Atmel C51 Installation and operating instructions

1
External Brown-out Protection for C51
Microcontrollers with Active High Reset Input
Features
•Low-voltage Detector
•Prevents Register and EEPROM/Flash Corruption
•One Discrete Solution
•Integrated IC Solution
•Low-power/Low-cost Solution
•Formulas for Component Value Calculations
•Complete with Sample Schematics
Introduction
This application note shows in detail how to prevent system malfunction during peri-
ods of insufficient power supply voltage. It describes techniques to prevent the MCU
from executing code during periods of insufficient voltage by using external low volt-
age detectors. These events are often referred to as “Brown-outs”, where power
supply voltage drops to an insufficient level, or “Black-outs” where power supply volt-
age totally disappear for a period of time.
One discrete solution is discussed in detail, allowing the user to calibrate the system
requirements. A complete guide to Integrated Circuit (IC) solutions is also included. By
implementing these techniques, the following can be prevented:
• MCU Register Corruption
• I/O Register Corruption
• I/O-pin Random Toggling
• On-chip Memory Corruption (SRAM, EEPROM, Flash)
• External Memory Corruption
C51
Microcontrollers
Application
Note
Rev. 4183A–80C51–11/02

2C51 MCUs 4183A–80C51–11/02
RST Pin Description A high on the RST pin for several machine cycles while the oscillator is running, resets
the device. An internal resistor to VSS permits a power-on reset using only an external
capacitor to VCC. While this pin is forced high, the MCU Core is halted from executing
code, the peripherals are stopped and the I/Os are tri-stated.
The RST pin is an output when the Hardware Watchdog Timer times out and forces a
system reset. When Watchdog Timer overflows, it drives an output RST HIGH pulse at
the RST pin. When the watchdog is used, a resistor Rrst mounted in serial with the exter-
nal capacitor Crst or reset circuit is necessary to allow the Hardware Watchdog to drive
the RST pin.
Theory of Operation For the MCU to successfully decode and execute instructions, the supplied voltage must
always stay above the minimum voltage level specified by the product datasheet. When
supplied voltage drops below this level, the MCU might to execute some instructions
incorrectly. The result is unexpected activity on the internal data and control lines. This
activity may cause MCU Registers, I/O Registers and Data Memories to get corrupted.
To avoid unexpected activity, the MCU should be prevented from executing code during
periods of insufficient supply voltage. This is best ensured by using a Power Supply Low
Voltage Detector. Below a fixed threshold voltage VT, the detector circuit forces the RST
pin high (active). Forcing RST high immediately stops the MCU from executing code.
While the supplied voltage is below the required threshold voltage VT, the MCU is
halted, making sure the system stays in a known state. When the supplied voltage rises
above this predefined voltage, the RST pin is again released, and the MCU starts to
execute code beginning at the Reset Vector.
Preventing SFR
Corruption When the Detector keeps the MCU in Reset, all MCU activity is halted. When released
from Reset, all the Special Function Registers (SFRs) are in their default state.
Without a Detector, random MCU activity such as described before might cause the
Special Function Registers to get corrupted. See “Volatile Memory”below.
Preventing I/O Register
Corruption When using a Detector to keep the MCU in Reset, all I/O Registers are kept in their
default state for the duration of the reset. Consequently, all on-chip peripherals will stay
in their reset state.
Without a Detector, random MCU activity such as described before might write an
unknown value to any I/O Register. This may cause unexpected behavior of the on-chip
peripherals.
Preventing I/O Pin
Random Toggling A Detector will keep the MCU in Reset, and all I/O pins are kept in their default state for
the duration of the Reset.
Without a Detector, random MCU activity such as described before might write a ran-
dom value to the I/O Registers. This may cause random toggling of the I/O pins.
Preventing Volatile On-
chip Memory Corruption Using a Detector to keep the MCU in Reset, there will be no access to the volatile on-
chip memory. The memory will keep its present content for the duration of the Reset.
Without a Detector, random MCU activity such as described in the introduction may
write an unknown value to any volatile on-chip memory location.

3
C51 MCUs
4183A–80C51–11/02
Preventing External
Components Corruption Using a Detector to keep the MCU in Reset, prevents access to the external compo-
nents such as memories, peripherals, latches, etc. These components will keep their
contents or status for the duration of the Reset.
Without a Detector, random MCU activity could write an unknown value to any External
Components address (SRAM locations, peripheral command registers, etc.).
Preventing Non volatile
On-chip Memory
Corruption
Non volatile memories such as EEPROM, and Flash are designed to keep their contents
even when power is completely removed from the system. By the use of a Detector to
keep the MCU in Reset, activity on the control lines ceases. The memory contents are
then prevented from unintentional writes from the MCU for the duration of the Reset.
Without a Detector, random MCU activity could initialize an unintended write to the non-
volatile memory. This may cause random corruption of the memory contents. Since the
C51 MCU is capable of writing to its own program memory, the internal Flash Program
memory contents could be affected by a power failure situation.
Notes: 1. For any write to non volatile memory, a minimum voltage is required to successfully
write the new values into the memory. If supplied voltage at any time during the write
cycle drops below the minimum voltage, the write might fail, corrupting the location
written to.
2. When the reset activates during a write to the internal EEPROM, the operation is
aborted. The result can be seen when the location being written get corrupted.
Design Criteria
Threshold Voltage It is recommended to set the threshold voltage below minimum VCC to allow for small
fluctuations in supplied voltage (VTMAX must be less than “Supply Voltage Min”). The
threshold voltage should always be selected to ensure that the Detector keeps the
device properly reset when supply voltage drops below the critical voltage required by
the MCU (VTMIN must be greater than “Component Specification VCC Min”). Care should
be taken to ensure sufficiently high detector threshold voltage even in worst case situa-
tions. See Table 4 on page 14 for examples.
Figure 1. Threshold Voltage Choice
Component
Specification
VccMAX
VccMIN
Supply
Voltage
VccMAX
VccMIN
Treshold
Voltage
VTMAX
VTMIN
Hysteresis
HIGH
LOW

4C51 MCUs 4183A–80C51–11/02
Hysteresis Hysteresis is the difference in the voltage between the positive-going switching thresh-
old and the negative-going switching threshold. Without hysteresis, it is possible that
system or environmental noise could cause a rising signal to toggle from low to high,
then high to low again in immediate succession. Hysteresis must ensure that any noise
on the VCC input is unable to cause a rising RST signal to toggle once it has gone from
low to high or a falling RST signal to toggle once it has gone from high to low. Typical
values for hysteresis are 0.2 - 0.3V, it must be selected according to VCC characteristics.
The hysteresis thresholds must be contained between VTMIN and VTMAX.
Operating Voltage Range Carefully monitor the MCU since it can operate with a supply voltage below its Vcc min.
However, its behavior may be erratic. The MCU oscillator can run with a supply voltage
near 1 volt. Under these conditions, the Brown-out circuit must generate a valid RST
signal.
Output Transition Delay Output transition (propagation delay and slew rate) must be fast enough to stop the
MCU before it causes memory corruption or unexpected behavior of peripherals.

5
C51 MCUs
4183A–80C51–11/02
Implementation A variety of Integrated Circuit (IC) solutions are available from different manufacturers.
These solutions offer high accuracy at a low price, typically they guarantee the threshold
voltage to be within ± 1%. Although the elementary three-pin fixed voltage detector is
available, there is also a whole range of devices offering additional features such as
Reset Pulse stretching, Power-on Reset Time-out, Watchdogs, Power regulation, dual
supply switching for uninterruptible Power Supply operation and more. Included in this
application note is a guide to integrated circuit solutions available. As an alternative, this
application note also presents one discrete Low-power Supply Voltage RESET
Detector.
•Alternative 1: Minimum Power Consumption. Well-suited for battery-powered
applications where power consumption is the most critical parameter.
•Alternative 2: High accuracy, low-power consumption using commercial
semiconductor ICs.
Design Hint: Supply
Voltage Filtering,
Oscillator Stability
Use low impedance capacitors (low ESR and ESL) on the VCC and multi-layer PCB with
power planes to improve transient rejection from the power supply.
To provide good MCU startup, the oscillator must be stable for several cycles before
releasing the RST signal (refer to the device documentation).

6C51 MCUs 4183A–80C51–11/02
Alternative 1: Low-
power Consumption
Characteristics •Very Low-power Consumption, (Typ 0.5 µA@3V, 1 µA@5V)
•Low-cost Solution
•Large Hysteresis, Typ. 0.3 Volts
•Fast Output Transitions
•Accuracy ± 5 - 10%
•High Component Count
•Long Response Time against VCC
Figure 2. Low-power Consumption Brown-out Detector (1) (2)
Notes: 1. Refer to the component specifications, some products such as T89C51CC02 have
an extended tolerance range from 20K to 200K.
2. HWDT: Hardware Watchdog Timer
Figure 3. Oscilloscope Plots Show how the Voltage on RST Varies with VCC
Vcc
Vcc
Vss
RST
C51 MCU
OPTIONAL
RESET SWITCH
(1)
(2)
Rrst
1K
R
50-200K
C2
T2
C1
C3
R1 T3
R4 R5
T1 HWDT
R3
R2
+
Crst
1µF
Vcc
Vcc
Vss
RST
C51 MCU
OPTIONAL
RESET SWITCH
Rrst
1-5K
R
50-200K
T2
C2
R1
C1
T3
R4 R5
T1 HWDT
R3
C3
R2
+
Crst
1µF
VCC Optional
Reset Switch
C51 MCU
Vcc
RST
RST
Vcc 3V

7
C51 MCUs
4183A–80C51–11/02
Introduction The circuit shown in Figure 2 on page 6 benefits from low-power consumption, which
makes it suitable for battery operated applications. Standard discrete components pro-
vide a low cost design.
The voltage transition on the RST pin is very steep. Combined with the large hysteresis,
the accuracy is high. However, the response time is slow, which makes it unsuitable for
rapidly varying supply voltages.
Theory of Operation The Brown-out Detector has two stages, the Detector and the Amplifier. In the Detector
stage, the threshold voltage is set by the resistors R1 and R2 in relation to the critical
voltage of transistor T1. Under normal operation, this transistor is conducting. When the
supply voltage drops below the threshold voltage, the transistor shuts off.
The output from this Detector leads to the input of the ultra low power Amplifier stage.
Under normal operation, the low voltage of the base of transistor T2 causes it to remain
shut, allowing resistor R5 to pull the RST input low. The Amplifier stage also contains a
hysteresis feedback loop through transistor T3, short-circuiting a resistor R3 in the
amplifier when the RST output is kept high.
Selecting Components
T1, T2, and T3 The production spread of current gain β(or hFE) in transistor T1 affects the threshold
voltage VT(typically ± 0.2 volts). Most small signal transistors can be used, but low pro-
duction spread transistors are recommended.
CAUTION: If transistor T1 is changed from one type to another. The emitter-base
threshold voltage of T1 affects the constant (VBE) in the equation for threshold voltage
(below). As a consequence, a change of transistor could cause a change in the thresh-
old voltage of the detector, which requires the voltage divider R1 + R2 to be re-
calculated.
R1 and R2 R1 and R2 form a voltage divider that defines the threshold voltage VT. As the threshold
voltage depends on these resistors, it is recommended to choose resistors with 1% tol-
erance or better. See “Noise Sensitivity”.
R1 is usually chosen equal to 10 MΩto ensure the lowest power consumption possible.
R2 is then found by the equation below. The constant (Vbe) in the equation may vary
slightly with variations in transistor T1:
Note: Vbe is the T1 base to supply voltage and typically it equals 0.4V to 0.6V.
R3 R3 is a non-critical pull-up resistor that has very little influence on the threshold voltage.
It should be selected as large as possible to minimize power consumption. A resistance
of R3 greater than 10 MΩis not recommended, see “Noise Sensitivity”below.
R4 Resistor R4 defines the hysteresis of the threshold voltage (VT). By choosing R4 to
3.3 MΩ, the resulting hysteresis will be approximately 0.3 volts. A smaller R4 will give a
larger hysteresis, a larger R4 gives smaller hysteresis. A larger R4 will also result in a
less sharp transition in the output slope of the RST signal. Large deviations from the rec-
ommended value will eventually alter the constant VBE in the threshold voltage equation
above. As the hysteresis is only slightly changed with variation in R4 resistance, the
accuracy is not critical.
VTR1 R2+()
Vbe
R2
-----------


or R2 Vbe R1⋅
VTVbe–
------------------------=
,⋅=

8C51 MCUs 4183A–80C51–11/02
R5 Resistor R5 pulls the RST pin low in normal operating mode. A value in the range of
100 kΩis recommended to tie RST securely to GND. As no current goes through this
resistor in normal operating mode, its value and accuracy is of little importance. When
RST is pulled high, this resistor will start conducting a relatively large current.
C1 and C2 Capacitors C1 and C2 reduce RF noise picked up in the circuitry and amplified by the
transistors. Both capacitors can be omitted, but a value greater than 1 nF is recom-
mended. For maximum noise immunity, 100 nF (LF) or capacitors with lower ESR (HF)
should be selected when possible. Also see “Response Time”below. The accuracy is
not critical, but to ensure proper RF decoupling, the capacitors should have Z5U dielec-
tric or better.
C3 Capacitor C3 decouples the power lines. It can be omitted if there is RF decoupling of
the power lines somewhere nearby on the circuit board, otherwise 1 nF is recom-
mended. For maximum noise immunity, 100 nF (LF) or low ESR (HF) should be
selected.
Crst Capacitor Crst (typically 1 µF) from the RST pin to the VCC supply rail is the simplest way
to achieve a reset time of approximatively 100 msS. This reset time is dependant on the
sink current defined by the microcontroller (refer to the device specification). Crst must
be selected to ensure that the RST signal is high during at least two machine cycles
after the power on (refer to the device specification).
Rrst Rrst is necessary when the Hardware Watchdog of the MCU is used. It allows the MCU
to drive the RST pin high and it limits the output current of the RST pin. The recom-
mended value is 1 to 5K.
Reset Switch/In-System
Programming If a push button reset is required, it is simply connected in parallel as shown in Figure 2.
As the switch/programmer will pull RST high, power consumption in R5 will be relatively
high for the duration of the event. Also see “Power Consumption”.
Response Time Choosing large values for capacitors C1 and C2 will slow down the circuit’s response
time. This is not a problem with battery driven applications where the supply voltage
decreases slowly over time. Allthough the response time also applies to the time imme-
diately following power-on. This could affect operation when a flat battery is loaded.
When power can drop more rapidly, the long response time due to C1 and C2 should be
taken into consideration.
Noise Sensitivity Choosing values of R1 and R3 greater than 10 MΩis not recommended, as it makes the
circuitry sensitive to thermal noise generated in the resistor. When noise is not critical,
the values of R1 and R3 can be raised to 20 MΩ. Choosing larger values will result in
the resistors not conducting sufficient current, giving in a non-functional Detector. If
more noise immunity is required, these resistors can be reduced with smaller ones, at
the expense of increased power consumption.
Capacitors C1, C2 and C3 are decoupling capacitors to minimize noise sensitivity to
both RF and 50/60 Hz fields. They can all be omitted, but the noise immunity greatly
depends on the values selected.

9
C51 MCUs
4183A–80C51–11/02
Threshold Accuracy Because the threshold voltage is defined mainly by R1 and R2, inaccuracies in these
resistors directly influence the threshold voltage accuracy. It is recommended to choose
these with ± 1% tolerance.
Power Consumption The current consumption in normal operating mode (sufficiently high VCC) is found by:
When reset switch or programmer force RST to GND, the current increases to:
When voltage drops to the level where the detector activates, transistor T1 closes, T2
opens and the current is:
As resistor R5 is usually chosen much smaller than the other resistors R1-R4, the last
two expressions both simplify to:
Note: The operator indicates resistors are parallel.
In Table 1 the values are calculated to minimize the power consumption.
Table 1. Example Values when MCU RST Internal Pull-down is in the Range 50 - 200K
Component
Example Values
Recommended Tolerance3.0V 4.5V
T1, T2 BC558/BC888/2N3906 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100
T3 BC548/BC848/2N3904 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100
R1 10 MΩ≤1%
R2 1.54 MΩ976 kΩ≤1%
R3 6.8 MΩ≤20%
R4 3.3 MΩ≤20%
R5 100 kΩ≤20%
C1, C2, C3 100 nF ≤20%, Z5U dielectric or better
IVCC
R1 R2+()R3 R4+()
||
----------------------------------------------------------


VCC 1
R1 R2+
--------------------- 1
R3 R4+
---------------------+


=≈
IVCC
R1 R2+()R3 R4+()R5 RRESET
|| || ||
-------------------------------------------------------------------------------------------------
≈
IVCC
R1 R2+()R5 RRESET
|| ||
-----------------------------------------------------------------≈
IVCC
R5 RRESET
||
---------------------------------≈

10 C51 MCUs 4183A–80C51–11/02
In Table 2 the transistor T2 can source more current. The Vbe constant is approximately
0.5V.
Table 2. Example Values when MCU RST Internal Pull-down is in the Range 20 - 200K
Component
Example Values
Recommended Tolerance3.0V 4.5V
T1, T2 BC558/BC888/2N3906 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100
T3 BC548/BC848/2N3904 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100
R1 6.04 MΩ6.19 MΩ≤1%
R2 1.3 MΩ820 kΩ≤1%
R3 1.5 MΩ≤20%
R4 510 KΩ≤20%
R5 100 kΩ≤20%
C1, C2, C3 100 nF ≤20%, Z5U dielectric or better