CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
337
Digital Blocks
17.1.2.1
Clock Resynchronization Summary
■
Digital PSoC blocks have extremely flexible clocking
configurations. To maintain reliable timing, input clocks
must be resynchronized.
■
The master clock for any clock in the system is either
SYSCLK or SYSCLKX2. Determine the master clock for
a given input clock and resynchronize to that clock.
■
Do not use divide by 1 clocks derived from SYSCLK and
SYSCLKX2. Use the direct SYSCLK or SYSCLKX2
clocking option available at the block.
17.1.3
Output Demultiplexers
Most functions have two outputs: a primary and an auxiliary
output, the meaning of which are function dependent. Each
of these outputs may be driven onto the row output bus.
Each demux is implemented with four tri-state drivers. There
are two bits in the output register to select one of the four tri-
state drivers and an additional bit to enable the selected
driver.
17.1.4
Block Chaining Signals
Each digital block has the capability to be chained and to
create functions with bit widths greater than eight. There are
signals to propagate information, such as Compare, Carry,
Enable, Capture and Gate, from one block to the next to
implement higher precision functions. The selection made in
the function register determines which signals are appropri-
ate for the desired function. User Modules that have been
designed to implement digital functions, with greater than 8-
bit width, will automatically make the proper selections of the
chaining signals, to ensure the correct information flow
between blocks.
17.1.5
Input Data Synchronization
Any asynchronous input derived from an external source,
such as a GPIO pin input, must be resynchronized through
the row input before use into any digital block clock or data
input. This is the default mode of operation (resynchroniza-
tion is on).
17.1.6
Timer Function
A timer consists of a period register, a
down
counter, and a capture/compare register, all of which are
byte wide. When the timer is disabled and a period value is
written into DR1, the period value is also loaded into DR0.
When the timer is enabled, the counter counts down until
positive terminal count (a count of 00h) is reached. On the
next clock edge, the period is reloaded and, on subsequent
clocks, counting continues. The terminal count signal is the
primary function output. (Refer to the timing diagram for this
function on page
.) This can be configured as a full or
half clock cycle.
This function also supports multi-shot mode. When the
multi-shot register is set to non-zero, the function is in multi-
shot mode. For example, if the multi-shot register is set to
01h, the function is disabled after it reaches the first 00
value in DR0. If the multi-shot register is set to 02h, when
the function reaches the first 00, DR0 is reloaded and runs
again. The function is disabled after the second 00 in DR0
register. The multi-shot supports up to a MAX number of 15
shots.
Hardware capture occurs on the positive edge of the data
input. This event transfers the current count from DR0 to
DR2. The captured value may then be read directly from
DR2. A software capture function is equivalent to a hard-
ware capture. A CPU read of DR0, with the timer enabled,
triggers the same capture mechanism. The hardware and
software capture mechanisms are ORed in the capture cir-
cuitry. Because the capture circuitry is positive edge sensi-
tive, during an interval where the hardware capture input is
high, a software capture is masked and will not occur.
The timer also implements a compare function between
DR0 and DR2. The compare signal is the auxiliary function
output. A limitation, in regards to the compare function, is
that the capture and compare function both use the same
register (DR2). Therefore, if a capture event occurs, it will
overwrite the compare value.
There is another mode, called NPS mode, supported at the
compare output. When it is set, the compare output is
delayed half clock cycle. It is used to achieve a higher reso-
lution when 48MHz clock is used as block clock.
This function also supports KILL function. There are two
KILL modes: KILL-Disable and KILL-Reload. In KILL-Dis-
able mode, the function is disabled immediately when kill is
asserted. The function must be restarted in firmware. In
KILL-Reload mode, the DR0 register and multi-shot counter
register stays in reload state when KILL is high, and the
function counts down when KILL is released. The function
outputs are gated to zeros when KILL is asserted. For more
detail see
Mode bit 1 in the function register sets the compare type
(DR0 <= DR2 or DR0 < DR2) and Mode bit 0 sets the inter-
rupt type (Terminal Count or Compare). There are also two
more bits used to control interrupts, located at bit 1 in CR0
and bit 0 in CR1.
Table 17-1. AUXCLK Bit Selections
Code
Description
Usage
00
Bypass
Use this setting only when SYSCLKX2 (48 MHz) is
selected. Other than this case, asynchronous clock
inputs are not recommended. This setting is also
required for blocks to remain active while in sleep.
01
Resynchronize
to SYSCLK
(24 MHz)
Use this setting for any SYSCLK-based clock. VC1,
VC2, VC3 driven by SYSCLK, digital blocks with
SYSCLK-based source clocks, broadcast bus with
source based on SYSCLK, row input and row out-
puts with source based on SYSCLK.
10
Resynchronize
to SYSCLKX2
(48 MHz)
Use this setting for any SYSCLKX2-based clock.
VC3 driven by SYSCLKX2, digital blocks with
SYSCLKX2-based source clocks, broadcast bus
with source based on SYSCLKX2, row input and
row outputs with source based. on SYSCLKX2.
11
SYSCLK Direct
Use this setting to clock the block directly using
SYSCLK. Note that this setting is not strictly related
to clock resynchronization; because YSCLK cannot
resynchronize itself, it allows a direct skew con-
trolled SYSCLK source.
Содержание CY8C28 series
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