Motorola MPC8260 User Manual

26-22

MOTOROLA

When the core enables the SMC receiver in transparent mode, it waits for synchronization
before receiving data. Once synchronization is achieved, the receiver transfers the incoming
data into memory according to the Þrst RxBD in the table. Synchronization can be achieved
in two ways. First, when the receiver is connected to a TDM channel, it can be synchronized
to a time slot. Once the frame sync is received, the receiver waits for the Þrst bit of its time
slot to occur before reception begins. Data is received only during the time slots deÞned by
the TSA. Secondly, when working with its own set of signals, the receiver starts reception
when SMSYNx is asserted. 

When the buffer full, the SMC clears the E bit in the BD and generates an interrupt if the I
bit in the BD is set. If incoming data exceeds the data buffer length, the SMC fetches the
next BD; if it is empty, the SMC continues transferring data to this BDÕs buffer. If the CM
bit is set in the RxBD, the E bit is not cleared, so the CP can automatically overwrite the
buffer on its next access.

The SMSYN signal offers a way to externally synchronize the SMC channel. This method
differs somewhat from the synchronization options available in the SCCs and should be
studied carefully. See Figure 26-11 for an example.

Once SMCMR[REN] is set, the Þrst rising edge of SMCLK that Þnds SMSYN low causes
the SMC receiver to achieve synchronization. Data starts being received or latched on the
same rising edge of SMCLK that latched SMSYN. This is the Þrst bit of data received. The
receiver does not lose synchronization again, regardless of the state of SMSYN, until REN
is cleared.

Once SMCMR[TEN] is set, the Þrst rising edge of SMCLK that Þnds SMSYN low
synchronizes the SMC transmitter which begins sending ones asynchronously from the
falling edge of SMSYN. After one character of ones is sent, if the transmit FIFO is loaded
(the TxBD is ready with data), data starts being send on the next falling edge of SMCLK
after one character of ones is sent. If the transmit FIFO is loaded later, data starts being sent
after some multiple number of all-ones characters is sent.

Note that regardless of whether the transmitter or receiver uses SMSYN, it must make
glitch-free transitions from high-to-low or low-to-high. Glitches on SMSYN can cause
errant behavior of the SMC. 

The transmitter never loses synchronization again, regardless of the state of SMSYN, until
the TEN bit is cleared or an 

ENTER

 

HUNT

 

MODE

 command is issued.