Z85233 Datasheet, PDF (308/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

SCC™/ESCC™ User’s Manual
Zilog SCC
Asynchronous mode:
External clock
6x mode (no BRG)
BRG
16x mode (TX + 0)
Synchronous mode:
Using external clock
Using DPLL, FM encoding
Using DPLL, MRZ/NRZI encoding
Using DPLL, FM, BRG
Using DPLL, NRZ/NRZI, BRG
4 MHz
6 MHz
8 MHz
10 MHz 16 MHz 20 MHz
250K
62.5K
375K
93.75K
1M
250K
125K
62.5K
32.25K
1.5M
375K
187.5K
93.75K
46.88K
500K
125K
635K
156.5K
2M
500K
250K
125K
62.5K
2.5M
625K
312.5K
156.25K
78.125K
1M
250K
1.25M
312.5K
4M
1M
500K
250K
125K
5M
1.25M
625K
312.5K
156.25K
Q. Can the maximum transfer rate using an external
clock be achieved?
A. Yes, but it is not trivial. In order to achieve the maximum
rate on transmit, the SCC should have a dedicated pro-
cessor or DMA. For example, at a 1 MHz rate, a byte
must be loaded into the SCC every 8 microseconds. To
achieve the maximum rate on receive, requires that the
receive clock and the SCC PCLK be synchronized.
(RTxC to PCLK setup time at maximum rate in the
Product Specification.) It is probably easier to use a
slightly faster PCLK SCC, or back off slightly from the
maximum rate.
FIFO
Q. How do you avoid an overrun in the received
FIFO?
A. The receive buffer must be read before the recently re-
ceived data character on the serial input is shifted into
the receive data FIFO. This FIFO is three bytes deep.
Thus, if the buffer is not read, the fifth character just ar-
rived causes an overrun condition. There is no bit that
can be set or reset to disable the buffering.
Q. What happens when you read an empty FIFO?
A. You read the last character in the buffer.
Q. When the FIFO gets locked due to an error condi-
tion, can it still receive?
A. The SCC continues to receive until an overrun occurs.
Q. Assuming that there are characters available in
the FIFO, what happens to them if the receiver
goes into the hunt mode?
A. They will remain in the FIFO until they are either read
by the CPU or DMA, or until the channel is reset.
UM010901-0601
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