380
CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
Digital Blocks
Clock Generation and Start Detection.
The input clock
selection is a free running, eight times over-sampling clock.
This clock is used by the clock divider circuit to generate the
block clock at the bit rate. As shown in
, the
clock block is derived from the MSb of a 3-bit counter, giving
a sample point as near to the center of the bit time as possi-
ble. This block clock is used to clock all internal circuits.
Because the RXD bit rate is asynchronous to the block bit
clock, these clocks must be continually re-aligned. This is
accomplished with the START bit detection.
When in IDLE state, the clock divider is held in reset. On
START (when the input RXD transitions are detected as a
logic 0), the reset is negated and the divider is enabled to
count at the eight times rate. If the RXD input is still logic 0
after three samples of the input clock, the status RXACTIVE
is asserted, which initiates a reception. If this sample of the
RXD line is a logic 1, the input '0' transition was assumed to
be a false start and the Receiver remains in the idle state.
shows that the internal bit clock (CCLK) is run-
ning slower than the external TX bit clock and the STOP bit
is sampled later than the actual center point. After the STOP
bit is sampled, the 24 MHz reset pulse forces the Receiver
back to an idle state. In this state, the next START bit search
is initiated, resynchronizing the RX bit clock to the TX bit
clock.
Figure 17-37. Clock Generation and Start Detection
CLKIN
RXD
(ASYNCH)
START
STATE
Input is sampled at the
center of the bit time.
Start detection enables
the clock divider.
CCLK
RESET
(CLK GEN)
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
COUNT
IDLE
BIT0
1
BIT0
START BIT
0
1
2
3
7
0
1
Actual
center of
STOP bit.
STOP bit
sample
point.
Width of reset is
one 24 MHz
clock pulse.
Next
START
bit
RXACTIVE
START is confirmed with
another sample at the
3rd sample clock.
STOP
IDLE
STOP
Reset to IDLE
and initiate
search for a new
START bit.
Содержание CY8C28 series
Страница 65: ...64 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G RAM Paging ...
Страница 85: ...84 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Internal Main Oscillator IMO ...
Страница 93: ...92 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G External Crystal Oscillator ECO ...
Страница 97: ...96 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Phase Locked Loop PLL ...
Страница 125: ...124 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G ...
Страница 311: ...310 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G IDAC_CR0 1 FDh ...
Страница 317: ...316 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G ...
Страница 393: ...392 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G ...
Страница 425: ...424 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Analog Reference ...
Страница 461: ...460 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Two Column Limited Analog System ...
Страница 477: ...476 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Digital Clocks ...
Страница 483: ...482 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Multiply Accumulate MAC ...
Страница 513: ...512 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Internal Voltage Reference ...
Страница 523: ...522 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Switch Mode Pump SMP ...
Страница 533: ...532 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G I O Analog Multiplexer ...
Страница 537: ...536 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G Real Time Clock RTC ...
Страница 561: ...560 CY8C28xxx PSoC Programmable System on Chip TRM Document No 001 52594 Rev G ...