Document # 001-20559 Rev. *D
179
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 De-Multiplexers
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 de-mux 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, 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 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.
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 OR’ed in the capture cir-
cuitry. Since the capture circuitry is positive edge sensitive,
during an interval where the hardware capture input is high,
a software capture is masked and does 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 over-
writes the compare value.
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).
Timers may be chained in 8-bit lengths up to 32 bits.
17.1.6.1
Usability Exceptions
The following are usability exceptions for the timer function.
1. Capture operation is not supported at 48 MHz.
2. DR2 is not writeable when the timer is enabled.
17.1.6.2
Block Interrupt
The timer block has a selection of three interrupt sources.
interrupt on terminal count (TC) and interrupt on compare
may be selected in Mode bit 0 of the function register. The
third interrupt source, interrupt on capture, may be selected
with the Capture Interrupt bit in the control register.
■
Interrupt on Terminal Count
: The positive edge of ter-
minal count (primary output) generates an interrupt for
this block. The timing of the interrupt follows the TC
pulse width setting in the control register.
■
Interrupt on Compare
: The positive edge of compare
(auxiliary output) generates an interrupt for this block.
■
Interrupt on Capture
: Hardware or software capture
generates an interrupt for this block. The interrupt occurs
at the closing of the DR2 latch on capture.
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
outputs 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, but since
SYSCLK cannot resync itself, it allows a direct
skew controlled SYSCLK source.
Содержание PSoC CY8C23533
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