AM8530 datasheet(97/194 Pages) AMD

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5–3

Support Circuitry Programming

CHAPTER 5

5.1

INTRODUCTION

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

CLOCK OPTIONS

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

Crystal Oscillator

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;

30 ppm @ 25

°C

<50 ppm over temperature range of –20 to 70

°C

<5 ppm/yr aging

5 mw drive level

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