原创 fpga学习日记30，Cyclone II全局时钟与PLL

除了电源时钟应该是fpga的第二重要的部分了，扮演着系统心脏的角色，是一切运算的基础。

使用HDL编程时往往就是以clk信号为驱动信号进行设计的，所以clk信号将直接影响整个芯片的逻辑。

搞清楚时钟的来龙去脉对软硬件的设计都是有好处的。

本文参考资料：

Cyclone II hand book

Altera时钟与PLL特性

http://www.altera.com/support/devices/pll_clock/pll-overview.html

Using PLLs in Cyclone Devices

http://www.altera.com/literature/hb/cyc/cyc_c51006.pdf

PLLs in Cyclone II Devices (PDF)

http://www.altera.com/literature/hb/cyc2/cyc2_cii51007.pdf

Clock Control Block (ALTCLKCTRL) Megafunction

http://www.altera.com/literature/ug/ug_altclock.pdf

先看下官方的描述（cyclone系列最多4个PLL  16条全局时钟网络）

Cyclone II devices provide a global clock network and up to four phase-locked loops (PLLs).

The global  clock network consists of up to 16 global clock lines that drive throughout the entire device.

The global clock network can provide clocks for all resources within the device, such as input/output elements (IOEs), LEs, embedded multipliers, and embedded memory blocks. The global clock lines can also be used for other high fan-out signals. Cyclon e II PLLs provide general-purpose clocking with clock synthesis and phase shifting as well as external outputs for high-speed differential I/O support.

Flexible clock management circuitry

● Hierarchical clock network for up to 402.5-MHz performance

● Up to four PLLs per device provide clock multiplication and

division, phase shifting, programmable duty cycle, and external

clock outputs, allowing system-level clock management and

skew control

● Up to 16 global clock lines in the global clock network that drive

throughout the entire device

关于全局时钟

Each Altera® FPGA has an advanced, low-skew clock network. The number of clock networks and features offered varies depending on the device family (see Tables 1 and 2). The clock networks can be driven by CLK input pins, phase-locked loop (PLL) outputs, or internal logic, and can be used for other device-wide signals with large fan-outs, such as asynchronous clears and clock enables.

Each global clock network has a clock control block to select from a number of input clock sources (PLL clock outputs, CLK[]  pins,  DPCLK[] pins, and internal logic) to drive onto the global clock network. Ta b l e 2 – 2 lists how many PLLs,  CLK[]  pins,  DPCLK[] pins, and global clock networks are available in each Cyclone II device.  CLK[] pins are dedicated clock pins and DPCLK[] pins are dual-purpose clock pins.

The 16 or 8 global clock networks drive throughout the entire device. Dedicated clock pins (CLK[]), PLL outputs, the logic array, and dual-purpose clock (DPCLK[]) pins can also driv e the global clock

network.

The global clock network can provide clocks for all resources within the device, such as IOEs, LEs, memory blocks, and embedded multipliers. The global clock lines can also be used  for control signals, such as clock enables and synchronous or asynchrono us clears fed from the external pin, or DQS signals for DDR SDRAM or QDRII SRAM interfaces. Internal logic can also drive the global clock network for internally generated global clocks and asynchronous clea rs, clock enables, or other control signals with large fan-out.

CYCLONE全局时钟与引脚

全局时钟引脚分为专用时钟引脚和两用时钟引脚

Larger Cyclone II devices (EP2C15 and larger devices) have 16 dedicated clock pins (CLK[15..0] , four pins on each side of the device). Smaller Cyclone II devices (EP2C5 and EP2C8 devices) have eight dedicated clock pins (CLK[7..0], four pins on left and right sides of the device). These

CLK  pins drive the global clock network (GCLK), as shown in Figures 2–11 and 2–12.

Cyclone II devices have either 20 dual-purpose clock pins, DPCLK[19..0] or 8 dual-purpose clock pins,  DPCLK[7..0]. In the larger Cyclone II devices (EP2C15 devices and higher), there are 20DPCLK pins; four on the left and right sides and six on the top and bottom of the device. The corner CDPCLK pins are first multiplexed before they drive into the clock control block. Since the signals pass through a multiplexer before feeding the clock control block, these signals incur

more delay to the clock control block than other DPCLK  pins that directly feed the clock control block. In the smaller Cyclone II devices (EP2C5 and EP2C8 devices), there are eight DPCLK pins; two on each side of the device

These dual-purpose pins can connect to the global clock network for high-fanout control signals such as clocks, asynchronous clears, presets, and clock enables, or protocol control signals such as TRDY and IRDY for PCI, or DQS signals for external memory interfaces.

全局时钟的来源（时钟控制块）

Notes to Figure 2–13:

(1) The CLKSWITCH signal can either be set through the configuration file or it can be dynamically set when using the manual PLL switchover feature. The output of the multiplexer is the input reference clock (fIN) for the PLL.

(2) The CLKSELECT[1..0]  signals are fed by internal logic and can be used to dynamically select the clock source for the global clock network when  the device is in user mode.

(3) The static clock select signals are se t in the configuration file and cannot be  dynamically controlled when the device is in user mode.

(4) Internal logic can be used to enabled or di sabled the global clock network in user mode.

Global Clock Network Distribution（全局时钟分布）

Cyclone II devices contains 16 global clock networks. The device uses multiplexers with these clocks to form six-bit buses to drive column IOE clocks, LAB row clocks,  or row IOE clocks (see Figure 2–14). Another multiplexer at the LAB level selects two of the six LAB row clocks to feed

the LE registers within the LAB.