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TMP92CM22
2007-02-16
92CM22-34
3.4.2 Micro DMA
In addition to general-purpose interrupt processing, the TMP92CM22 also includes a
micro DMA function. Micro DMA processing for interrupt requests set by micro DMA is
performed at the highest priority level for maskable interrupts (Level 6), regardless of the
priority level of the interrupt source.
Because the micro DMA function is implemented through the CPU, when the CPU is
placed in a stand-by state by a Halt instruction, the requirements of the micro DMA will be
ignored (pending).
Micro DMA is supports 8 channels and can be transferred continuously by specifying the
micro DMA burst function as below.
(1) Micro DMA operation
When an interrupt request is generated by an interrupt source specified by the micro
DMA start vector register, the micro DMA triggers a micro DMA request to the CPU at
interrupt priority level 6 and starts processing the request. The eight micro DMA
channels allow micro DMA processing to be set for up to eight types of interrupt at
once.
When micro DMA is accepted, the interrupt request flip-flop assigned to that
channel is cleared. Data in one-byte, two-byte or four-byte blocks, is automatically
transferred at once from the transfer source address to the transfer destination
address set in the control register, and the transfer counter is decremented by 1. If the
value of the counter after it has been decremented is not 0, DMA processing ends with
no change in the value of the micro DMA start vector register. If the value of the
decremented counter is 0, a micro DMA transfer end interrupt (INTTC0 to INTTC7) is
sent from the CPU to the interrupt controller. In addition, the micro DMA start vector
register is cleared to 0, the next micro DMA operation is disabled and micro DMA
processing terminates.
If micro DMA requests are set simultaneously for more than one channel, priority is
not based on the interrupt priority level but on the channel number: the lower the
channel number, the higher the priority (channel 0 thus has the highest priority and
channel 7 the lowest).
If an interrupt request is triggered for the interrupt source in use during the interval
between the time at which the micro DMA start vector is cleared and the next setting,
general purpose interrupt processing is performed at the interrupt level set. Therefore,
if the interrupt is only being used to initiate micro DMA (and not as a general-purpose
interrupt), the interrupt level should first be set to 0 (i.e., interrupt requests should be
disabled).
If micro DMA and general purpose interrupts are being used together as described
above, the level of the interrupt which is being used to initiate micro DMA processing
should first be set to a lower value than all the other interrupt levels. (Note) In this
case, edge triggered interrupts are the only kinds of general interrupts which can be
accepted.
Note: If the priority level of micro DMA is set higher than that of other interrupts, CPU operates as follows.
In case INTxxx interrupt is generated first and then INTyyy interrupt is generated between checking
“Interrupt specified by micro DMA start vector” (in the
Figure 3.4.1) and reading interrupt vector with
setting below. The vector shifts to that of INTyyy at the time.
This is because the priority level of INTyyy is higher than that of INTxxx.
In the interrupt routine, CPU reads the vector of INTyyy because cheking of micro DMA has finished.
And INTyyy is generated regardless of transfer counter of micro DMA.
INTxxx: level 1 without micro DMA
INTyyy: level 6 with micro DMA