EM773
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NXP B.V. 2012. All rights reserved.
Product data sheet
Rev. 2 — 3 January 2012
18 of 51
NXP Semiconductors
EM773
Energy metering IC
7.16 System control
7.16.1 Start logic
The start logic connects external pins to corresponding interrupts in the NVIC. Each pin
shown in
Table 2 as input to the start logic has an individual interrupt in the NVIC interrupt
vector table. The start logic pins can serve as external interrupt pins when the chip is
running. In addition, an input signal on the start logic pins can wake up the chip from
Deep-sleep mode when all clocks are shut down.
The start logic must be configured in the system configuration block and in the NVIC
before being used.
7.16.2 Reset
Reset has four sources on the EM773: the RESET pin, the Watchdog reset, Power-On
Reset (POR), and the BrownOut Detection (BOD) circuit. The RESET pin is a Schmitt
trigger input pin. Assertion of chip reset by any source, once the operating voltage attains
a usable level, starts the IRC and initializes the flash controller.
A LOW-going pulse as short as 50 ns resets the part.
When the internal Reset is removed, the processor begins executing at address 0, which
is initially the Reset vector mapped from the boot block. At that point, all of the processor
and peripheral registers have been initialized to predetermined values.
An external pull-up resistor is required on the RESET pin if Deep power-down mode is
used.
7.16.3 Brownout detection
The EM773 includes four levels for monitoring the voltage on the VDD pin. If this voltage
falls below one of the four selected levels, the BOD asserts an interrupt signal to the
NVIC. This signal can be enabled for interrupt in the Interrupt Enable Register in the NVIC
in order to cause a CPU interrupt; if not, software can monitor the signal by reading a
dedicated status register. Four additional threshold levels can be selected to cause a
forced reset of the chip.
7.16.4 Code security (Code Read Protection - CRP)
This feature of the EM773 allows user to enable different levels of security in the system
so that access to the on-chip flash and use of the Serial Wire Debugger (SWD) and
In-System Programming (ISP) can be restricted. When needed, CRP is invoked by
programming a specific pattern into a dedicated flash location. IAP commands are not
affected by the CRP.
In addition, ISP entry via the PIO0_1 pin can be disabled without enabling CRP. For
details see the EM773 user manual.
There are three levels of Code Read Protection:
1. CRP1 disables access to the chip via the SWD and allows partial flash update
(excluding flash sector 0) using a limited set of the ISP commands. This mode is
useful when CRP is required and flash field updates are needed but all sectors can
not be erased.