Hardware Description
Circuit Description
USB08 Evaluation Board
Designer Reference Manual
MOTOROLA
Hardware Description
31
2.4.1 MCU Core Circuit and USB Interface
The operating voltage, V
DD
, is supported by the capacitors C6 and C3
close to the MCU. Out of this primary operating voltage of approximately
5 V, the MCU produces an internal operating voltage, V
REG
, of 3.3 V,
using an integrated voltage regulator. This voltage is supported by two
capacitors, C4 and C5, and continues in the circuit as V
CC
.
In particular, the internal voltage V
CC
/V
REG
is used as the USB interface
driver voltage supply. V
CC
/V
REG
is accessible over the expansion plug
connector X1. However, it must be noted that V
CC
/V
REG
can be
additionally loaded only with a few milliamps.
For clock generation, the external elements Q1, C1, C2, and R18 are
used. These elements form a Pierce oscillator together with the active
elements integrated in the MCU. This oscillator produces a clock
frequency of 6 MHz. The internal bus clock of the MCU (3 MHz) as well
as the USB clock (1.5 MHz) are derived from the main clock frequency.
The USB data lines are connected to the MCU pins PTE3 (USB D+) and
PTE4 (USB D–). So that the USB hub will be able to classify this
equipment as a low-speed USB device, a pullup resistance of 1.5 k
Ω
(R7) to the data line D– is required.
On the demo board, R7 is not installed. This is because the
MC67HC908JB8 has an additional internal pullup at PTE4 which can be
activated and de-activated by software.
To optimize the connection adjustment, the serial resistors in the data
lines R16 and R17 and inductances (ferrite beads) in the current supply
path L1 and L2 are used. However, these measures are optional.
The reset system of the M68HC08 shows clear differences from other
Motorola MCUs (M68HC11 and M68HC12). For example, the capacitor
C19 at the reset pin of this circuit could never be used in an M68HC11
system. This is because the MC68HC908JB8 has an integrated
low-voltage inhibit (LVI) circuit. Therefore, no external reset controller is
required.
