12-16
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Functional Description
12
The operation of clearing the TE bit disables the transmitter after completion of
transmission of the current data word. Setting the TE bit again enables transmission of the
next word. During the time TE = 0, the STD signal is tri-stated. The TE bit should be
cleared after the TDE bit is set, ensuring all pending data is transmitted.
The Network mode transmitter generates interrupts every time slot (unless the TSM
registers are utilized) and requires the DSP program to respond to each time slot. These
responses may be one of the following:
Write the data register with data to enable transmission in the next time slot
Write the time slot register to disable transmission in the next time slot
Do nothing—transmit underrun occurs at the beginning of the next time slot and
the previous data is re-transmitted
Table 12-8.
Note
Source
Signal
Destination
Signal
Description
1
——
Example of a five time-slot frame, transmitting in time-slots 0 and 3.
2SC2
—
Example with word-length frame sync and standard timing (TFSI=0, TFSL=0,
and TEFS=0). Frame timing begins with the rising edge of SC2.
3
—
TDE Status Flag
and Interrupt
This flag is set at the beginning of each word to indicate that another data
word should be supplied by the software. When the transmit interrupt is
enabled, the processor is interrupted to request the data. The flag and
interrupt are cleared when data is written to either the STX or STSR
registers.1
4
STX / STSR
Register
TXSR
Register
On each word clock boundary a decision is made concerning what to
transmit on the next time-slot.
If the STSR register was written during the previous time-slot the STD pin is
tri-stated.
If the STSR register was NOT written during the previous time-slot the
contents of the STX register is transferred to the TXSR register and this data
is shifted out. If the STX register has not been written in the previous
time-slot the previous data is reused.
If neither of these registers were written in the previous time-slot the TUE
status bit will be set and the hardware will operate as if the STX register had
been written. The STD pin will be enabled and the contents of the STX will
be transmitted again. This may lead to drive conflicts on the transmit data
line.
5
TXSR
Register
STD Pin
On active time-slots, the TXSR register contents are shifted out on the STD
pin, one bit per rising edge of SCK.
On inactive time-slots, the STD pin is tri-stated so it can be driven by another
device.
1.
Section 12.12 provides a complete description of interrupt processing.