Functional Description
MOTOROLA
Enhanced Synchronous Serial Interface (ESSI)
12-21
Preliminary
12
12.5.4.2 Operation Using RSM Register
When the RSM register is included in the design, interrupt overhead can be reduced. If all
bits of the RSM register are set, the ESSI receiver will continue to operate as previously
described. The RSM register is used to automatically discard data from selected time slots.
This is accomplished by writing the RSM with 0 in the selected time-slot bit location. This
means no data is transferred from the Receive Data Shift Register (RXSR) on the 0
time-slots, no status flags change, and no interrupts are generated.
Receiver timing in the Network mode, and using RSM registers for an 8-bit word with a
continuous clock, results in the disabled FIFO five words per frame sync. In this example
there are only two receive interrupts per frame instead of five as in the previous example
where the RSM register is not used. This process is illustrated in Figure 12-8 and
Table 12-10.
Notes for Transmit Timing with Mask Register in Figure 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 of a 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
For enabled time slots, this flag is set at the beginning of each word to
indicate the STX data has been used and another data word should be
supplied by the software. If 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 TSMn register bit is zero for the next
time slot, the STD pin is tri-stated and the time slot is ignored.
When the TSMn bit is one for the next time slot, the contents or the STX
register are 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.
Note:
If the STSR is written instead of the STX, the STD pin is
When neither of these registers were written in the previous time slot (where
TSMn=1), 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, in another device is transmitting data during this
time slot.
5
TXSR
Register
STD Pin
On active time slots, the TXSR register contents are shifted out on the STC
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 the interrupt process.