erratic behavior. In other words, if the GFMR MODE = Ethernet or ATM, 
TTX must equal TRX.