Universal Serial Bus (USB)
18-6
Am186™CC/CH/CU Microcontrollers User’s Manual
primary system clock must be a minimum of 24 MHz when using the USB peripheral
controller.
To select the dedicated USB clock source, assert either the USBSEL2 or USBSEL1 pinstrap
during reset (power-on or assertion of RES). These pinstraps select either 4x or 2x PLL
operation, allowing the use of a 12-MHz or 24-MHz crystal, respectively, as the USB clock
input on pins USBX1 and USBX2. Table 18-2 lists the permutations of the USB PLL mode
pinstraps.
18.3.1.4
Isochronous Synchronization Signals
The USBSCI signal input and USBSOF signal output provide for isochronous transfer
synchronization, which is described on page 18-23. These signals are multiplexed on the
same pin with the UART external clock input signal (UCLK).
Enabling the USBSOF signal output (by setting the ESOF_EN bit in the Isochronous
Synchronization Control (ISCTL) register) overrides the USBSCI signal input if that signal
is also selected (through the SAM_CLK_SEL field in the ISCTL register.)
Do not enable the USBSOF signal output at the same time as the UART external clock
input (UCLK). The UCLK signal is enabled by the XTRN bit in the High-Speed Serial Port
Control 1 (HSPCON1) or Serial Port Control 1 (SPCON1) registers.
The USBSCI and UCLK signal inputs can be enabled at the same time, but it is unlikely
that the same signal source can be used as an input for both of these functions.
18.3.2
DMA Trade-Offs
The microcontroller contains two different kinds of DMA channels, general-purpose DMA
and SmartDMA channels. The USB data endpoints can use either kind. Choosing which
type of DMA channel to use, if any, involves the following system trade-offs:
■
The integrated HDLC controllers in the Am186CC microcontroller can use only the
SmartDMA channel. Consequently, if all four HDLC controllers are to be used with DMA
(for high-bandwidth HDLC connections), then the USB can use only general-purpose
DMA or no DMA.
■
Other integrated peripherals such as the UARTs and the external DMA request lines
can use only the general-purpose DMA channels.
■
For USB bulk endpoints, SmartDMA channels have advantages over general-purpose
DMA channels that can result in higher performance and lower software overhead,
especially when each transaction is relatively small. When most transactions are
relatively large, general-purpose DMAs may have a small performance advantage over
SmartDMA channels.
■
For USB isochronous endpoints with true streaming data, general-purpose DMAs are
slightly easier to use than SmartDMA channels.
Table 18-2
USB PLL Mode Pinstraps
{USBSEL1} {USBSEL2} USB PLL Mode
1
1
Use CPU clock, USB PLL disabled (default)
0
1
4x, USB PLL enabled
1
0
2x, USB PLL enabled
0
0
Reserved
CC
Содержание Am186 CC
Страница 1: ...Am186 CC CH CU Microcontrollers User s Manual Order 21914B...
Страница 4: ...iv Am186 CC CH CU Microcontrollers User s Manual...
Страница 18: ...Table of Contents xviii Am186 CC CH CU Microcontrollers User s Manual...
Страница 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Страница 39: ...Architectural Overview Am186 CC CH CU Microcontrollers User s Manual 1 15 Figure 1 6 32 Channel Linecard CH CC...
Страница 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Страница 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Страница 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Страница 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Страница 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...