AM8530H AMD [Advanced Micro Devices], AM8530H Datasheet - Page 97

• Manufacturer Part Number: AM8530H
• Description: Serial Communications Controller
• Manufacturer: AMD [Advanced Micro Devices]

CHAPTER 5
5.1
The SCC incorporates additional logic on-chip which dramatically reduces the need for
external hardware. This includes clocking options, baud rate generators, clock recovery
logic, on-chip oscillators, and internal loopback modes. This chapter discusses how to
program these functions.
5.2
The SCC may be programmed to select one of several sources to provide the transmit
and receive clocks. In addition, the SCC contains a crystal oscillator in each channel, as
well as the ability to echo one of several internal clock sources off chip. These options are
controlled by the bits in WR11 as shown below.
WR11 is the Clock Mode Control register for both the receive and transmit clocks. It de-
termines the type of signal on the SYNC and RTxC pins and the direction of the TRxC
pin. This register also controls the output of the baud rate generator, the DPLL output,
and the selection of either an input clock or XTAL output for the RTxC pin.
5.2.1
The crystal oscillator option is controlled by bit D7 in WR11. When this bit is set to ‘0’, the
crystal oscillator is disabled and all pins function normally. When this bit is set to ‘1’, the
crystal oscillator is enabled and a high-gain amplifier is connected between the RTxC pin
and the SYNC pin. While the crystal oscillator is enabled, anything that has RTxC se-
lected as its clock source will automatically be connected to the output of the crystal oscil-
lator.
While the crystal oscillator is enabled, the SYNC pin is unavailable for other use. In Syn-
chronous modes no sync pulse is output, and the External Sync mode cannot be se-
lected. In Asynchronous mode, the state of the SYNC/HUNT bit in RR0 is no longer con-
trolled by the SYNC pin. Instead, the SYNC/HUNT bit is forced to ‘0’. Note that the crystal
oscillator requires some finite time to stabilize (20 ms) and so must be allowed to stabilize
before being used as a clock.
For best results, a crystal oscillator with the following specifications should be used;
Support Circuitry Programming
30 ppm @ 25 C
<50 ppm over temperature range of –20 to 70 C
<5 ppm/yr aging
5 mw drive level
INTRODUCTION
CLOCK OPTIONS
Crystal Oscillator
5–3