SECTION 15: MESSAGE DATA LINK CONTROLLER
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15.2.3
MDLC STATUS REGISTER (MSR) $0F
This register indicates the transmit and receive status of the MDLC.
All bits may be read in all modes of MCU operation.
Bits 0,1, 4, 5, 6, and 7 will always read as zeros and can never be written to.
Bits 2 and 3 are reflective of CPU interrupt request signals and are asserted as long as their
respective CPU interrupt request is pending. Neither bit is affected by CPU interrupt
requests generated when the MDLC ’wakes up’ from MDLC Stop or MDLC Wait mode.
All writes to this register are ignored in all modes of MCU operation.
15.2.3.1
TXMS - Transmitted Message Successfully
When set this bit indicates that the Tx Buffer contents have been sent successfully.
An access of the Tx Control Register (MTCR) will clear this bit.
15.2.3.2
RXMS - Received Message Successfully
When set, this bit indicates that one of the Rx Buffers contains a new message.
This bit can only be cleared if both Rx Buffers are empty. If only one message has been
received into an Rx Buffer by the MDLC, any access (read or write) of the MDLC Rx Status
Register (MRSR) will clear the RXMS bit.
If a second message is received after the RXMS bit has been cleared by the CPU as it is
retrieving the first message, the RXMS bit will immediately be set again, and will remain set
until the MRSR is accessed once the last message received is made available to the CPU.
If both Rx Buffers contain a received message, the RXMS bit will simply remain set until the
MRSR register is accessed once the last message received is made available to the CPU.
15.2.3.3
TXMS and RXMS Interrupts
If either the TXMS or the RXMS bit is set, and if interrupts are enabled, an interrupt request
will be made to the CPU. The interrupt service routine may poll these bits to establish the
cause of the interrupt. The mechanism for clearing each of these bits will also clear the
associated CPU interrupt request.
Alternatively, if interrupts are disabled, the CPU may poll these bits periodically to see if a
change in status has occurred.
Figure 15-4: MDLC Status Register (MSR)
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
0
0
RXMS
TXMS
0
0
0
0
RESET
0
0
0
0
0
0
0
0
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