System Overview
3-34
Am186™CC/CH/CU Microcontrollers User’s Manual
3.7.2
PLL Bypass Mode
The Am186CC/CH/CU microcontrollers provide a PLL Bypass mode that allows the X1
input frequency to be anything from 0 to 24 MHz. Select PLL Bypass by asserting CLKSEL1
and CLKSEL2.
When the microcontroller is in PLL Bypass mode, the CLKOUT frequency equals the X1
input frequency. When changing frequency in PLL Bypass mode, the X1 input must not
have any short or “runt” pulses. At 24 MHz, the nominal High/Low time is 21 ns. The actual
High times and Low times must not fall below 16 ns. These values allow a 60%/40% duty
cycle at X1.
In the Am186CC and Am186CU microcontrollers, the USB PLL and USBX1 determine the
USB clock. USB requires the CPU clock to be 24 MHz or greater. Therefore, disable the
USB peripheral controller before slowing the CPU clock to less than 24 MHz. If USB is not
used, you can pull down USBX1.
In the Am186CC and Am186CH microcontrollers, the system clock must be at the same
or a greater frequency than the HDLC clock and UCLK (if using UCLK). Therefore, if
reducing the system clock frequency, disable these interfaces or run them at a lower
frequency.
In the Am186CC microcontroller, the system clock must be the same or twice the frequency
of the GCI clock. Therefore, if reducing the system clock frequency, disable the GCI interface
or run it at a lower frequency.
3.8
HARDWARE-RELATED CONSIDERATIONS
■
Pins latched on reset (pinstraps) are not resampled during a watchdog-timer reset.
■
If the external reset (RES) signal is asserted while the watchdog timer is performing a
watchdog-timer reset, the external reset takes precedence over the watchdog-timer
reset. This means that the RESOUT signal asserts as with any external reset and the
WDTCON register does not have the RSTFLAG bit set. In addition, the microcontroller
exits reset based on the external reset timing (i.e., 4.5 clocks after the deassertion of
RES rather than 2
16
clocks after the watchdog timer time-out occurred).
3.9
COMPARISON TO OTHER DEVICES
■
The 80C186 microcontroller provided a 16-bit wide data bus over its entire address
range, memory, and I/O, but did not allow accesses to an 8-bit wide bus. However, the
data bus width on the Am186CC/CH/CU microcontrollers is programmable to be 8 bits
or 16 bits.
■
Earlier Am186 microcontrollers included a power save clock mode. The
Am186CC/CH/CU microcontrollers are not designed for low-power applications and
therefore do not incorporate the power save clock mode. However, the Am186CC/CH/CU
microcontrollers do have a PLL Bypass mode that allows the X1 clock input frequency
to be anything from 0 to 24 MHz.
3.10
INITIALIZATION
On both an external and internal reset, the following occurs:
■
The SYSCON register defaults to 00h, which has the following effects: sets normal timing
on DEN for read and writes, disables PWD mode, enables memory and I/O addresses
on the AD15–AD0 bus, and enables CLKOUT.
■
The PRL register defaults to the processor revision level.
■
Multiplexed signals default as shown in Table 3-7 on page 3-10.
CU
CC
CH
CC
CC
Summary of Contents for Am186 CC
Page 1: ...Am186 CC CH CU Microcontrollers User s Manual Order 21914B...
Page 4: ...iv Am186 CC CH CU Microcontrollers User s Manual...
Page 18: ...Table of Contents xviii Am186 CC CH CU Microcontrollers User s Manual...
Page 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Page 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Page 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Page 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Page 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Page 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...