SCC HDLC Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
23-18
Freescale Semiconductor
Figure 23-11. Typical HDLC Bus Single-Master Configuration
23.14.1 HDLC Bus Features
The main features of the HDLC bus are as follows:
•
Superset of the HDLC controller features
•
Automatic HDLC bus access
•
Automatic retransmission in case of collision
•
May be used with the NMSI or a TDM bus
•
Delayed RTS mode
23.14.2 Accessing the HDLC Bus
The HDLC bus protocol ensures orderly bus control when multiple transmitters attempt simultaneous
access. The transmitter sending a zero bit at the time of collision completes the transmission. If a station
sends out an opening flag (0x7E) while another station is already sending, the collision is always detected
within the first byte, because the transmission in progress is using zero bit insertion to prevent flag
imitation.
While in the active condition (ready to transmit), the HDLC bus controller monitors the bus using CTS. It
counts the one bits on CTS. When eight consecutive ones are counted, the HDLC bus controller starts
transmitting on the line; if a zero is detected, the internal counter is cleared. During transmission, data is
continuously compared with the external bus using CTS. CTS is sampled halfway through the bit time
using the rising edge of the Tx clock. If the transmitted bit matches the received CTS bus sample,
transmission continues. However, if the received CTS sample is 0 and the transmitted bit is 1, transmission
stops after that bit and waits for an idle line before attempting retransmission. Since the HDLC bus uses a
HDLC
Controller
RXD
TXD
A
RCLK
HDLC Bus
Controller
RXD
CTS
TXD
B
HDLC Bus
Controller
RXD
CTS
TXD
C
Clock1
HDLC Bus LAN
+ 5 V
R
Slave
Slave
Master
Notes:
1. Transceivers may be used to extend the LAN size.
2. The TXD pins of slave devices should be configured to open-drain in the port C parallel I/O port.
3. Clock1 is the master RCLK and the slave TCLK.
Clock2
TCLK
RCLK
TCLK
RCLK
TCLK
4. Clock2 is the master TCLK and the slave RCLK.
Summary of Contents for PowerQUICC MPC870
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