Compaq 21264 - Manual (Page 260)

11–2 Testability and Diagnostics Alpha 21264/EV67 Hardware Reference Manual
SROM/Serial Diagnostic Terminal Port
11.2 SROM/Serial Diagnostic Terminal Port
This port supports two functions. During power-up, it supports automatic initialization
of the Cbox configuration registers and the Icache from the system serial ROMs. After
power-up, it supports a serial diagnostic terminal.
11.2.1 SROM Load Operation
The following actions are performed while the SROM is loaded:
• The SromOE_L pin supplies the output enable as well as the reset to the serial
ROM. (Refer to the serial ROM specifications for details.) The 21264/EV67 asserts
this signal low for the duration of the Icache load from the serial ROM. When the
load has been completed, the signal remains deasserted.
• The SromClk_H pin supplies the clock to the SROM that causes it to advance to
the next bit. Simultaneously, it causes the existing data on the SromData_H pin to
be shifted into an internal shift register. The cycle time of this clock is 256 times the
CPU clock rate. (If the FASTROM flag is set, the rate is 16 times the CPU clock
rate.) The hold time on SromData_H is 2* CPU cycle time with respect to
SromClk_H.
• The SromData_H pin reads data from the SROM.
Every data and tag bit in Icache is loaded by that sequence.
11.2.2 Serial Terminal Port
After the SROM data is loaded into the Icache, the three SROM interface signals can be
used as a software UART and the pins become parallel I/O pins that can drive a system
debug or diagnostic terminal by using an interface such as RS422.
The serial line interface is automatically enabled if the SromOE_L pin is wired to the
following pins:
• An active high enable RS422 (or 26LS32) driver, driving to SromData_H
• An active high enable RS422 (or 26LS31) receiver, driven from SromClk_H
After reset, SromClk_H is driven from the Ibox I_CTL[SL_XMIT]. This register is
cleared during reset, so it starts driving as a 0, but it can be written by software. The
data becomes available at the pin after the HW_MTPR instruction that wrote
I_CTL[SL_XMIT] is retired.
SromClk_H Output SROM clock/Diagnostic terminal data output
SromOE_L Output SROM enable/Diagnostic terminal enable
TestStat_H Output BiST status/timeout output
Table 11–1 Dedicated Test Port Pins (Continued)
Pin Name Type Function