C8051F987-GMR Silicon Labs, C8051F987-GMR Datasheet - Page 188

• Manufacturer Part Number: C8051F987-GMR
• Description: 8-bit Microcontrollers - MCU 8kB 512B RAM
• Manufacturer: Silicon Labs

Specifications of C8051F987-GMR

• Rohs: yes
• Core: 8051
• Processor Series: C8051
• Data Bus Width: 8 bit

C8051F99x-C8051F98x
Important Note on External Crystals: Crystal oscillator circuits are quite sensitive to PCB layout. The
crystal should be placed as close as possible to the XTAL pins on the device. The traces should be as
short as possible and shielded with ground plane from any other traces which could introduce noise or
interference.
When using an external crystal, the external oscillator drive circuit must be configured by software for Crys-
tal Oscillator Mode or Crystal Oscillator Mode with divide by 2 stage. The divide by 2 stage ensures that the
clock derived from the external oscillator has a duty cycle of 50%. The External Oscillator Frequency Con-
trol value (XFCN) must also be specified based on the crystal frequency. The selection should be based on
Table 19.1. For example, a 25 MHz crystal requires an XFCN setting of 111b.
When the crystal oscillator is first enabled, the external oscillator valid detector allows software to deter-
mine when the external system clock has stabilized. Switching to the external oscillator before the crystal
oscillator has stabilized can result in unpredictable behavior. The recommended procedure for starting the
crystal is as follows:
1. Configure XTAL1 and XTAL2 for analog I/O and disable the digital output drivers.
2. Configure and enable the external oscillator.
3. Poll for XTLVLD => 1.
4. Switch the system clock to the external oscillator.
188
XFCN
000
001
010
100
101
011
110
111
Table 19.1. Recommended XFCN Settings for Crystal Mode
155 kHz  f 415 kHz
415 kHz  f 1.1 MHz
1.1 MHz  f 3.1 MHz
3.1 MHz  f 8.2 MHz
8.2 MHz  f 25 MHz
58 kHz  f 155 kHz
20 kHz f 58 kHz
Crystal Frequency
Figure 19.2. 25 MHz External Crystal Example
f  20 kHz
25 MHz
15 pF
15 pF
Rev. 1.1
Bias Current
10 Mohm
120 µA
550 µA
2.6 mA
0.5 µA
1.5 µA
4.8 µA
14 µA
40 µA
XTAL1
XTAL2
Typical Supply Current
3.0 µA, f = 32.768 kHz
4.8 µA, f = 32.768 kHz
9.6 µA, f = 32.768 kHz
193 µA, f = 400 kHz
3.9 mA, f = 25 MHz
28 µA, f = 400 kHz
71 µA, f = 400 kHz
940 µA, f = 8 MHz
(VDD = 2.4 V)