CY8C28xxx PSoC Programmable System-on-Chip TRM, Document No. 001-52594 Rev. *G
513
30. System Resets
This chapter discusses the System Resets and their associated registers. PSoC
®
devices support several types of resets.
The various resets are designed to provide error-free operation during power up for any voltage ramping profile, to allow for
user-supplied external reset and to provide recovery from errant code operation. For a complete table of the System Reset
registers, refer to the
“Summary Table of the System Resource Registers” on page 462
. For a quick reference of all PSoC
registers in address order, refer to the
Register Details chapter on page 125
30.1
Architectural Description
When reset is initiated, all registers are restored to their
default states. In the
Register Details chapter on page 125
this is indicated by the POR column in the register tables
and elsewhere it is indicated in the Access column, values
on the right side of the colon, in the register tables. Minor
exceptions are explained here.
The following types of resets can occur in the PSoC device:
■
Power on Reset (POR). This occurs at low supply volt-
age and is comprised of multiple sources.
■
External Reset (XRES). This active high reset is driven
into the PSoC device, on parts that contain an XRES pin.
■
Watchdog Reset (WDR). This optional reset occurs
when the watchdog timer expires, before being cleared
by user firmware. Watchdog reset defaults to off.
■
Internal Reset (IRES). This occurs during the boot
sequence, if the SROM code determines that Flash
reads are not valid.
The occurrence of a reset is recorded in the Status and Con-
trol registers (CPU_SCR0 for POR, XRES, and WDR) or in
the System Status and Control Register 1 (CPU_SCR1 for
IRESS). Firmware can interrogate these registers to deter-
mine the cause of a reset.
30.2
Pin Behavior During Reset
Power on Reset and External Reset cause toggling on two
GPIO pins, P1[0] and P1[1], as described here and illus-
trated in
. This allows program-
mers to synchronize with the PSoC device. All other GPIO
pins are placed in a high impedance state during and imme-
diately following reset.
30.2.1
GPIO Behavior on Power Up
At power up, the internal POR causes P1[0] to initially drive
a strong high (1) while P1[1] drives a resistive low (0). After
256 sleep oscillator cycles (approximately 8 ms), the P1[0]
signal transitions to a resistive low state. After additional 256
sleep oscillator clocks, both pins transition to a high imped-
ance state and normal CPU operation begins. This is illus-
trated in
.
Figure 30-1. P1[1:0] Behavior on Power Up
30.2.2
GPIO Behavior on External Reset
During External Reset (XRES = 1), both P1[0] and P1[1]
drive resistive low (0). After XRES deasserts, these pins
continue to drive resistive low for another 8 sleep clock
cycles (approximately 200
s). After this time, both pins
transition to a high impedance state and normal CPU opera-
tion begins. This is illustrated in
Figure 30-2. P1[1:0] Behavior on External Reset (XRES)
Internal
Reset
P1[0]
P1[1]
HiZ
HiZ
Vdd
POR Trip
Point
S1
R0
R0
R0
T1
T2
T1 = T2 = 256 Sleep Clock Cycles
(approximately 8 ms)
XRES
P1[0]
P1[1]
HiZ
HiZ
R0
R0
T1
T1 = 8 Sleep Clock Cycles
(approximately 200
s)
Содержание CY8C28 series
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