Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 620

Serial Interface
Bits Name
7–9 CSEL Channel select. Indicates which channel the time slot is routed to.
000 This time slot is not used. Tx data signal is three-stated; Rx data signal is ignored.
001 This time slot is not used.
010 SCC2
011 SCC3
100 SCC4
101 SMC1
110 SMC2
111 This time slot is not used. Also used in SCIT mode to indicate the D channel grant bit.
10–13 CNT Count. The number of bits/bytes (the unit is determined by BYT) that the routing and strobe select
of this entry controls. If CNT = 0b0000, 1 bit/byte is routed; if CNT = 0b1111, 16 bits/bytes are
routed.
14 BYT Byte resolution
0 Bit resolution. CNT indicates the number of bits in this entry.
1 Byte resolution. CNT indicates the number of bytes in this entry.
15 LST Last entry in a TDM channel's Rx or Tx SI RAM. LST must be set in the last entry even if all the
entries are used.
0 Not the last entry in this TDM channel's Rx or Tx SI RAM.
1 Last entry. After this entry, the SI waits for SYNC to start the next frame.
16–31 — Reserved, should be cleared.
For applications needing to receive data from a Tx signal and send data on an Rx signal, set
SIRAMn[SWTR]. For example, stations A and B in
channel. Even though they share Rx and Tx data lines, stations A and B can communicate using the SWTR
option.
Rx Tx
Station A
Tx and Rx SI RAMn[SWTR] = 0
The SWTR option allows station B to listen to transmissions from and send data to station A. By setting
SWTR in its Rx route RAM entry, station B receives data from L1TXD and, if the time slot's Tx route
RAM entry allows, sends data on L1RXD. To prevent sending on L1RXD while listening to station A,
clear the CSEL bits in the corresponding Tx route RAM entries. Conversely, to prevent receiving on
L1TXD while sending on L1RXD, clear the CSEL bits in corresponding Rx SI RAM entries. Note that
using the SWTR option may cause data collisions with other stations unless an empty (quiet) time slot is
used.
20-14
Table 20-2. SIRAM Field Descriptions (continued)
L1RXD
L1TXD
Rx Tx
Station B
Figure 20-11. Example Using SI RAM n [SWTR]
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Description
Figure 20-11 use different time slots on one TDM
Rx Tx Tx
Station A Station B
Tx and Rx SI RAMn[SWTR] = 1
L1RXD
L1TXD
Rx
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