Cypress DCT-1D Betriebsanweisung PDF herunterladen (Seite 32)

CHAPTER 3. LAB TASK 1 - INTERFACING TO THE WISHBONE BUS

In the return path the addressed slave’s s-bus is connected to all the masters. This

is handled by the block DEC. The ack-signal is, however, only asserted at the master

that won the arbitration.

3.3 A Simple Computer

3.3.1 General

For the lab you will have to download tsea44.tgz if you haven’t done so already.

Uncompress the zip-ﬁle to your home directory. Inspect the directory hw and you will

ﬁnd:

• the ﬁle lab1/lab1_uart_top.sv, a skeleton for the top ﬁle.

• the ﬁle lab1.ucf, a User Constraints File.

• the directory or1200 containing the CPU. The top ﬁle is or1200_top.sv.

• the directory monitor containing both HW and SW for the boot monitor.

• the directory wb containing the Wishbone interconnect.

• the directory include containing some include ﬁles.

• the directory firmware, which contains the example program dct_sw/dct_sw

that can be downloaded to your computer with the boot monitor.

3.3.2 A Wishbone Interface for the UART

Let’s start our computer design with the UART. In the introductory lab you designed a

simple UART. All that is needed now is to attach a Wishbone interface to your design,

see Figure 3.4.

Since you will use a boot monitor that is written for the standard 16550 UART,

you will want to make your design emulate that UART.

Luckily our device driver does not use much of the functionality in the 16550.

The main enhancement in the 16550 are 16 character FIFOs in both directions. This is

more or less mandatory when you run an OS, which always has some interrupt latency.

The driver routine expects three bytesized registers:

1. transmit register, adr=0, write-only

2. receive register, adr=0, read-only

3. status register, adr=5, read-only

In the status register, you will only need two F/Fs:

• rx_full, set when the stop-bit is received and reset when the receive register is

read. Use signal wb_sel[3] to determine when the receive register is read.

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Cypress DCT-1D Betriebsanweisung Seite 32