cop8ame9 National Semiconductor Corporation, cop8ame9 Datasheet - Page 55

• Manufacturer Part Number: cop8ame9
• Description: 8-bit Cmos Flash Microcontroller With 8k Memory, Dual Op Amps, Virtual Eeprom, Temperature Sensor, 10-bit A/d And Brownout Reset
• Manufacturer: National Semiconductor Corporation

15.0 A/D Converter
Source impedances greater than 3 kΩ on the analog input
lines will adversely affect the internal RC charging time
during input sampling. As shown in Figure 29, the analog
switch to the Sample & Hold capacitor is closed only during
the 3 A/D cycle sample time. Large source impedances on
the analog inputs may result in the Sample & Hold capacitor
not being charged to the correct voltage levels, causing
scale errors.
*The analog switch is closed only during the sample time.
16.0 Temperature Sensor
16.1 GENERAL DESCRIPTION
The Temperature Sensor on this device operates over a
−40˚C to +125˚C temperature range and produces an output
voltage proportional to the device temperature. The transfer
function is approximately linear. Refer to the A/D Converter
section to see how the Temperature Sensor is integrated
with the Programmable Gain Amplifier and A/D Converter.
The equation for V
where T is the temperature in ˚C.
The user can achieve greater temperature sensor accuracy
by performing a two temperature calibration to compensate
for device-to-device variations in slope and base value for
V
16.2 OPERATION
The Temperature Sensor is used in conjunction with the
on-chip Programmable Gain Amplifier and A/D converter to
read the Temperature Sensor output voltage. The Program-
mable Gain Amplifier must be used and the gain can be set
at either 1 or 2, depending on the operating voltage of the
device. To use a gain of 2, V
The output voltage given in the above equation is for a gain
of 1. The Temperature Sensor is connected to channel 7 on
the A/D Converter multiplexor. See the A/D Converter sec-
tion for more details on using the ENAD and ADGAIN regis-
ters.
The Temperature Sensor is enabled by setting the ENTS bits
in the ADGAIN register to a 1. The circuit will draw power
when it’s enabled. When disabled, the current drawn is
OUT
.
V
OUT
= [(−8.0 mV/˚C) X T] + 1.65V
OUT
vs. temperature is:
CC
should be greater than 4.5V.
(Continued)
FIGURE 29. A/D Pin Model (Single Ended Mode)
55
If large source resistance is necessary, the recommended
solution is to slow down the A/D clock speed in proportion to
the source resistance. The A/D Converter may be operated
at the maximum speed for R
than 3 kΩ, A/D clock speed needs to be reduced. For ex-
ample, with R
at half the maximum speed. A/D Converter clock speed may
be slowed down by either increasing the A/D prescaler
divide-by or decreasing the CKI clock frequency. The A/D
minimum clock speed is 64 kHz.
extremely low. When using the HALT mode of the device, the
Temperature Sensor will draw current unless it is disabled by
software. Therefore, for minimal current in HALT mode, the
Temperature Sensor should be disabled prior to entering
HALT. A Reset will disable the Temperature Sensor.
16.2.1 Procedure for Reading the Temperature Sensor
Voltage
The following steps should be followed for measuring the
temperature sensor voltage:
1. Enable the Temperature Sensor by setting the ENTS bit
2. Wait 350 µs for the temperature sensor to stabilize. This
3. Load the ENAD register with the channel number for the
4. Wait for the Programmable Gain Amplifier to stabilize
5. Set the ADBSY bit in the ENAD register. The other bits in
6. Wait for the ADBSY bit to go to 0 and then read the
7. Subsequent readings of the temperature sensor can be
in the ADGAIN register to 1. The Programmable Gain
Amplifier gain should also be selected to be either 1 or 2.
is only required after ENTS bit is changed from 0 to 1.
temperature sensor, and the desired prescale value. The
ADMOD, MUX, and ADBSY bits should be 0.
with the voltage for the newly selected channel.
the ENAD register should be the same as in step 3.
output of the A/D Converter result registers, ADRSTH
and ADRSTL.
done by repeating steps 5 and 6, as long as the channel
number in ENAD has not changed from that of the
temperature sensor. If the channel number has been
changed to measure other channels, in between two
successive temperature sensor readings, then steps
1–6 should be followed.
S
= 6 kΩ, the A/D Converter may be operated
S
less than 3 kΩ. For R
20006362
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S
greater