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APPENDIX A
SERIAL COMMUNICATION PERFORMANCE
Operating at 25MHz, the MPC823e is designed to support unrestricted operation of the
high-level data link control (HDLC) or transparent protocol running on the serial
communication controllers at 2.048Mbps. The MPC823e can also support one Ethernet
channel at 10Mbps and one HDLC or transparent channel at 1Mbps. The physical clocking
limit of the serial communication controllers is higher than the sustained serial bit rate. This
limit is given as a 1:2.25 ratio between the sync clock, which is a clock generated in the clock
synthesizer that can be as fast as the 25MHz system clock and the serial clock. For
example, with a sync clock of 25MHz, the serial communication controllers can be clocked
at 11.1MHz. This clocking scheme allows the serial communication controllers to handle
high-speed bursts of data bits for short periods of time subject to the FIFO sizes.
When the serial communication controllers are connected to a time-division multiplexed
channel using the time-slot assigner on the MPC823e, a serial communication controller's
physical clocking limit is a 1:2.5 ratio between the sync clock and serial clock. Therefore, the
serial communication controllers can be connected to a 10.0MHz time-division multiplexed
channel with a 25MHz MPC823e. This clocking scheme allows it to handle high-speed
bursts of data bits for short periods of time subject to the FIFO sizes. Other devices that offer
a higher HDLC performance than the MPC823e are the Motorola MC68605 1984 CCITT
X.25 LAPB controller and MC68606 CCITT Q.921 multilink LAPD controller. The MC68605
and MC68606 perform the full data-link layer protocol and support various transparent
modes within HDLC-framed operation at a minimum 10Mbps. The performance figures
listed in Table A-1 are for a 25MHz system clock. Notice that, in general, performance scales
linearly with the frequency, so that a combination of protocols over the MPC823e's
performance limitation at 25MHz can occur at 50MHz.
MOTOROLA
MPC823e REFERENCE MANUAL
A-1
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